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284 Cards in this Set
- Front
- Back
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What do disorders involving amino acids result from?
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Either a problem in amino acid uptake
or Incorrect amino acid metabolism |
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What is the ultimate source of amino acid nitrogen?
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Atmospheric nitrogen (N2 gas)
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Can N2 gas be incorporated into amino acids?
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No, it cannot be directly incorporated into amino acids
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How can N2 gas be incorporated into amino acids?
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The nitrogen needs to be fixed
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How can nitrogen be fixed?
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There are two ways:
1) By microorganisms 2) Process of lightning |
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What is so special about the bond in N2?
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It is a triple bond that is incredibly stable as it is so difficult to break it.
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How much energy is required to break the triple bond in N2?
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945 kJ
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What type of bacteria can break the triple bond in N2?
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Diazotrophs
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What does the action of diazotrophs lead to?
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Production of NH3
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What are the three main ways to get NH3 into the biosphere?
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Synthesis of glutamate (GLU)
Synthesis of glutamine (GLN) Synthesis of carbarnyl phosphate |
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How is over 90% of dietary nitrogen taken in?
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90% of dietary nitrogen is taken in the form of amino acids.
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What happens to the nitrogen produced when lightning breaks the triple bond of N2?
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The nitrogen produced is very reactive and reacts immediately with oxygen to form No2, and NO3 (nitrate)
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What happens to the nitrate?
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Some microorganisms, known as nitrifying bacteria, can convert nitrates to ammonia (NH3)
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Where are most nitrifying bacteria found?
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Most nitrifying bacteria are found in the soil.
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Which organism do nitrifying bacteria form a symbiotic relationship with?
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Leguminous plants (ex: beans, lentils, not corn or wheat)
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How does the symbiotic relationship between the nitrifying bacteria/plant affect the plant structure?
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Nodules are formed at the root ends
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Why do legunimous plants allow the bacteria to grow at their root ends?
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This is beneficial for them because the bacteria take nitrates and provide the plant with the ammonia (NH3) necessary to make amino acids.
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What are fertilizers used for?
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As a means of artificially enriching our fields with NH3
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What type of reaction is that of the fertilizer action on atmospheric nitrogen?
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It is dangerous, requires H2 gas which directly reduces N2, and a high temperature, high pressure, and iron catalyst
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Do diazotrophs require alot of energy for N2 fixation?
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Yes, the reaction of N2 fixation is very energetically expensive for diazotrophs
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Which enzymes of the diazotrophs are responsible for performing the nitrogen fixation?
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Nitrogenases.
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What is the catalyst/cofactor for the nitrogen fixation performed by nitrogenases in diazotrophs?
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Iron
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How many NH3 molecules are produced per molecule of N2 gas?
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Two
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How much ATP is required for the fixation of nitrogen by nitrogenases in diazotrophs?
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16 ATP
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How many electrons are required for the fixation of nitrogen by nitrogenases in diazotrophs?
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8 electrons
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What does "nitrogen balance" mean?
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Nitrogen balance means the state of an individual over a 24 hour perioud, it is the difference between the amount of nitrogen taken in (N intake) and the amount of nitrogen expelled (N outflow)
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What is positive nitrogen balance?
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Retaining more nitrogen tthan secreting
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What are some cases where positive nitrogen balance can occur?
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Growth, pregnancy, weight lifting, anabolic steroids
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What is negative nitrogen balance?
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More nitrogen is secreted than taken in
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When are some cases where negative nitrogen balance occurs?
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Illness, fever
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How do amino acids exist in the cell?
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In two forms:
-in body protein form -free pool of amino acids |
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For a 70kg individual, how much of the weight can be attributed to body proteins?
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10kg
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For a 70kg individual, how much of the weight can be attributed to the free pool of aa?
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100 g
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How much proteins do you make and break every day?
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300 g
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Besides protein synthesis, what can the free pool of aa be used for?
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Making N containing compounds
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What are some examples of N containing compounds?
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Purines (adenine, guanine), Pyrimidine (cytosine, thymine), creatine, heme
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Why do we degrade excess free amino acid after synthesis of body proteins and free amino acids?
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-It is energetically expensive to maintain the free aa pool
-If aa accumulate- can be toxic to the blood and damage tissues |
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How do we get free aa?
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-Body protein degradation
-Consume in diet (essential aa) -Pathways to make nonessential aa |
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What are the essential amino acids in humans?
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Arginine
histidine isoleucine leucine lysine methionine phenylalanine threonine tryptophan valine |
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What are the nonessential amino acids in humans?
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Alanine
Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Proline Serine Tyrosine |
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What does the "essential" mean in essential amino acids?
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You have to eat this amino acid in your diet because we are missing the enzymes required to synthesize them.
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Why is arginine conditionally essential?
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As adults, we can make arginine
But children grow too fast and need it in their diet |
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Which NON-ESSENTIAL amino acids require ESSENTIAL amino acids in order to be synthesized?
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Cysteine and tyrosine
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Which essential amino acid does cysteine require?
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Methionine
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Which essential amino acid does tyrosine require?
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Phenylalanine
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What happens if you miss even a single amino acid?
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You will be in a negative nitrogen balance
The body will react like it is starving and start degrading muscle (skeletal, cardiac, smooth) |
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What happens if you have an excess of aa?
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To maintain homeostasis, we use amino acid oxidation to degrade the aa
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What are the two types of oxidation pathways?
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Regulated and obligatory oxidation pathway
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What is the obligatory oxidation pathway?
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It is turned on in the "post-absorptive state" and is the basal rate of aa oxidation
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What is the "post-absorptive state"?
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10-12 hour after you eat, i.e. before breakfast
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When is regulated oxidation turned on?
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To maintain N equlibrium, it is turned on directly after a meal to regulate levels of N
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What are the two ways that aa are used to produce N-containing compounds?
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Using whole amino acids or just the alpha amino acid amino group
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What are two examples of amino acids that are used as a whole to synthesize N containing compounds?
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Glycine and Tyrosine
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What are some examples of N containing compounds that glycine makes?
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Heme, Purine rings, Creatine
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What are some examples of N-containing compounds that tyrosine makes?
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Proteins + neurotransmitters such as dopamine, norepinephrine, epinephrine
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Normally, Where does the alpha amino group come from when synthesizing N containing compounds using just the aa amino group?
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Glutamine
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How is glutamine produced?
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Glutamate is converted to the intermediate gamma-glutamylphosphate by glutamine synthetase upon addition of ATP.
This intermediate is converted to Glutamine by the addition of ammonia and the release of a phosphate |
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What type of reaction is the production of glutamine?
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It is the first ammonia assimilation reaction
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What is the most common aa disorder?
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PKU- phenylketouria
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What type of reaction is the synthesis of glutamine from glutamate?
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An amidation reaction
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Which amino acids cannot transfer their alpha amino group to alpha-ketoglutarate to form glutamate?
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Threonine and Lysine
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Which enzymes mediate the transfer of an amino group onto alpha ketoglutarate to form glutamate?
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Transaminases or aminotransferases
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What are the two mechanisms by which glutamate can be converted to glutamine?
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Its amino groupcan be released by glutamate dehydrogenase or it can be used directly by glutamine synthetase
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Which other reactants are required for the synthesis of of glutamine besides glutamate?
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ATP, amino group
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Which enzyme converts Glutamine to Glutamate and what is released/consumed in the process?
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An H2O molecule is used in the reaction by glutaminase to convert glutamine to glutamate. An amino group is released in the process
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Why is glutamine an important AA?
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It is used in protein synthesis
It is used as a source of N in biosynthetic reactions It is a non-toxic storage vesicle for NH3, an alternate to the urea (alanine) cycle |
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What is glutamine synthetase activated by?
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Alpha keto glutarate
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What is the intermediate formed by the glutamine synthetase reaction?
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Gamma-glutarnyl phosphate (formed by taking the terminal phosphate from ATP)
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What is special about the phosphate on this intermediate?
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It is a good leaving group that is readily replaced by NH3
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How can all amino acids be used for the production of N containing compounds?
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Their alpha amino groups can undergo a "funneling" reaction
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How does this funneling reaction work?
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The alpha amino group from any amino acid is transfered to alpha keto glutarate to form glutamate
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What else is the glutamate dehydrogenase reaction important for?
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Production of urea in amino acid metabolism
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How does the regulation of glutamine synthetase by alpha ketoglutarate work?
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Alpha ketoglutarate allosterically regulates glutamine synthetase. The enzyme senses an accumulation of NH3 in the blood stream, through akga. When there is too much ketoacid of glutamate (akga)- will stimulate glutamine synthetase to make glutamine from this.
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Why is the funnelling of any aa alpha amino group by transaminases so important?
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Glutamine (Glu) is the only amino acid that can undergo oxidative deamination at an appreciable rate
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What is the coenzyme of transaminases?
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PLP (Pyridoxal Phosphate)
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What is PLP a derivative of?
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Vitamin B6
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What is PMP?
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A derivative of PLP formed in the transamination reaction
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What do we need to do to have PLP and be able to use it?
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Ingest Vitamin B6 in our diet
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What happens to Vitamin B6 once we ingest it?
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It enters the blood stream, enters cells, and phosphorylation and addition of an aldehyde occur, forming PLP
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What are some other reactions that PLP containing enzymes mediate?
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a-decarboxylation, racemization, a.b-elimination, b,g-elimination, aldolization, b-decarboylation of Aspartate
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All the enzymes that involve PLP as a coenzyme are involved in amino acid metabolism except for one, which?
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Glycogen phosphorylase
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What intake of VB6 is adequate for most adults?
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1.5-2 mg/day
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What are the four most important components of the PLP molecule?
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C 4' aldehyde group, pyridine ring, phosphate group, 3' OH group
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What is the C 4' aldeyde group important for
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Forming a bond with the enzyme and with the substrate
Forms a covalent (imine) link with alpha amino group |
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Why is the pyridine ring important?
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It acts as an electron sink, used to delocalize electron pairs during the rection. Bonds are always bokren and formed in enzymatic reactions. In intermediates, when electrons need to be delocalized, the ring will absorb the electron pair and stabilize the intermediates, so rxn can proceed
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Why is the phosphate group important?
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It acts as a handle by which the apoenzyme holds onto the PLP
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Why is the hydroxyl group important?
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Transamination reactions require the establishment of a planar geometry of the molecule.
THe OH forms an H bond with the amino group (from acid or enzyme) that will be added onto the aldehyde group. By doing so, extrends the surface area and can delocalize e pairs by resonance: improve catalytic efficiency |
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How is the PLP linked to the enzyme?
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Covalently
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What is a Schiff base?
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The type of bond that is the covalent link through the C'4 aldehyde to the Enzyme. It is a carbon-nitrogen double bond, with the N atom connected to an aryl or alkyl group
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What are some other features of ttransamination reactions?
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The formation of an external aldimine bond and carbanion intermediate
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What is the difference between an external aldimine bond and an internal aldimine bond?
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Initial Schiff base= internal aldimine bond
switches to an external aldimine bond, where the PLP is bound to the alpha amino group of the amino acid. |
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What happens in the carbanion intermediate?
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E pair is delocalized throughout whole of PLP- stablizes whole molecule
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How many phases do transamination reaction exist in?
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Two
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What type of mechanism is the transamination reaction?
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Pingpong Bibi
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What is the first phase of transamination?
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Formation of a PMP enzyme and alpha keto acid
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What is the second phase of transamination?
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Formation of glutamate and a regeneration of the PLP enzyme
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Describe the first phase: where a PMP enzyme and alpha-keto acid are formed
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Amino acid bind to enzyme/PLP-> release of enzyme-> tautomerization after formation of carbanion intermediate, hydrolysis to have release of PMP-enzyme + alpha keto acid of amino acid that started with.
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Describe the 2nd phase
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Reversible reaction at every step
In this case the substrate is an alpha keto acid forming another amino acid |
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What type of geometry is around the imine C4 bond of the molecule? (PLP/aa or PLP/enzyme?)
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Planar geometry
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How does this planar geometry exist?
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The 3' OH of the pyridine forms a planar chelate ring with a proton of the alpha- amino group
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What is this planar geometry important for?
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Enzyme activity
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What is the overall reaction of transaminases?
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AA1 + alpha-ketoacid2 -> AA2 + alpha-ketoacid 1
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Which alpha keto acid do most enzymes prefer to use as a keto substrate in the inverse direction?
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Alphaketoglutarate
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Explain the ping-pong bi bi mechanism and how it can be used to produce glutamate.
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Any amino acid (A1) comes in, interacts with the PLP/Enzyme to release alphaketoacid 1. The PMP (which has an amino group from AA1)/Enzyme can now perform the reverse reaction on the alphaketoacid, but in this case, takes alphaketoacid 2 (alphaketoglutarate), and produces amino acid 2 (glutamate). So you start off with AA1, akga (aka2), and end up with glutamate (AA2) and the unused aka1
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What is an example of a disorder that arises from transamination defects?
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Type II Tyrosemia
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What is Type II tyrosemia caused by?
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Non-sense mutations (where a stop codon is introduced) in one of the genes that encodes a key enzyme in the processs
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What type of genetic disorder is this?
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It is an inherited autosomal recessive defect in Tyrosine Catabolism
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Which enzyme is deficieny in Type II tyrosemia?
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Tyrosine aminotransferase
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What are some symtoms of this disorder?
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Photophobia, sensitive skin, blistering, mental retardation/microencephaly, behavioral problems
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What are three reasons why transaminases are important?
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Synthesis of N containing compounds
To rid the body of excess free AA Oxidative Deamination |
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Why are transaminases important in the synthesis of N containing compounds?
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Need to funnel the AA pool through glutamate
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Why are transaminases important in getting rid of excess free AA?
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Glutamate is the only AA that can undergo oxidative deamination at an appreciable rate in the cell
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How is oxidative deamination of glutamate mediated?
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By Glutamate dehydrogenase
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Where is GDH found and in which tissue is its activity most important?
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GDH is widely distributed in mammalian tissues- peripheral cells, mostly involved in production of N contraining compounds.
It is most important in the liver mitochondria for the production of urea |
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How much of urea synthesis occurs in liver mitrochondria?
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99% (the other 1% is in the intestines)
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In the liver, what happens when an amino group is released?
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It will immediately get incorporated and detoxified in the form of urea. Urea is passed through the urine
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How do we get rid of ammonia in peripheral tissues?
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Because of toxicity, we cannot just put the NH3 through the blood stram. The body has developed the GLUCOSE/ALANINE cycle to transport NH3
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What is the maximum ammount of ammonia the blood can have?
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75 mmol
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What do the muscles use the process of the glucose/alanine cycle for?
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To elimate nitrogen during AA catabolism
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When is this process particularily important?
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In the post absorbtive state: we are using the amino acids for energy, and therefore need to detoxify the tissues
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How can alanine be produced in skeletal muscle?
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Body protein degradation
Transamintation of pyruvate by muscle alanine transaminase |
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What is different about the alanine transaminase?
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Normal transaminases prefer alphaketoglutarate as aka2, but in this case, alanine transaminase prefers pyruvate, resulting in the production of alanine instread of glutamate
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What is the rate of alanine formation from pyruvate proportional to?
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Pyruvate intracellular levels
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What are the six steps of the glucose/alanine cycle?
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1) The alpha amino group of GLU is transfered to pyruvate by MLT, forming alanine
2) Alanine is not toxic and can travel to liver through blood 3) Alanine undergoes transamination with alphaketoglutarate to regenerate GLU and pyruvate 4. Glutamate unddergoes oxidative deamination by GDH. NH3 is incorporated into Urea 5. Pyrvate is converted to glocse, which is transported to the muscle to be used for energy 6. Once in the muscle, glucose is converted to pyruvate, restarting cycle |
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How is nitrogen also transporter to the liver?
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In the form of GLN (glutamine)
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Where are the two locations that glutamine can release its ammonia group?
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Liver and Kidney
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What happens to the ammonia released by glutamine in the LIVER?
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Produce urea
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What happens to ammonia released by glutamine in the KIDNEY?
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NH3 can be released directly to urine by enzyme glutaminase
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What is the 2nd ammonia assimilation reaction?
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The reverse reaction of GDH- formation of glutamate
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What type of intermediate is formed?
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a-iminoglutarate intermediate
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What type of reaction is the GDH reaction?
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Oxidation, reversible
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What type of coenzyme can GDH use?
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NAD+ or NADP+
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What type of enzyme is GDH and how is its activity modulated?
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Allosteric enzyme and its activity is modulated according to the energy of the cell
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How does GTP affect GDH activity?
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Decreases it
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How does ADP affect activity?
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Increases it
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How is the energy charge of the cell most of the time?
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High- due to our western diet
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What is the flux of glutamate like though GDH?
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It is continuous, even when enzyme activity is low.
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What is an example of a disease caused in GDH mutation?
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Congenital HyperInsulinism/HyperAmmonemia
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What are some symptoms of this disorder?
|
high insulin/hypoglycemia
elevated blood ammonia evels |
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What type of mutation causes this disorder?
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Missense mutation (1aa is changed to another) in the GTP allosteric site
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What is a result of this mutation?
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Enzyme is insensitive to GTP and therefore is not inhibtied as much
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What does this lack of inhibition by GTP lead to?
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An increase in alphaketoglutarate and NH3
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What does an increase in alphaketoglutarate resutlt in?
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TCA cyle stimulation, oxphos, resulting in increased insulin production and hypoglycemia
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What is the main method to get ammonia to the liver in a nontoxic form?
|
The alanine/glucose cycle
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Which cycle was the first metabolic pathway to be defined?
|
The urea cycle
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What is urea cycle used for?
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The synthesis of urea is the main mechanism that the body uses to rid itself of surplus alpha amino groups generated through amino acid catabolism
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How much of excess nitrogen is passed through the urea cycle?
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90%
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How is the urea cycle compartmentalized?
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The first two reactions in urea production takes place in the MITOCHONDRIA
The last 3 reactions take place in the cytoplasm |
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What is the first reaction of urea synthesis mediated by?
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Carbarnyl Phosphate Synthetase 1
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Why does it make sense for the first two reactions of urea cyce to be in the mitochondria?
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This is where GDH is located, and therefore where NH3 is released
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What does CPS1 do?
|
Catalyzes the formation of carbarnoyl phosphate from NH4+ and HCO3- +ATP
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What is CPS1 similar to in TCA?
|
It is similar to PDH in that it provides the first substrate to enter the cycle
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What is the second reaction of the urea cycle?
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Carbarnoyl phosphate is combined with ornithine to produce CITRULLINE
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Which is the MOST IMPORTANT reaction of the urea cycle?
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The first reaction
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What is the rate limiting step of urea production?
|
Rxn mediated by CPS1 (first reaction)
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What are the three ammonia assimilation reactions?
|
1) Glutamine formation (from glutamate by glutamine synthetase)
2) GLutamate formation (from the reversible GDH action) 3) Carbonyl phosphate formation (from ammonia and bicarbonate) |
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What are the three steps to carbarnoyl phosphate formation by CPS1?
|
1) Formation of Carbonyl phosphate
2) Attack of NH3 on carbonyl phosphate 3)Phosphorylation of caramate |
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Describe the formation of carbonyl phosphate
|
Transfer of a phosphate group to bicarbonate from ATP. WHen transferring a terminal phosphate onto a molecule, it is usualyl a good leaving group and can easily be replaced by NH3
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Describe the formation of carbamate
|
Pi leaves, NH3 comes in
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Describe the formation of carbarnoyl phosphate
|
Another ATP molecule is consumed to phosphorylate carbamate to get carbarnoyl phosphate
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Consequently, how many ATP are consumed in the reaction to form Carbarnoyl Phosphate
|
2 ATP
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What does CPS1 require to function?
|
An allosteric activator: NAG (N-acetylglutamate)
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How is NAG produced?
|
Glutamate and Acetyl-Coa from N-acetylglutamate synthetase
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Where is NAG produced?
|
In the mitochondria
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How does NAG work in mediating CPS1 activity?
|
It is an allosteric activator that is a sensor for the amount of amino acid catabolism:
the more aa are funnelled to form glutamate, the more glutamate is produced, therefore the more NAGS is formed, and therefore the more formation of carbarnoyl phosphate |
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Besides compartmentalizing urea cycle to allow CPS1 to be where GDH is, what is another reason?
|
There are two different cellular (CPS) Carbarnyl Phosphate Synthetase
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What is CPS2?
|
It is cytoplasmic, involved in pyrimidine synthesis, does not need NAG and uses glutamine as an NH3 donor
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Why would you want to keep CPS1 and CPS2 separate?
|
Ensures that they make both end products at optimal rates
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What can mutations in CPS1 result in?
|
A disease associated with HYPERAMMONEMIA, liver failure, and death
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What type of mutations can result of CPS1?
|
Nonsense or missense
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What is reaction 2 of the urea cycle and what is it mediated by?
|
Ornithine Transcarbarnoylase transfers the carbarnoyl moiety of carbarnoyl phosphate onto ornithine to form CITRILLINE
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What type of molecule is the carbarnoyl moiety?
|
A bicarbonate derivitave
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What happens when the carbarnoyl moiety is transferred onto ornithine?
|
A phosphate is released
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What happens to citrulline after it is synthesized?
|
It is exported to the cytoplasm through a transporter in the mitrochondrial membrane
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What do mutations in OTC (ornithine transcarbarnoylase) result in?
|
Hyperammonemia
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What is the third reaction of the urea cycle?
|
Arginosuccinate synthetase incorporates 2nd amino group into urea (1st was by CPS1) to form arginosuccinate from citrulline
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Where does the 2nd amino group of urea come from?
|
Aspartate
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What molecule is consumed in the formation of arginosuccinate from citrulline?
|
ATP
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What is released in the formation of arginosuccinate from citrulline?
|
AMP and PPi
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How many steps are there to form arginosuccinate from citrulline?
|
two
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Describe the first step in arginosuccinate synthesis from citrulline
|
ASS mediates this step: pyrophosphate (PPi) is released from ATP and AMP is attached to citrulline to form citrullyl-AMP
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What happens to the PPi released from ATP?
|
It is hydrolyzed by pyrophosphatases
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What is different from using ATP here than in CPS1 action or in glutamine production?
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In those reactions, the terminal phosphate of ATP was used to attach to molecule, here it is the AMP moiety that is attached!
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What is the 2nd step in the formation of arginosuccinate?
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Citrullyl-AMP can be easilly attacked by amino group of aspartate, and AMP is released
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What do mutations in ASS lead to?
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Type I HyperCitrunillemia
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What is an effect of Type I Hypercitrunillemia?
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Hyperammonemia
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What is the fourth urea cycle reaction?
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Arginosuccinate will be split to fumarate and arginine by ARGINOSUCCINASE
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What happens to the arginine?
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In the fifth reaction, it is hydrolyzed by ARGINASE, to release UREA and regenerate ORNITHINE
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What will happen to the released urea?
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Urea will be released to the blood and go to the kidney to be released in the urine
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What will happen to the ornithine?
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It will be recycled
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In the UREA cycle, where do the two amino groups come from?
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One from NH4 (oxidative deamination of GLU by GDH)
one from aspartate (transamination of oxaloacetate via GLU to form aspartate) |
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How is aspartate formed in the liver, to act as a substrate for urea cycle?
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Fumarate is converted to Malate (by fumarase). Malate is converted to OAA by malate dehydrogenase. OAA is the alphaketoacid of asparatate. Transaminases can also accept OAA as an alphaketoacid, and this is how aspartate is regenerated
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What are the energetics of the urea reactions?
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It is energetically expensive to form Urea. Incorporation of each alpha amino group (NH3 or aspartate) requires the hydrolysis of 2 high E bonds (pyrophosphate)
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How is energy recuparated?
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ATP regeneration by oxphos
Gluconeogenesis |
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How does Oxphos production of ATP help to regenerate the energy lost in the urea cycle?
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When GDH is working to deaminate glutamate (to form NH4 for cycle), NADH is produced.
When malate dehydrogenase is converting malate to OAA, NADH is also produced NADH can be used in oxphos as a source of electrons, or in gluconeogenisis by glyceraldhedye 3phosphate dehydrogenase |
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How is the urea cycle regulated? (3 ways)
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1) GDH is regulated by the energy charge of the cell
2) CPS-1 requires the allosteric activator NAG 3) NAGS activity is regulated by Glutamate and Arginine |
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How does GDH sense the amount of energy in the cell?
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If you have alot of GTP in the cell, you will downregulate GDH (because you do not really need to get rid of aa, to degrade)
-if you have low levels of energy, and consequently an accumulation of ADP,->-will stimulate GDH activity |
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How is NAG regulated?
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It is regulated by the AVAILABILITY of the SUBSTRATES. If glutamate levels increase than NAG increases. This is a feedback mechanism to tell the system that there is alot of aa catabolism occuring)
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How can regulation of urea cycle be long term?
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At the level of transcription
All five urea cycle enzymes and NAGS transcription can be induced, |
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When will induction of transcription occur?
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If AA load increases, ex: increased intake in diet
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When will transcription levels DECREASE?
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When the diet has less proteins
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What does it mean that the UC enzymes are coregulated?
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If the first enzyme is induced two-fold, then all the other enzymes will be induced two fold
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Why is there a transient peak in transcription levels in starvation, if normally transcription levels decrease in a low protein diet???
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In starvation, the body will resort to metabolizing body proteins for energy. Body will sense this, as if one was consuming a high protein diet. Eventually, the body will realize there is no food, and transcription will decrease
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Which enzymes involved in the UC have disorders associated with mutations of these enzymes?
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ALl 5 UC enzymes, NAGS, ORNT1, GDH
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Out of all these mutations, which would result in the worst condition?
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CPS1 and OTC- acute neonatal hyperammonemia
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What is the normal concentration of urea?
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30-70uM
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What is the condition of urea when one has hyperammonemia?
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240uM and up!
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What type of damage is that of ammonia to the nervous system?
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Irreversible
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What is the treatment of hyperammonemia?
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To modify the diet to reduce protein intake. For instance, PKU patients will be given a diet low in phenylalanine
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Besides modification of the diet, what is another way to rid the body of NH3?
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Stimulate alternate pathways of NH3 secretion
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What are two such methods to reduce NH3?
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Feed with Benzoate and phenylacetate
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What does benzoate do?
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Couples with glycine to form hippurate, and hippurate is released through the kidneys
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What does phenylacetate do?
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COuples with glutamine to form phenylglutamine which can be eliminated to kidneys
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What is the name of the drug that is used to administer benzoate and phenylacetate together?
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Ammonul
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Why does this approach of feeding benzoate and phenylacetate work?
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BEcause glycine and glutamine are two of the most abundant aa in the free pool.
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What is Sudden Onset Hyperammonemia?
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Patients with hyperammonemia that are fed benzoate/phenylacetate have their problem fixes temporarily. But if they catch a cold,etc and the body is in negative nitrogen balance, bodies will need to catabolize proteins to get E to fight off the cold, and there is a transient peak of ammonia- patients cannot cope with this
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What is an alternate therapy for hyperammonemia? (mutations in UC enzymes)
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Gene therapy
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How can we use a chemical/biological approach to regulate a UC enzyme?
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Create a comppind that is similar to allosteric regulators of the enzyme to resestablish proper enzyme activity
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When we are in a post-absorptive state, we degrade proteins to get energy. How does this work?
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The carbon skeletons (w/o alpha amino group) are converted to CAC intermediates-where they are metabolised to CO2 and water, or used for glucose production
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How can a carbon skeleton vary among the 20 aa?
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They could have different SIDE CHAINS
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What is the difference between the carbon skeleton of glycine and that of phenylalanine?
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Glycine- hydrogen island sidechain
Phenylalanine- benzene ring + alkyl group side chain |
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What remains after we remove the amino groups from aa?
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An oxidized hydrocarbon
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What type of molecule is this oxidized hydrocarbon?
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An amphibolic intermediate (can be used as an intermediate for biosynthesis or can be degraded by catabolism)
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How many amphibolic intermediates exist?
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SEVEN
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How can the 20 amino acids be classified?
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On the basis of their 1st amphibolic intermediate at the end point of catabolism
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What are the seven amphibolic intermediates?
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Pyruvate
Acetyl CoA Acetoacetate Alphaketoglutarate Succinyl-COA Fumarate Oxaloacetate |
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What is an example of an AA that can be degraded to two amphibolic intermediates?
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Aspartate can enter as fumarate, or oxaloacetate
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How do glutamate and aspartate enter the TCA cycle?
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By deamination: Glutamate is deaminated to form alphaketoglutarate
Aspartate is deaminated to form oxaloacetate |
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How can asparatate enter the TCA as fumarate?
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Aspartate is converted to arginosuccinate by arginosuccinate synthetase and then converted to fumarate by arginosuccinase
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How can asparagine enter the TCA?
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Asparagine is deaminated to Aspartate by Asparaginase, deaminated to oxaloacetate
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How can glutamine enter the TCA?
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Glutamine can be deaminated to form glutamate, which can be further deaminated to form alphaketoglutarate
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Which amino acids enter as pyruvate?
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Alanine, cysteine, glycine, serine, threonine, tryptophan
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Which amino acids enter as acetyl coa?
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Isoleucine, leucine, Threonine
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Which aminoacids enter as acetoacetate?
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Leucine, Lysine, Pheylalanine, tryptophan, tyrosine
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Which amino acids enter as alphaketoglutarate?
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Arginine, Glutamate, Glutamine, Histidine, Proline
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Which amino acids enter as succinyl coa?
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Methionine, Isoleucine, Valine
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Which amino acids enter as fumarate?
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ALanine, phenylalanine, tyrosine
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Which amino acids enter as oxaloacetate?
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asparagine, aspartate
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Besides their amphibolic intermediates, how else can one classify amino acids?
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On their catabolic pathway
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What are the three classifications based on catabolic pathway?
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Glucogenic, Ketogenic, or Both
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What does glucogenic mean?
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The AA will be eventually converted to glucose through gluconeogenisis
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What does ketogenic mean?
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AA broken down to acetyl coa or aceto acetate- which can be used to form ketone bodies
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Which aa are glucogenic?
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Glycine, Serine, Valine, Histidine, Arginine, Cysteine, Proline, Alanine, Glutamate, Glutamine, Aspartate, Asparagine, Methionine
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Which aa are ketogenic?
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Leucine
Lysine |
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Which aa are both ketogenic and glucogenic?
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Threonine, isoleucine, phenylalanine, tyrosine, tryptophan
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What are the three types of ketone bodies?
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Acetone
Acetoacetate B-hydroxybutiric acid |
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What are ketone bodies?
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They are the soluble equivalent of lipids. During starvation, can be used as an energy source for the brain.
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Can acetyl coa and acetoacetate be converted to form glucose?
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No! There is no direct metabolic pathway to do so
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Is net synthesis of glucose possible from leucine and lysine?
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NO. These aa are ketogenic, meaning that they cannot be degraded to amphibolic intermediates that result in gluconeogenisis
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Why were aa structures selected for in evolution?
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1- Particular properties important for protein struture and function
2-Metabolism of AAs could be integrated into the metabolism of glucose and fatty acids |
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How many reactions does methionine metabolism have?
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10
(9 degradation, 1 regeneration) |
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What does the catabolism of methionine lead to?
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Production of amphibolic intermediate succinyl CoA
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What is the name of reactions 1-4 of methionine catabolism?
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Active Methyl Cycle
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Why is methionine important?
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It is involved in
1-protein synthesis 2-Synthesis of cysteine 3-Methylating agent |
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What is the purpose of the active methyl cycle?
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To regenerate some methionine for the methylation of reactions
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What is the first reaction in methionine catabolism?
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Conversion of Methionine to SAM (S-adenosylmethionine) by Methionine Adenosyl transferase, using 1ATP and H2O
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What does SAM act as?
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Can be used in methylation reactions by methylases
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What is an important component of SAM for its methylation capability?
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In its generation, a sulfonium (+) charged sufur ataom) is formed. THis sulfur is bound to three methyl groups, one of which is activated and can be easily transferred to other molecules by attack of nucleophiles
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What happens to SAM after its formation?
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It is demethlyated in biosynthetic methylation reactions, and cconverted to S-adenosylhomocysteine
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What happens to S-adenosylhomocysteine?
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S-adenosylhomocysteine is hydrolyzed to form homocysteine and adenosine
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What is special about homocysteine?
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It is at a branch point between methionine regeneration and degradation
Many disorders associated with methionine catabolism in olve upregulation of homocysteine in the plasma |
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What can happen to homocysteine?
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1-converted back to methionine
2-targeted for degradation by undergoing transulfuration pathway to generate cysteine |
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What is homocysteine converted to in the transulfuration pathway? (Rxn5)
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Uses a serine to form Cystathionine
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How is cysteine formed by cystathionine? (Rxn6)
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Cystathionine is converted to alpha-ketobutyrate and cysteine
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How can succinyl-coa be produced? (Rxn7,8,9,10)
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Rxn 7 is the conversion of alpha-ketobutyrate to propionyl-coA. through a series of rxns, 8,9,10, is converted to succinyl-CoA
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Which coenzyme is required by Rxn 4 (conversion of homocysteine back to methionine) and Rxn 10 (final rxn to form succinyl Coa)?
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Vitamin B12
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What does rxn 4 (conversion of homocysteine to methionine) also require, in addition to VB12?
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Folic acid
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Which reactions use PLP and therefore require VB6?
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Rxns 5 (conversion of homocysteine to cystathionone) and Rxn 6 (conversion of cystathionone to cysteine and alphaketobutyrate)
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Which cofactor is required for rxn 8?
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Biotin
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What can VB12 deficiency lead to?
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Elevated homocysteine levels and health problems
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Who is susceptible to disorders of VB12 deficiency?
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Vegans
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How can hyperhomocysteinemia result?
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Active methyl cycle is not functioning properly or degradation to amphibolic intermediates is not working
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What can cause these pathways to malfunction?
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Folic acid deficiency (required for Rxn 4) or mutations in enzyme converting homocysteine to cystathionine (Cystathionine-beta-synthase)
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What are three diseases that can result from hyperhomocysteinemia?
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Cardiovascular, cognitive impairment/dementia, Developmental defects
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How does homocysteine cause cardiovascular problems?
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Homocysteine interferes with formation of connective tissue, which affects the walls of arteries. Lesions cause fat deposits, plaque formation, and artery obstruction
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How does homocysteinemia cause cognitive impairment?
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Reduces the ability to reason, concentrate
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How does homocysteinemia cause developmental problems?
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Causes neural tube defects ot anencephaly
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What is an example of a disorder resulting from neural tube defects caused by homocysteinemia? What is it characterized by?
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Spina Bifida- failure of neural tube formation or closure
-due to defects in vertebrae formation |
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What is anencephaly characterized by?
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Failure of brain development, leading cause of infant death due to congenital abnormalities
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What are two ways to reduce amount of homocysteine?
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Stimulate regneration or catabolism
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How would you stimulate regeneration?
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With VB6 or Folic acid
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What happens when you mutate methylene tetrahydrofolate reductase?
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Normally functions as tetramer, but here tetramer collapses. Still functions but activity is alot lower
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How does folic acid help the mutation in MTR?
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Drives it to form a tetramer
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What is the role of MTR?
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Generate 5-methyl-THF, used to remethylate homocysteine to form methionine
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