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Amino Acid Metabolism: Objectives 1. Few questions… a. Which amino acids are the essential ones and which are nonessential? i. Essential – there are no biosynthetic pathways or not synthesized in adequate quantities 1. Arginine **Conditionally essential (can be made from glutamate + aspartate) 2. Histidine **Conditionally essential 3. Isoleucine 4. Leucine 5. Lysine 6. Methionine 7. Phenylalanine 8. Theronine 9. Tryptophan 10. Valine ii. Nonessential 1. Alanine (made from pyruvate and an amino acid) 2. Asparagine (made from aspartate + glutamine) 3. Aspartate (made from oxaloacetate) 4. Cysteine **Made from methionine (+ serine) 5. Glutamate (made from aspartate + alpha- ketogluterate) 6. Glutamine (made from glutamate and ammonia) 7. Glycine (made from threonine) 8. Proline (made from glutamate) 9. Serine (made from glutamate or glycine or choline) 10. Tyrosine**Made from phenylalanine b. Which nonessential amino acids are derived directly from an essential amino acid? i. They are readily made from intermediates of central pathways c. Which amino acids are conditionally essential and why? i. Arginine – not essential if have adequate glutamate and aspartate
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Page 1: Amino Acid Metabolism: Objectives - UTCOM2013 - …utcom2013.wikispaces.com/file/view/Amino+Acid+Metabolism... · Web viewAmino Acid Metabolism: Objectives 9 9 Few questions… Which

Amino Acid Metabolism: Objectives

1. Few questions…a. Which amino acids are the essential ones and which are nonessential?

i. Essential – there are no biosynthetic pathways or not synthesized in adequate quantities

1. Arginine **Conditionally essential (can be made from glutamate + aspartate)

2. Histidine **Conditionally essential 3. Isoleucine4. Leucine5. Lysine6. Methionine7. Phenylalanine8. Theronine9. Tryptophan10. Valine

ii. Nonessential1. Alanine (made from pyruvate and an amino acid)2. Asparagine (made from aspartate + glutamine)3. Aspartate (made from oxaloacetate)4. Cysteine **Made from methionine (+ serine)5. Glutamate (made from aspartate + alpha-ketogluterate)6. Glutamine (made from glutamate and ammonia)7. Glycine (made from threonine)8. Proline (made from glutamate)9. Serine (made from glutamate or glycine or choline)10. Tyrosine**Made from phenylalanine

b. Which nonessential amino acids are derived directly from an essential amino acid?i. They are readily made from intermediates of central pathways

c. Which amino acids are conditionally essential and why?i. Arginine – not essential if have adequate glutamate and aspartate

ii. Histidine – essential in infants (pathway is present as age) d. Methionine: need in higher amounts (solely responsible for making Cysteine)e. Phenylalanine: need in higher amounts (solely responsible for making Tyrosine)

2. How are amino acids absorbed in small intestine?a. Amino acids are taken up by enterocytes in the small intestine

i. AAs transfer through enterocytes to the portal circulation by diffusion, facilitated diffusion, or active transport

3. What molecules serve as carriers of nitrogen?a. Glutamine (in a number of pathways)b. Alanine (inter-organ transfer of nitrogen…as in glucose-alanine cycle)c. Urea (transfers N in blood for excretion in the urine is its ONLY function)

4. What properties of urea make it well suited as a carrier of nitrogen?

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Amino Acid Metabolism: Objectives

a. Urea is nontoxic.b. Urea is highly water soluble.c. Its only purpose in life is to excrete excess Nitrogen out of the body and into the toilet.

5. Explain how transaminases and glutamate dehydrogenase play a central role in nitrogen metabolism.

a. N is carried in blood serum as an amino group on amino acids i. Primarily, alanine and glutamine

b. Transaminases: transfer amino groups to alpha-ketogluterate to make glutamatei. Amino Acid + alpha-ketogluterate keto acid + Glutamate

c. Glutamate dehydrogenase: releases ammonia for urea synthesisi. Glutamate + NADP +H2O alpha-ketogluterate + NADPH + NH3

1. Enzyme: glutamate dhdd. Thus, a free NH3 is formed… which can be brought together with HCO3- and attach to

urea to be excretedi. NH3 + HCO3- Urea to be excreted

6. Under what conditions are ammonium ions excreted in relatively high concentrations in the urine?

a. During metabolic acidosis:i. Ex: Ketoacidosis due to uncontrolled diabetes mellitus is most common

ii. Urea production is suppressediii. Ammonium is excreted directly by the kidney!

b. Glutamine + H2O Glutamate +NH4+i. What enzyme plays an important role under these conditions?

1. Glutaminase7. Explain how the glucose alanine cycle operates in inter-organ metabolism. Why is this cycle

important?a. Provides for the nontoxic transport of nitrogen (produced by metabolizing amino acids)

from skeletal muscleb. Produces pyruvate for the production of glucose by gluconeogenesis (in the liver)

i. Especially useful in times of fasting or starvation

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Amino Acid Metabolism: Objectives

8. Explain how pyridoxal phosphate functions in amino group transfer by transaminases.a. Pyridoxal phosphate acts as an electrophile for covalent catalysis

i. Essential for the transfer of N (as an amine group) that goes eventually to ureab. Pyridoxal phosphate and transaminase attach

i. Then, transaminase and the amino acid switch places1. So the pyridoxal phosphate is attached now to the amino acid

ii. Release of the amino acid, and the free pyridoxal phosphate are produced.c. THUS: Pyridoxal phosphate acts as an electrophile

9. How are the reactions of the urea cycle compartmentalized inside of cells?a. Occurs in the Kidney and the Liver:

i. Occurs within the mitochondria:1. HCO3- + NH3 + 2 ATP Carbamoyl Phosphate

a. Enzyme: Carbamoyl phosphate synthetase I2. Ornithine + Carbamoyl phosphate Citrulline and PO4H2-

a. Ornithine needs a membrane carrier to enter or exit the mitochondria

i. Ornithine is made in the cytosolii. Occurs within the cytosol:

1. Citrulline + Aspartate + ATP Argininosuccinatea. Citrulline is transported into the cytosol (via membrane carrier)

2. Argininosuccinate Fumerate + Arginineb. In the liver, the reactions will continue:

i. Arginine + H2O Ornithine + Urea

10. Explain the role of the shuttles for ornithine/citrulline and malate/aspartate in the urea cycle.

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Amino Acid Metabolism: Objectives

a. Glutamate is an anti-transporter with Aspartatei. Aspartate into the cytosol, glutamate into the mitochondria

b. Alpha-ketogluterate is an anti-transporter with Malatei. Alpha-ketogluterate into the cytosol, malate into the mitochondria

c. Citrulline and ornithine are antiports as welli. Citrulline into the cytosol, ornithine into the mitochondria

d. See #9 diagram for more info…11. How are the urea cycle and the TCA cycle linked?

a. Malate (from Fumerate)b. Aspartatec. Alpha-ketogluterate formation (can enter at that point in the TCA)

12. How is the urea cycle regulated?a. Long term regulation:

i. Increased metabolism of AAs Increased synthesis of Urea cycle enzymes1. Due to high protein diet (Carbon skeletons are oxidized and converted

to fat or glycogen)2. Due to starvation (Carbon skeletons from protein catabolism for energy

are oxidized)b. Short term regulation:

i. Arginine activates N-acetylglutamate (signals that high levels of AAs present)ii. Acetyl-CoA + glutamate N-acetylglutamate + CoA

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Amino Acid Metabolism: Objectives

13. Explain why ammonia is toxic. Explain the elements of inter-organ metabolism that contribute to the self-perpetuating nature of ammonia intoxication.

a. Ammonia is toxic, because when its concentration is high…i. Stimulates glucagon release from the pancreas

1. Which stimulates gluconeogenesis from AAs in the kidneysa. Results in even higher ammonium level

i. Further stimulating glucagon release1. Stimulate release of glucose (gluconeogenesis)

a. Promotes insulin releasei. Stim uptake of branched chain

AAS by muscleb. Muscle uses AAs as oxidizable

substrates2. That metabolism causes release of more

ammonium ionsii. In the brain, stimulates glutamine synthesis, promotes

tryptophan transportb. Increase of tryptophan, increase the synthesis of 5-OH-

tryptamine (an important neurotransmitter)2. Causing depletion of alpha-ketogluterate and glutamate in brain!

b. It obviously has a cascading effect! Higher ammonia []causes even higher ammonia []!c. Impaired Mitochondrial Function with Ammonia Intoxication

i. Increased Ca++ uptake, causing the pump to work harder (which requires ATP)ii. Decrease ETC activity, and thus the ATP production is slowed

iii. Increased formation of free radicals and also NOiv. Increase in lipid peroxidation

14. What TCA cycle intermediate is depleted in the brain during ammonia intoxication and how does this happen?

i. Alpha-ketogluterate is the TCA intermediate which is depleted in the brainii. How it happens: See #13 for the cascade of happenings

b. What amino acids are depleted in the brain?i. Alpha-ketogluterate

ii. Glutamatec. What are the consequences of depletion of these metabolites for the brain?

i. Inhibits TCA cycle and thus lowers ATP production, BUT increase in ATP need15. How is ammonia intoxication treated and how do these treatments work?

a. Diet: Low protein, high carb (which will reduce ammonia production)b. Levulose: Bacteria utilize this in colon a more acidic env. excrete NH4+ in the fecesc. Antibiotics: Kills intestinal ammonia-producing bacteria in the gutd. Sodium Benzoate and Sodium Phenylbutyrate: form covalent products with glycine and

glutamine; promoting N excretion in the feces

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Amino Acid Metabolism: Objectives

16. Explain the distinction between a ketogenic vs a gluconeogenic fate of an amino acid. Which amino acids are exclusively ketogenic and to what are they ultimately metabolized?

a. Ketogenic fate: Yield precursors which go to either fatty acids or ketone bodiesi. Amino acids which are metabolized to Acetyl-CoA or Acetoacetyl-CoA

ii. Amino acids which are exclusively ketogenic include:1. Leucine: goes to Acetyl-CoA2. Lysine: goes to alpha-ketoadipate, which goes to Acetyl-CoA

b. Gluconeogenic fate AKA Glucogenic fate: amino acids which yield precursors that can be used for gluconeogenesis (can produce glucose)

i. Metabolize to:1. Pyruvate

a. Include: Alanine, Cysteine, Serine, Threonine, Glycine2. Alpha-ketogluterate

a. Include: Arginine, Histidine, Glutamine, Proline3. Succinyl-CoA

a. Include: Methionine, Valine4. Fumarate

a. Include: Tyrosine5. Oxaloacetate

a. Include: Asparagine, Asparatatec. Both Ketogenic and glucogenic fates are possible

i. Isoleucine: Acetyl CoA Propionyl CoA (Succinyl CoA)ii. Tyrosine: Acetoacetate Fumarate

iii. Phenylalanine: Acetoacetate Fumarate1. –forms into tyrosine

iv. Tryptophan: Acetyl CoA Others…17. To what TCA cycle intermediates are individual amino acids metabolized and where do these

metabolites enter the TCA cycle.

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Amino Acid Metabolism: Objectives

18. What metabolites accumulate in individuals with a defect in the branched chain keto acid dehydrogenase? What amino acids are metabolized by the pathway involving this enzyme?

a. Branched Chain Keto Aciduria (AKA Maple Syrup Disease)i. Metabolites which accumulate:

1. Alpha-ketoiscaproate, alpha-keto-beta-methylvalerate, and alpha-ketoisovalerate

ii. Enzyme with the defect is alpha-ketoacid(branched chain) dehydrogenase1. AKA: BCKAD

iii. Amino acids which are metabolized by the pathway involving BCKAD include:1. Leucine, Valine, Isoleucine

iv. Manifestation: severe mental retardation1. Rare, recessive mutation isolated in Asian and Mennonite communities

19. What metabolites accumulate in alkaptonuria and what enzyme is defective in this disease?a. Accumulating metabolites include:

i. Homogentisate 1. Causes urine to turn black on reaction with O2, at an alkaline pH

ii. Phenylalanineiii. Tyrosine

b. Enzyme: Homogentisate Oxidasei. Homogentisate Maleylacetoacetic acid

c. Manifestation: damage to joints (arthritis), calcifications in the CVS, reddish tint (ochre) to the skin

20. How is S-adenosyl methionine synthesized?

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Amino Acid Metabolism: Objectives

21. How is methionine regenerated from homocysteine? What coenzyme is involved in this reaction? What other enzyme catalyzed reaction in mammals utilized this coenzyme?

a.

b. Coenzyme: B12c. B12 is also needed for Methylmalonyl CoA Succinyl CoS (along with the enzyme:

Methylmalonyl-CoA Mutase)22. What enzyme is defective in individuals with homocystinuria? What metabolite accumulates?

How is this disease usually treated?a. Defective enzyme: Cysathionine Beta-Synthase

i. Which is a vitamin B6 (pyridoxal phosphate) dependent enzymeb. Accumulating Metabolite:

i. Homocysteinec. Treatment:

i. If Vitamin responsive: treat with Vit B6 (pyridoxine), folic acid, vit B12ii. If Vitamin Unresponsive: Restrict methionine

1. Supplement with cystine and betainea. Promotes re-methylation of homocysteine to methionine

23. How are homocysteine thiolacetone and S-nitrosohomocysteine formed?a. They are formed from homocysteine’s reaction with protein amino groups

i. Makes the modified protein immunogenicb. S-nitrosohomocysteine is formed from homocysteine and NOc. Homocysteine thiolacetone is formed from homocysteine

i. By the enzyme: Methionyl-tRNA Synthetase

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Amino Acid Metabolism: Objectives

24. Which amino acids can be made from intermediates of a central pathway and what intermediate gives rise to which amino acids?

a. AAs made from intermediates of a central pathway:i. Transamination from a keto acid:

1. Pyruvate alanine2. Alpha-ketogluterate glutamate3. Oxaloacetate aspartate

ii. Transamination of a keto acid followed by amidation:1. Alpha-ketogluterate glutamate glutamine2. Oxaloacetate aspartate Asparagine

iii. Reductive Amidation:1. Alpha-ketogluterate Glutamate (Enzyme: Glutamate Dhd)

iv. From other intermediates of a central metabolic pathway:1. Phosphoglycerate Serine2. Phosphoglycerate Serine Glycine3. Citrulline Argininorsuccinate Arginine

v. From other amino acids:1. Phenylalanine Tyrosine2. Glutamate Proline3. Methionine Cysteine

25. Explain the role of the small intestine and kidney in the synthesis of arginine.a. Synthesis of arginine begins in the enterocytes of the small intestine

i. Glutamine y-semialdehyde + Glutamate Ornithine Citrullineii. Citrulline is carried by the circulation to the kidney

b. Synthesis of arginine continues in the kidneyi. Citrulline+ Aspartate + ATP Argininosuccinate Fumerate + Arginine

ii. Enzymes: 1st – argininosuccinate synthetase 2nd – argininosuccinate lyasec. The kidney lacks arginase of the urea cycle

i. Thus, the kidney is a major site of arginine productiond. This pathway is AKA “Intestinal-Renal Axis”

26. What reaction is catalyzed by phenylalanine hydroxylase and what is the role of tetrahydrobiopterin in this reaction. What is phenylketonuria?

a. Phenylalanine Tyrosine Enzyme: Phenylalanine hydroxylaseb. Tetrahydrobiopterin is the substrate (along with phenylalanine) for the formation of

tyrosinei. Needs to be produced from dihydro-biopterin + NADH

1. Enzyme: Dihydropteridine Reductasec. Phenylketonuria: Defect in a phenylalanine hydroxylase gene

i. Occasionally, this defect involves the dihydrobiopterin cofactorii. Tx: Supplement with tyrosine, avoid phenylalanine

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Amino Acid Metabolism: Objectives

27. How is nitric oxide made? What are some of the functions of this short-lived molecule?a. Arginine + O2 + NADPH Citrulline + NADP+ + NOb. Functions:

i. iNOS (AKA NOSI): Immune and inflammatory responsesii. nNOS (AKA NOSII): Neurotransmitter in the GI tract, in penile erection, and in

sphincter relaxation - acts on smooth muscleiii. eNOS (AKA NOSIII): Acts on endothelial cells; blood flow, blood pressure, and

platelet activation

c.