Deck 19: Amino Acid Metabolism

Question 1

A defect in the metabolism of branched chain amino acids known as maple syrup urine disease is caused by a deficiency in

A) methylglutaconyl-CoA hydratase.
B) leucine transaminase.
C) methylcrotonyl-CoA carboxylase.
D) alpha-keto acid dehydrogenase (decarboxylating).
E) enoyl-CoA hydratase.

Answer 1

alpha-keto acid dehydrogenase (decarboxylating).

Question 2

Tetrahydrobiopterin is a cofactor required for which one of the following conversions?

A) Dihydrofolate -> tetrahydrofolate
B) Phenylalanine -> tyrosine
C) Methionine -> cysteine
D) Phenylalanine -> phenylpyruvate
E) None of the above.

Answer 2

Phenylalanine -> tyrosine

Question 3

What enzyme(s) is/are mainly responsible for catabolism of serotonin?

A) L-aromatic amino acid decarboxylase
B) Catechol-O-methyl transferase (COMT)
C) Monoamine oxidase
D) Both A and B
E) Both B and C

Answer 3

Monoamine oxidase

Question 4

The excretion of ammonia in urine results directly from action of the enzyme

A) glutamine synthetase.
B) glutaminase.
C) L-amino acid oxidase.
D) glutamate dehydrogenase.
E) D-amino acid oxidase.

Question 5

Vitamin B₁₂ is required for the normal catabolism of

A) glutamine.
B) asparagine.
C) alanine.
D) phenylalanine.
E) methionine.

Question 6

The most serious consequence of failure of the urea cycle is

A) accumulation of urea in extracellular fluid.
B) accumulation of aspartic acid which cannot be utilized in the urea cycle.
C) accumulation of ammonia (ammonium ion) in the blood.
D) jaundice.
E) high levels of uric acid in the blood due to increased formation of purines from carbamoyl phosphate in the cytosol.

Question 7

The mitochondrial carbamoyl phosphate synthetase which catalyzes the first step in urea synthesis requires ____________ as a cofactor

A) N-acetyl glutamic acid
B) ascorbic acid
C) N?acetyl neuraminic acid
D) lipoic acid
E) pyridoxal phosphate

Question 8

The principal pathway diminishing the ammonia pool in the liver is

A) urine formation.
B) glutamic acid formation.
C) urea formation.
D) asparagine synthesis.
E) none of the other answers is correct.

Question 9

Glutamine

A) is not incorporated into proteins because there is no codon or tRNA for glutamine.
B) is the principal donor of amino groups in transamination reactions.
C) donates its alpha-amino group in the de novo biosynthesis of both purine and pyrimidine nucleotides.
D) is synthesized from glutamate primarily in the liver.
E) represents an important non-toxic transport form of ammonia.

Question 10

Which of the following is the major excretory by-product of the catabolism of norepinephrine?

A) Normetanephrine
B) 3,4-Dihydroxymandelic acid
C) 3-Methoxy, 4-hydroxyphenylacetic acid
D) 3-Methoxy, 4-hydroxyphenylglycol
E) 3-Methoxy, 4-hydroxymandelic acid

Question 11

A genetic disease characterized by accumulation of excess phenylalanine in the tissues, mental retardation, and excretion of phenylalanine metabolites in the urine is caused by a deficiency in

A) phenylalanine transaminase.
B) tyrosine transaminase.
C) phenylalanine hydroxylase.
D) homogentisic acid oxidase.
E) phenylalanine decarboxylase.

Question 12

Hydroxyphenylpyruvic acid is a product of

A) the deamination of tyrosine.
B) the action of phenylalanine hydroxylase on phenylpyruvic acid.
C) the action of homogentisic acid oxidase on homogentisic acid.
D) a genetic mutation.
E) the action of tyrosinase on tyrosine.

Question 13

Pyridoxal phosphate acts as coenzyme in most

A) transmethylation reactions.
B) carbon dioxide fixation reactions.
C) transamination reactions.
D) kinase reactions.
E) none of the above.

Question 14

The reactions of the urea cycle

A) convert carbamoyl phosphate to ammonia.
B) are inhibited by N-acetyl glutamate.
C) convert ADP + Pi to ATP.
D) hydrolyze arginine to form urea.
E) involve enzymes found in both mitochondria and cytosol of skeletal muscle cells.

Question 15

The major nitrogenous product excreted in the urine is

A) urea.
B) ammonia.
C) creatinine.
D) uric acid.
E) glutamine.

Question 16

In man the primary organ of urea formation is the

A) brain.
B) kidney.
C) liver.
D) heart.
E) bladder.

Question 17

Which amino acids is NOT classified as glucogenic?

A) isoleucine
B) leucine
C) phenylalanine
D) alanine
E) histidine

Question 18

The utilization of ammonia for synthesis of the alpha-amino group of non-essential amino acids is

A) dependent upon the action of glutamate dehydrogenase.
B) achieved by reversal of the urea cycle.
C) mediated by carbamoyl phosphate.
D) effected through the intermediate of carbamino acids.
E) dependent upon the intestinal bacterial flora.

Question 19

The products of the reaction of alpha-ketoglutarate and tyrosine in the presence of an aminotransferase are which of the pairs listed below?

A) aspartate, phenylpyruvate
B) aspartate, p-hydroxyphenylpyruvate
C) glutamate, phenylpyruvate
D) glutamate, p-hydroxyphenylpyruvate
E) glutamate, p-hydroxyphenylacetate

Question 20

A deficiency in which ONE of the following enzymes would be most likely to cause hyperammonemia?

A) Creatine kinase
B) L-amino acid oxidase
C) Serine transhydroxymethylase
D) Carbamoyl phosphate synthetase I
E) Phenylalanine hydroxylase

Question 21

An amino acid which humans cannot synthesize from the keto analog is

A) leucine.
B) isoleucine.
C) phenylalanine.
D) lysine.
E) methionine.

Question 22

Which amino acid donates the carbon chain in the biosynthesis of cysteine?

A) methionine
B) cystine
C) glycine
D) serine
E) half-cystine

Question 23

A vitamin which can be synthesized in man from an amino acid precursor is

A) biotin.
B) cobalamine.
C) niacin.
D) pyridoxine.
E) thiamine.

Question 24

The carbon atom in urea synthesized in the liver arises most directly from

A) aspartate.
B) CO₂.
C) fumarate.
D) ornithine.
E) glutamate.

Question 25

Which of the following is not derived from tyrosine?

A) dihydroxyphenylalanine
B) thyroid hormone
C) epinephrine
D) serotonin
E) dopamine

Question 26

The coenzyme derived from which vitamin participates most directly in the decarboxylation of glutamic acid?

A) biotin
B) pyridoxine
C) pantothenic acid
D) thiamine
E) vitamin K

Question 27

In mammalian metabolism, which molecule can be converted to serine in a one-step reaction?

A) cysteine
B) pyruvate
C) alanine
D) glycine
E) choline

Question 28

Which of the following is the rate-limiting enzyme in pigment formation in melanocytes?

A) tyrosine hydroxylase
B) tyrosinase
C) phenylalanine hydroxylase
D) homogentisic oxidase
E) none of the other answers is correct

Question 29

Which of the following amino acids is a precursor of the pyridine ring portion of NAD?

A) Alanine
B) Glycine
C) Tyrosine
D) Tryptophan
E) Phenylalanine

Question 30

Enzymic transamination involves

A) participation of the coenzyme biotin.
B) production of ammonia.
C) participation of an alpha-keto carboxylic acid and an alpha-amino acid.
D) only non-essential amino acids.

Question 31

Albinism is caused by a deficiency in an enzyme involved in the metabolism of

A) histidine.
B) hydroxyproline.
C) tryptophan.
D) arginine.
E) tyrosine.

Question 32

Ammonia entering the liver can be incorporated most directly into

A) alanine.
B) glutamate.
C) adenosine.
D) glutathione.
E) aspartate.

Question 33

Glutamic acid can be formed in one enzyme-catalyzed step from

A) malic acid
B) succinic acid
C) alpha-ketoglutaric acid
D) oxaloacetic acid
E) fumaric acid

Question 34

The dietary requirement for nicotinic acid may be spared by an adequate amount of which one of the following amino acids?

A) tryptophan
B) glutamine
C) histidine
D) tyrosine
E) phenylalanine

Question 35

All of the following compounds participate directly in the biosynthesis of urea EXCEPT which one?

A) Citrulline
B) Carbamoyl aspartate
C) Ornithine
D) Carbamoyl phosphate
E) Arginine

Question 36

The sources of nitrogen in urea are

A) carbamoyl phosphate only.
B) carbamoyl phosphate and lysine.
C) aspartic acid only.
D) aspartic acid and carbamoyl phosphate.
E) aspartic acid and glutamine.

Question 37

Urea is the product of an enzyme in liver which catalyzes the hydrolysis of

A) carbamoyl phosphate.
B) citrulline.
C) ornithine.
D) argininosuccinate.
E) arginine.

Question 38

S-Adenosyl methionine is required for which conversion?

A) tyrosine --> dihydroxyphenylalanine (L-DOPA)
B) L-DOPA --> dopamine
C) tryptophan --> serotonin
D) norepinephrine --> epinephrine
E) dopamine --> norepinephrine

Question 39

In humans, carbon atoms from serine can be incorporated into each of the following EXCEPT

A) glucose.
B) threonine.
C) cholesterol.
D) sphingomyelin.
E) lecithin.

Question 40

Which of the branched chain amino acids is metabolized to acetoacetic acid and acetyl-CoA?

A) Valine
B) Leucine
C) Isoleucine
D) Both valine and isoleucine
E) Both leucine and isoleucine

Question 41

The conversion of propionyl CoA derived from the metabolism of odd? chain length fatty acids or branched chain amino acids to succinyl CoA requires which of the following cofactors?

A) NADPH
B) FADH₂
C) pyridoxal phosphate
D) cobalamine
E) N-acetyl glutamate

Question 42

Ammonia formation in the kidney is via the enzyme

A) urease.
B) uricase.
C) arginase.
D) carbamoyl phosphate synthetase.
E) glutaminase.

Question 43

The rate-limiting reaction in urea production is that catalyzed by

A) argininosuccinase.
B) argininosuccinate synthetase.
C) arginase.
D) ornithine transcarbamoylase.
E) carbamoyl phosphate synthetase.

Question 44

Which of the following is a membrane-bound enzyme?

A) Glutathione synthetase
B) Gamma-glutamyl transpeptidase
C) Cysteinyl-glycinase
D) 5-Oxoprolinase
E) Gamma-glutamyl cyclotransferase

Question 45

In mammals, the methyl group of S-adenosylmethionine

A) can be transferred to deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate.
B) is transferred to putrescine to form higher polyamines.
C) can react with tetrahydrofolate to form 5-methyl tetrahydrofolate.
D) can react with malonyl CoA to form methylmalonyl CoA.
E) is incorporated directly into cysteine via the trans-sulfuration pathway.

Question 46

Serine is a non-essential amino acid. Which of the following is its appropriate direct precursor?

A) Glutamic acid
B) Methionine
C) 3-Phosphoglyceric acid
D) Pyruvic acid
E) Oxaloacetate

Question 47

Which compound is not a participant in the urea cycle?

A) argininosuccinate
B) ornithine
C) ureidosuccinate
D) citrulline
E) carbamoyl phosphate

Question 48

When cysteine is catabolized the carbon atoms appear as

A) alpha-ketoglutarate
B) homocysteine
C) pyruvate
D) cystathionine
E) oxaloacetate

Question 49

When glutamate is catabolized the carbon atoms appear first as

A) glutamine
B) proline
C) oxaloacetate
D) fumarate
E) alpha-ketoglutarate

Question 50

The choline of acetylcholine, a neurotransmitter, is obtained from which amino acid?

A) Serine
B) Alanine
C) Tyrosine
D) Tryptophan
E) Lysine

Question 51

A person is in negative nitrogen balance when his

A) dietary nitrogen drops below the recommended daily allowance.
B) fecal nitrogen excretion exceeds his urinary nitrogen excretion.
C) diet contains more nonessential amino acids than essential amino acids.
D) urinary nitrogen excretion exceeds his dietary nitrogen intake.
E) dietary nitrogen intake exceeds his urinary nitrogen excretion.

Question 52

The cofactor which participates directly in reactions catalyzed by both phenylalanine and tyrosine hydroxylases is

A) tetrahydrofolate.
B) tetrahydrobiopterin.
C) thioredoxin.
D) ferrodoxin.
E) ascorbic acid.

Question 53

Which statement about glutamate is FALSE?

A) It is an excitatory neurotransmitter.
B) It is decarboxylated to form gamma-aminobutyric acid (GABA).
C) Its deamination is catalyzed by glutamate dehydrogenase.
D) It can be converted to glutamine in peripheral tissues but not in the brain.
E) It is a source of ammonia for the synthesis of urea in the liver.

Question 54

The major non-dietary source of glycine is synthesis from which amino acid?

A) Threonine
B) Alanine via pyruvate
C) Serine
D) Glutamic acid
E) None of the above; glycine is a dietary essential amino acid.

Question 55

The major normal route of phenylalanine catabolism proceeds through

A) phenylpyruvic acid.
B) tyrosine.
C) hydroxyphenyllactic acid.
D) serotonin.
E) homogentisic acid.

Question 56

If an adult human who is in nitrogen balance is put on a diet deficient only in phenylalanine, which one of the following statements will be MOST CORRECT?

A) Nitrogen intake will continue to equal nitrogen excretion.
B) Nitrogen balance will temporarily become negative but the individual will adapt and nitrogen balance will gradually return to zero.
C) Nitrogen balance will become positive and remain that way as long as the deficiency exists.
D) Nitrogen balance will temporarily become positive but the individual will adapt and nitrogen balance will gradually return to zero.
E) Nitrogen balance will become negative and remain that way as long as the deficiency exists.

Question 57

Which statement about enzyme-catalyzed transamination is FALSE?

A) The equilibrium constant is approximately 1.
B) The reaction involves a Schiff base intermediate.
C) The reaction is important in the conversion of amino acid nitrogen to urea.
D) NAD⁺ or NADP⁺ is required.
E) There is no transient release of ammonia.

Question 58

A non-essential amino acid which serves as the biological precursor of the catecholamine neurotransmitters.

A) tyrosine
B) arginine
C) leucine
D) methionine
E) tryptophan

Question 59

The equilibrium constant for most of the transaminases is about 1.0, suggesting that physiologically

A) they probably provide no really useful function.
B) there can be no net synthesis in either direction.
C) the direction of the reaction is determined by the pyridoxine derivatives.
D) the direction of the reaction is determined by the removal of one or more of the products.
E) the enzyme has a very low turnover number.

Question 60

Vitamin C (ascorbate) is a cofactor in the enzymatic transformation of

A) glutamate to 2?ketoglutarate.
B) hydroxylysine to allysine.
C) lysine to desmosine.
D) proline to hydroxyproline.
E) glutamate to gamma-carboxyglutamate.

Question 61

The three branched chain amino acids (valine, leucine and isoleucine)

A) share a common enzyme for the oxidative decarboxylation of their alpha-keto analogues.
B) share a common synthetic pathway in man.
C) all give rise to the same end products to feed into the tricarboxylic acid cycle.
D) cannot be transaminated.
E) must be deaminated by L-amino acid oxidase.

Question 62

Which molecule can donate a one-carbon unit most directly to tetrahydrofolate?

A) serine
B) S-adenosylmethionine
C) glycine
D) histidine
E) choline

Question 63

The carbon and nitrogen atoms in the polyamine, spermidine, are derived from

A) citrulline and methionine.
B) carbamoyl phosphate and ornithine.
C) methionine and glutamic acid.
D) ornithine and methionine.
E) ornithine and S-adenosyl homocysteine.

Question 64

Kynurenine is an intermediate in the catabolism of

A) histidine.
B) tyrosine.
C) tryptophan.
D) lysine.

Question 65

Which is NOT an intermediate in the biosynthesis of cysteine from methionine?

A) cystathionine
B) S-adenosylhomocysteine
C) glycine
D) homocysteine
E) S-adenosylmethionine

Question 66

The essential amino acids for man

A) must be supplied in the diet because the corresponding keto acids cannot be aminated.
B) are all ketogenic.
C) are all glucogenic.
D) cannot be made at a rate adequate to meet the individual's metabolic needs.
E) cannot be converted to nonessential amino acids.

Question 67

The biosynthesis of cysteine in normal humans requires which of the following amino acids in the diet?

A) phenylalanine
B) methionine
C) histidine
D) threonine
E) leucine

Question 68

In mammals, which enzyme(s) must act to catalyze the production of ammonium ion most directly from alanine?

A) transaminase, glutaminase
B) transaminase
C) transaminase, glutamate dehydrogenase
D) aminopeptidase, glutamate dehydrogenase
E) alanine oxidase.

Question 69

All of the following amino acids take part in reversible transamination in mammals EXCEPT which one?

A) tryptophan
B) leucine
C) glutamic acid
D) lysine
E) phenylalanine

Question 70

The synthesis of carbamoyl phosphate for urea synthesis occurs in the

A) mitochondria of liver cells.
B) cytosol of liver cells.
C) cytosol of kidney cells.
D) mitochondria in all cells.
E) endoplasmic reticulum of kidney cells.

Question 71

In normal humans, the major site of transamination of the branched-chain amino acids is in

A) skeletal muscle.
B) kidney.
C) liver.
D) brain.
E) adipose tissue.

Question 72

GABA (gamma-aminobutyrate), a neurotransmitter of inhibitory neurons, is synthesized by

A) transamination of alpha-ketobutyrate.
B) carboxylation of alanine.
C) deamination of glutamine.
D) decarboxylation of glutamate.
E) decarboxylation of aspartate.

Question 73

The synthesis of creatine is made possible by the contributions of three amino acids. They are

A) arginine, glycine, methionine.
B) arginine, serine, valine.
C) arginine, methionine, glutamic acid.
D) arginine, alanine, histidine.
E) none of the other answers is correct.

Question 74

What amino acid is a precursor of norepinephrine?

A) Glycine
B) Tyrosine
C) Leucine
D) Methionine
E) Tryptophan

Question 75

In an individual who lacks the enzyme phenylalanine hydroxylase, which of the following non-essential amino acids becomes essential in the diet?

A) serine
B) aspartic acid
C) glutamine
D) tyrosine
E) proline

Question 76

A reaction, known to take place almost exclusively in mitochondria of LIVER, is the

A) conversion of ornithine to citrulline.
B) hydrolysis of arginine to ornithine and urea.
C) formation of argininosuccinic acid.
D) deamination of AMP.
E) oxidation of glutamate by glutamate dehydrogenase.

Question 77

Choline and methionine are both sources of methyl groups for transmethylation but

A) only methionine needs to be activated.
B) the activation of methionine is via oxidation while that of choline is not.
C) the product of activation of choline transfers its methyl group only to homocysteine while activated methionine has many acceptors.
D) in their appropriate forms, all three methyls of choline can be transferred to a methyl acceptor while methionine can transfer only one.
E) the activation of choline requires tetrahydrofolate while that of methionine requires ATP.

Question 78

The protective mechanism preventing accumulation of ammonia in most tissues resides in which enzyme?

A) aspartate aminotransferase (GOT)
B) glutaminase
C) gamma-glutamyl transpeptidase
D) glutamine synthetase
E) argininosuccinate synthetase

Question 79

The biosynthesis of carnitine requires

A) S-adenosylmethionine (SAM).
B) transamidination.
C) phosphocreatine breakdown.
D) all of the above.
E) A and B only.

Question 80

The coenzyme which serves as both an acceptor and donor of one-carbon units in metabolic reactions is a derivative of which of the following vitamins?

A) biotin
B) folic acid
C) thiamine
D) riboflavin
E) niacin