Mtc.ex2.f.aminoacidmetabolism

Front 1

What are some of the many roles that amino acids have in the body?

Back 1

Amino acids
provide the building blocks for
proteins, and they also supply the
nitrogen required for the
nucleotide building blocks for RNA and DNA.
These nucleotides also have very
central roles in metabolism as
well. Other important stuff like neurotransmitters is made from amino acids too.

In addition, the carbon skeletons of amino acids are important energy sources in some dietary situations. Use of these carbon skeletons requires proper disposal of ammonia (NH3), a toxic by-product of amino acid catabolism, so we need to know about how our bodies transport, handle, and eventually excrete nitrogen waste (this involves the urea cycle in liver, and ultimately your kidneys and bladder).

Front 2

Why is protein metabolism considered dynamic?

Back 2

Virtually all proteins are degraded and replaced. In general, the rate of synthesis equals the rate of degradation (steady-state). However, some amino acids are also used for energy production and storage and for synthesis of non- protein molecules, so there is a constant need for dietary intake of protein (or essential amino acids).

Front 3

What are the daily metabolic requirements for protein turnover?

Back 3

The metabolic energy requirements for synthesis of proteins to replace those turning over (about 400 g/day) and those excreted as digestive enzymes or lost from cells lining the small intestine (about 20 g/day) is considerable, as much as 20% of the total daily energy requirement.

Front 4

What is the response of lysosomes to atrophy during starvation?

Back 4

Lysosomes in tissues that waste away or atrophy during starvation (e.g., liver and muscle, but thankfully not brain and testes) have a special degradative pathway activated only during prolonged fasting – this involves selective degradation of cytosolic
proteins containing the pentapeptide sequence
Lys-Phe-Glu-Arg-Gln, thus sparing more essential
proteins from increased non-selective degradation.

Front 5

What are the steps in the Ubiquitin-Proteasome system?

Back 5

1. Protein destined for degradation tagged with ubiquitin molecules (tandem links)

2. Ubiquitinated proteins are recognized by proteasome, which unfolds and transports protein to its proteolytic core

3. Peptide fragments produced by proteasome degraded to amino acids by non-specific proteases

Front 6

What controls the rate and degree of ubiquitination and thus the rate of protein degradation?

Back 6

Factors include the N-terminal residue (Met and Pro = very slow; Arg, Leu = very fast), as well as “cyclin- destruction boxes” and PEST sequences (Pro-Glu- Ser-Thr) in proteins.

Front 7

How are proteins destined for proteasome degradation marked?

Back 7

Proteins destined for proteasome degradation are marked by addition of ubiquitin molecules, a small (~8.5kd) protein that is attached to the ε-amino groups of lysine on target proteins. Three different enzymes add progressively more Ub molecules, in tandem chains, an energy-requiring process (ATP). The more Ub molecules attached, the more rapid the degradation.

Front 8

What is the role of alanine in disposing of C-skeletons and waste NH3?

Back 8

lanine carries C- skeletons and waste NH3, both derived from skeletal muscle proteolysis, to liver where it serves as a major substrate for gluconeogenesis during fasting (Alanine-Glucose Cycle)

Front 9

What is the role of glutamate in disposing of waste NH3?

Back 9

Glutamate is a central molecule in amino acid and N metabolism. Transaminases funnel waste NH3 to α-ketoglutarate to form glutamate (α-ketoglutarate accepts the NH3 group from all other amino acids, forming glutamate in the process). Glutamate can be oxidatively deaminated to release free ammonia, which enters the urea cycle (liver).

Front 10

What is the role of aspartate in disposing of waste NH3?

Back 10

Aspartate, formed by transamination of oxaloacetate, provides the other NH3 group for urea; the C-skeleton of what used to be aspartate is eventually recycled to oxaloacetate, then transaminated to regenerate aspartate. The newly formed urea travels in the blood to the kidneys, where it is excreted in the urine. Some NH3 groups also come from deamination of glutamine in liver (for urea synthesis) and in kidney (for excretion into urine; involved in acid-base balance).

Front 11

What is the purpose of transamination reactions?

Back 11

Transamination reactions serve two purposes – first, they help maintain adequate levels of non-essential amino acids required for protein synthesis. Importantly, they also funnel amino groups from catabolized amino acids to glutamate and aspartate for eventual excretion as urea.

Front 12

What is the most common NH3 acceptor among transaminases?

Back 12

Nearly all of transaminases use α-ketoglutarate as the NH3 acceptor, producing glutamate and an α-keto acid.

Front 13

What are the two most important transaminases (aka aminotransferases)?

Back 13

The two most important transaminases are ALT (alanine aminotransferase; alanine is converted to pyruvate), and AST (aspartate aminotransferase; usually an exception to the rule that amino acids transfer their amino groups to form glutamate).

Front 14

What is the most important enzyme involved in oxidative deamination of glutamate?

Back 14

Glutamate dehydrogenase in liver is the most important enzyme involved in oxidative deamination of glutamate (which yields an α-keto acid with release of the amino group as free ammonia). Glutamate is the only amino acid that is rapidly deaminated – remember α-ketoglutarate collects amino groups from other amino acids as glutamate. Glutamate dehydrogenase then produces ammonia, regenerating α-ketoglutarate. The released ammonia provides one of the two NH3 groups for urea synthesis.

Front 15

What are the two major mechanisms for transporting ammonia to the liver for its conversion to urea and ultimate excretion in the urine?

Back 15

Mechanism #1: Glutamine synthetase puts free ammonia on glutamate to form glutamine (requires energy). Glutamine travels in the blood to the liver, where glutaminase releases free ammonia which can enter the urea cycle, and regenerates glutamate.

Mechanism #2: The glucose-alanine cycle. Amino acids derived from skeletal muscle protein breakdown are converted to alanine, which is transported to liver where it is deaminated to form pyruvate. Waste NH3 groups enter the urea cycle, while pyruvate is used for gluconeogenesis.

Front 16

How do the kidneys play a role in maintaining whole-body acid-base balance?

Back 16

The kidneys can also form ammonia from glutamine by action of renal glutaminase. This NH3 is excreted into the urine as NH4+, an important mechanism for maintaining whole- body acid-base balance.

Front 17

What is the urea cycle?

Back 17

The urea cycle is how mammals get rid of excess nitrogen arising mostly from metabolism of amino acids (it accounts for about 90% of N-containing compounds in urine).

1. In the liver, ammonia, produced by oxidative deamination of glutamate (which collects amino groups from other amino acids) and the amine group from aspartate combine with carbon dioxide (actually a HCO3- ion) to form urea.

2. The urea is carried through the blood to the kidney, which sequesters it for excretion in the urine.

Front 18

What are the steps of the urea cycle?

Back 18

Front 19

What are some of the possible fates of C-skeletons of amino acids?

Back 19

The carbon skeletons of amino acids can be used for energy storage and/or energy production. Some can be used to synthesize glucose and are termed glucogenic amino acids, while some are converted to acetyl~CoA (ketogenic amino acids – these CANNOT be used to synthesize glucose.)

Front 20

What happens to glucogenic amino acids?

Back 20

Glucogenic amino acids are converted to either pyruvate or some intermediate of the Krebs (citric acid) cycle. (gluconeogenesis begins with oxaloacetate, a component of the Krebs cycle.) Anything that feeds into an intermediate of the cycle, or that can be used to synthesize an intermediate (importantly, this includes pyruvate, which can form oxaloacetate via the pyruvate carboxylase Rx) can be used to synthesize new glucose.

Front 21

What is the fate of ketogenic amino acids?

Back 21

Ketogenic amino acids can be oxidized for energy, converted to fatty acids for storage as triglyceride and later oxidation (fed state), or to ketone bodies (made in liver mitochondria; mostly during fasting) for oxidation by a number of tissues, importantly including brain. Ketogenic amino acids can also be used by the liver to synthesize cholesterol.

Front 22

What is the Alanine-glucose cycle?

Back 22

During fasting, amino acids derived from skeletal muscle protein breakdown provide most of the substrates for gluconeogenesis by liver.

Adequate blood [glucose] must be maintained to feed brain and RBC, and liver glycogen stores are limited.

1. Glucogenic amino acids in muscle are converted to keto-acids that are either part of the Krebs cycle or can enter the cycle.

2. These eventually become oxaloacetate, which is converted via phosphoenolpyruvate to pyruvate.

3. The pyruvate is transaminated to alanine, which is shipped to liver where it is converted back to pyruvate, which then serves as the substrate for gluconeogenesis.

4. The released glucose could conceivably go back to muscle and be oxidized to pyruvate, then transaminated to alanine.

Front 23

Which 11 of the 20 required amino acids are "non-essential"?

Back 23

We can synthesize Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine, and Tyrosine.

Front 24

Which 9 of the 20 amino acids are "essential"?

Back 24

We must eat adequate quantities of these essential amino acids: Histine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.

Front 25

What are the two carrier compounds involved in transferring 1-C units?

Back 25

1. S-adenosylmethionine (SAM) transfers methyl groups.

2. Tetrahydrofolic acid (THF) transfers other 1-C units.

Front 26

How is tetrahydrofolate acid formed?

Back 26

Tetrahydrofolate acid is formed from the vitamin folic acid by dihydrofolate reductase. Inhibition of this enzyme is an important chemo- therapeutic approach, since 1-C THF derivatives are required for synthesis of nucleotides necessary for cell division.

Front 27

What is the major methyl-group donor of 1-C metabolism and how is it formed?

Back 27

S-adenosylmethionine (SAM) is the major methyl-group donor of 1–C metabolism.

It is formed by conversion of methionine. Methionine condenses with ATP to form S-adenosyl-methionine, an unusual hi-energy compound without ~P bonds. Synthesis is driven by hydrolysis of all 3 –P bonds of ATP.

Front 28

What is homocysteine? How is it formed? How is it related to heart disease?

Back 28

Homocysteine is a metabolite synthesized by donation of the methyl group from S-adenosylmethionine (SAM).

Elevated serum homocysteine levels are an independent risk factor for coronary artery disease. Dietary supplementation with folate, and vitamins B12 and B6 reduces circulating homocysteine, a good thing, at least for those with a high risk of vascular disease.

Homocysteine can be re-methylated back to methionine in a series of reactions involving methyl-THF and vitamin B12, or it can be used to form cysteine via a trans- sulfation reaction involving serine (vitamin B6 required).