Biochemistry Exam Ii - Amino Acids

Front 1

Phenylalanine
Valine
Threonine
Tryptophan
Isoleucine
Methionine
Histidine
Arginine (adult nonessential)
Lysine
Leucine

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PVT TIM HALL

All nonessential amino acids are glucogenic

Cys requires Sulfur from Met

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Tyrosine requires ______ for its synthesis

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Phenylalanine

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Internal cleavage of proteins

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Endopeptidase

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Cleaveage at N or C terminus

a. Carboxypeptidase - removes amino acid at C-terminus

b. Aminopeptidase - removal of amino acid at N-terminus

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Exopeptidase

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Proteolytic digestive enzymes help digest dietary protein.

Epithelial cells are sloughed off and proteins are metabolized

Proteases secreted as zyomogens (protects the body)

Efficent - only lose 1-2 grams of N/day

Occurs in lumen of GI tract

Absorps free amino acids, di and tri peptides

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Digestion and absorption of proteins

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1. Gastric phase (digestion of proteins by stomach protease)

2. Pancreatic lipase (Digestion of proteins by pancreatic enzymes in the duodenum)

3. Intestinal phase (Digestion of peptides by intestinal surface enzymes)

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Digestion of dietary protein

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Major protease in the stomach.

Active at acid pH

Secreted by chief cells of stomach as zymogens

Cleaves peptide bonds in which the carboxyl group is provided by:
- Phe
- Tyr
- Glu
- Asp

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Pepsin (initial phase of protein digestion)

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Acid in stomach

1. Kills microorganisms

2. Denatures proteins - makes proteins more susceptible to hydrolysis

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Role of acid in gastric phase.

Activates pepsinogen release from chief cells

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Acidic contents of stomach. Contains:

1. Polypeptides
2. Oligopeptides
3. Amino acids (Phe, Tyr, Glu, Asp)

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Chyme

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Stimulates intestinal mucosal epithelial cel.

Activates enteropeptidase, which cleaves trypsinogen to trypsin.

Trypsin cleaves off Arg and Lys (both very basic amino acids)

Trypsin activates chymotrypsin, elastase and carboxypeptidase

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Mechanism of CCK activation

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Stimulates pancreatic acinar cell.

Secretes NaHCO3, trypsinogen, chymotripsinogen, proelastase and procarboxypeptidase.

Enteropeptidase activates trypsinogen, which activates the other zymogen proteolyases.

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Mechanism of Secretin action

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Cleaves arginine and lysine (basic amino acids)

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Trypsin

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Cleaves phenylalanine, tryptophan and tyrosine (aromatic amino acids).

Also cleaves Leucine

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Chymotrypsin

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Cleaves serine, alanine, and glycine

If you don't want to SAG, you need...

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Elastase

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Zymogens activated before leaving the pancreas

Result = destruction of pancreatic cells

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Acute pancreatitis

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The input of energy to give rise to an ion gradient.

Examples:

Na, K ATPase (gives rise to the Na+ gradient)

H,K-ATPase (gives rise to the H+ gradient)

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Primary active transport

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Transport uses energy from Na+ or H+ gradient

example: amino acid transporters

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Secondary active transport

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These peptides use the neutral dipeptide transporter

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Neutral dipeptides (aminopenicillins)

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2 K+ in/ 3 Na+ out for one ATP

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Na, K-ATPase

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The body does not recognize ___-amino acids

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D

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Specific proteins (facilitated transporter) in the cell membrane facilitate net movement of amino acid in the thermodynamically favored direction (delta G < 0)

Specific transport proteins display a measurable affinity and specificty for the transported amino acid

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Facilitated diffusion

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Structural proteins turnover slow (halflife of hours to days, ie actin and myosin)

Regulatory proteins tend to turnover rapidly (halflife of 3-20 minutes)

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Proteins are synthesized and degraded at different rates.

Turnover of body protein is about 400g/day

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Cathepsins (acid peptidases) play a major role in this organelle.

A proteolytic degradation pathway

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Lysosomal degradation pathway

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1. Ubiquitin-dependent protein degradation

2. Lysosomal degradation pathways (cathepsins)

3. Other pathways (calpains, capsases, not well established - not responsible for pathway*)

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Proteolytic degradation pathways

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1. Ubiquitin is added to the e-amino groups of Lys residues of improperly folded proteins via three-enzyme system which requires ATP

2. Ubiquinated protein is targeted to the proteasome.

a. Regulatory subunits recognize, unfold and transport the protein to its proteolytic core.

b. Catalytic subunits carry out ATP-dependent proteolysis

3. Peptides are released and
ubiquitin is regenerated

4. Peptides are degraded to amino acids

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Ubiquitin-proteasome proteolytic pathway.

Occurs in cytosol.

Ubiquitin attaches to epsilon amino groups of lysine.

Attaches to improperly folded proteins via the three enzyme system (requires ATP)

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a. Mainly degrades extracellular and long lived protein

b. Relatively unselective

c. Cathepsins – acid proteases (many) are required (main protease)

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Lysosomal degradation

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1) Endocytosis (extracellular proteins)

2) Autophagy (engulfing organnels no longer needed. Source of intracellular proteins)

3. Phagocytosis (use other cells as sources of proteins)

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Delivery methods of protein to lysosome

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Nitrogen is obtained from dietary proteins or the breakdown of body proteins.

Reduced nitrogen (ammonia) produced by intestinal tract bacteria or amino acid catabolism is incorporated into two key amino acids (glutamate and glutamine).

Ammonia is also incorporated into carbamoyl phosphate

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How nitrogen is obtained and incorporated into amino acids

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Nitrogen is transported as amino acids.

Glutamine and alanine play key roles in carrying nitrogen from peripheral tissues to the liver.

Glutamate collects nitrogen from amino acids.

Glutamate donates amine groups (nitrogen) to form amino acids.

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How nitrogen is transported in the body

Alanine is highly glucogenic

Glutamate acts as a nitrogen collector

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Nitrogen is predominantly excreted as urea.

Ammonia excreted

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How nitrogen is excreted.

Ammonia plays important role in acid-base balance

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1. Dietary proteins are digested in the gut

2. Amino acids are transported to the liver via hepatic portal vein

3. Amino acids are used to synthesize proteins
nitrogenous compounds

4. Excess amino acids are converted to glucose
stored as glycogen
released into the blood

5. Excess amino acids are converted to triacylglycerols
packaged and secreted in VLDL

6. Amino acids that pass through the liver to other tissues are converted to proteins

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Roles of various tissues in amino acid metabolism during the fed state

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1. Muscle protein is digested to amino acids

2. Some amino acids enter the blood

3. Others are partially oxidized and converted to alanine and glutamine which enter the blood

4. In kidney, glutamine is converted to ammonia (released into urine) and alanine (released into blood)

5. In gut, glutamine is converted to alanine (released into blood)

6. In liver, amino acid nitrogen is converted to urea, which is excreted into the urine

7. In liver, amino acid backbones are converted to glucose and ketone bodies

8. Glucose and ketone bodies are oxidized by various tissues for energy

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Roles of various tissues in amino acid metabolism in fasting state

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- Most amino acids are obtained by muscle breakdown

- Body synthesizes non-essential amino acids

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During fasting state

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Nitrogen of amino acid oxidzed in TCA will be carried away as ____ or ____

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glutamate or alanine

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- remove a-amino group from amino acid

- transfer a-amino group to an a-ketoacid

Uses PLP as a cofactor (also used in glycogen phosphorylase in glycogenolysis)

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Aminotransferases

Nitrogen attacks

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Aminotransferase reactions are readily reversible

All amino acids except lysine and threonine can undergo transamination reactions

Predominant amino acid/a-ketoacid pair is glutamate/a-ketoglutarate

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Properties of aminotransferases

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Glutamate donates N for urea production.

Function - urea and energy production in the liver

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Alanine aminotransferase (ALT) reaction

Products: Pyruvate and glutamate

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Formation of aspartate is used to form urea.

Remember directionality in the liver.

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Aspartate Aminotransferase (AST) reaction.

In the liver, favors aspartate and alpha-ketoglutarate production

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This disease state has a higher AST ratio than ALT

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Alcoholism

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This disease state has a lower AST/ALT ratio (ALT is increased)

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Viral hepatitis

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Most important functional group is the aldehyde

Forms Schiff-base intermediates with amino acid substrates

Absence of substrate, the aldehyde is covalently linked to a Lys residue of the enzyme

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Pyridoxal phosphate (PLP)

Reacts with epsilon carbon of lysine

Schiff base is a double bond between carbon and nitrogen

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1. Aminotransferase reactions

2. Decarboxylase reactions

3. Aldolase reactions

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Reactions of a schiff base

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Free ammonia is not generated.

Regulates ammonia levels

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PLP-dependent aminotransferase properties

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- Fixed into alpha-ketoglutarate to produce glutamate

- fixed into glutamate for formation of glutamine

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Ammonia levels are carefully controlled

pKa for ammonia is 9.3

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30-60 uM. What's the molecule?

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Ammonia serum levels

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Fate of amino groups from many amino acids are transferred to a-ketoglutarate for formation of glutamate

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Aminotransferase play a central role in handling nitrogen

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1. Dehydrogenation of the C-N bond to produce the Schiff base

2. Hydrolysis of the Schiff base

Regulation.

a-KG production is increased by:
- ADP and GDP

Glutamate production increased by:
- ATP and GTP

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Oxidative deamination reaction catalyzed by glutamate dehydrogenase