BRS: BIOCHEM NITROGEN METABOLISM ?S EXAM QUESTIONS
AND ANSWERS WITH COMPLETE SOLUTIONS
1. The answer is A. Trypsin cleaves and thus activates the pancreatic zymogens,
converting chymotrypsinogen to the active form, chymotrypsin, and the
procarboxypeptidases to the active carboxypeptidases. If trypsin were inactive,
the other proteases could not be activated because enteropeptidase is specific
for trypsinogen. Pepsin is found in the stomach, whereas aminopeptidases are
intestinal enzymes found on the brush border membrane, facing the lumen of the
intestine.
1. A deficiency of which one of the following proteolytic enzymes would have the
greatest effect on the digestion of proteins?
(A) Trypsin
(B) Chymotrypsin
(C) Carboxypeptidase A
(D) Pepsin
(E) Aminopeptidase
2. The answer is E. These transaminases convert amino acids to their
corresponding α-keto acids in reactions that are readily reversible. α-
Ketoglutarate and glutamate serve as the other α-keto acid/amino acid pair.
Pyruvate (the end product of glycolysis) is the α-keto acid corresponding to
alanine, and oxaloacetate (an intermediate of the TCA cycle) is the partner of
,aspartate. PLP is the cofactor. Thus, AST will convert aspartate and α-
ketoglutarate to oxaloacetate and glutamate, and ALT will convert alanine and α-
ketoglutarate to pyruvate and glutamate.
2. In liver disease, the enzymes AST and ALT leak into the blood from damaged liver
cells. Both of these enzymes have which one of the following in common?
(A) They both transfer ammonia to α-keto acids to form amino acids.
(B) They both form intermediates of glycolysis from amino acids.
(C) They both require thiamine pyrophosphate as a cofactor.
(D) They both catalyze irreversible reactions.
(E) They both convert α-ketoglutarate to glutamate.
3. The answer is D. Carbamoyl phosphate within the mitochondria is formed from
NH4+, CO2, and ATP. Carbamoyl phosphate synthetase II catalyzes carbamoyl
phosphate synthesis from glutamine for pyrimidine synthesis in the cytoplasm.
Carbamoyl phosphate reacts with ornithine to form citrulline, which reacts with
aspartate to form argininosuccinate. Fumarate is released from
argininosuccinate, and arginine is formed. Urea is produced from the
guanidinium group on the side chain of arginine, not from the amino group on the
α-carbon. Ornithine is regenerated. N-Acetylglutamate is an allosteric activator of
carbamoyl phosphate synthetase I. Ornithine transcarbamylase is not a regulated
enzyme in mammals, and in bacteria, N-acetylglutamate is not an allosteric
effector of ornithine transcarbamylase.
3. Which one of the following occurs in the urea cycle?
(A) Carbamoyl phosphate is derived directly from glutamine and CO2.
, (B) Ornithine reacts with aspartate to generate argininosuccinate.
(C) The α-amino group of arginine forms one of the nitrogens of urea.
(D) Ornithine directly reacts with carbamoyl phosphate to form citrulline.
(E) N-Acetylglutamate is a positive allosteric effector of ornithine transcarbamylase.
4. The answer is D. The formation of glutamate from glucose involves the TCA
cycle intermediate α-ketoglutarate, which is formed from isocitrate in a reaction
catalyzed by isocitrate dehydrogenase. α-Ketoglutarate is converted to glutamate
either by glutamate dehydrogenase or by a transaminase. The formation of
serine, alanine, aspartate, and cysteine from glucose does not require the activity
of isocitrate dehydrogenase. Serine is derived from 3-phosphoglycerate, alanine
from pyruvate, aspartate from oxaloacetate, and cysteine from methionine (only
the sulfur) and serine (the carbon atoms). The oxaloacetate needed for aspartate
synthesis can be generated from pyruvate via the pyruvate carboxylase reaction.
4. Starting with glucose, the synthesis of which one of the following would require the
participation of isocitrate dehydrogenase?
(A) Serine
(B) Alanine
(C) Aspartate
(D) Glutamate
(E) Cysteine
5. The answer is A. Glutamate cannot produce histidine because histidine is an
essential amino acid in humans. Glutamate can fix ammonia to form glutamine in
a reaction catalyzed by glutamine synthetase. Glutamate can be synthesized from
AND ANSWERS WITH COMPLETE SOLUTIONS
1. The answer is A. Trypsin cleaves and thus activates the pancreatic zymogens,
converting chymotrypsinogen to the active form, chymotrypsin, and the
procarboxypeptidases to the active carboxypeptidases. If trypsin were inactive,
the other proteases could not be activated because enteropeptidase is specific
for trypsinogen. Pepsin is found in the stomach, whereas aminopeptidases are
intestinal enzymes found on the brush border membrane, facing the lumen of the
intestine.
1. A deficiency of which one of the following proteolytic enzymes would have the
greatest effect on the digestion of proteins?
(A) Trypsin
(B) Chymotrypsin
(C) Carboxypeptidase A
(D) Pepsin
(E) Aminopeptidase
2. The answer is E. These transaminases convert amino acids to their
corresponding α-keto acids in reactions that are readily reversible. α-
Ketoglutarate and glutamate serve as the other α-keto acid/amino acid pair.
Pyruvate (the end product of glycolysis) is the α-keto acid corresponding to
alanine, and oxaloacetate (an intermediate of the TCA cycle) is the partner of
,aspartate. PLP is the cofactor. Thus, AST will convert aspartate and α-
ketoglutarate to oxaloacetate and glutamate, and ALT will convert alanine and α-
ketoglutarate to pyruvate and glutamate.
2. In liver disease, the enzymes AST and ALT leak into the blood from damaged liver
cells. Both of these enzymes have which one of the following in common?
(A) They both transfer ammonia to α-keto acids to form amino acids.
(B) They both form intermediates of glycolysis from amino acids.
(C) They both require thiamine pyrophosphate as a cofactor.
(D) They both catalyze irreversible reactions.
(E) They both convert α-ketoglutarate to glutamate.
3. The answer is D. Carbamoyl phosphate within the mitochondria is formed from
NH4+, CO2, and ATP. Carbamoyl phosphate synthetase II catalyzes carbamoyl
phosphate synthesis from glutamine for pyrimidine synthesis in the cytoplasm.
Carbamoyl phosphate reacts with ornithine to form citrulline, which reacts with
aspartate to form argininosuccinate. Fumarate is released from
argininosuccinate, and arginine is formed. Urea is produced from the
guanidinium group on the side chain of arginine, not from the amino group on the
α-carbon. Ornithine is regenerated. N-Acetylglutamate is an allosteric activator of
carbamoyl phosphate synthetase I. Ornithine transcarbamylase is not a regulated
enzyme in mammals, and in bacteria, N-acetylglutamate is not an allosteric
effector of ornithine transcarbamylase.
3. Which one of the following occurs in the urea cycle?
(A) Carbamoyl phosphate is derived directly from glutamine and CO2.
, (B) Ornithine reacts with aspartate to generate argininosuccinate.
(C) The α-amino group of arginine forms one of the nitrogens of urea.
(D) Ornithine directly reacts with carbamoyl phosphate to form citrulline.
(E) N-Acetylglutamate is a positive allosteric effector of ornithine transcarbamylase.
4. The answer is D. The formation of glutamate from glucose involves the TCA
cycle intermediate α-ketoglutarate, which is formed from isocitrate in a reaction
catalyzed by isocitrate dehydrogenase. α-Ketoglutarate is converted to glutamate
either by glutamate dehydrogenase or by a transaminase. The formation of
serine, alanine, aspartate, and cysteine from glucose does not require the activity
of isocitrate dehydrogenase. Serine is derived from 3-phosphoglycerate, alanine
from pyruvate, aspartate from oxaloacetate, and cysteine from methionine (only
the sulfur) and serine (the carbon atoms). The oxaloacetate needed for aspartate
synthesis can be generated from pyruvate via the pyruvate carboxylase reaction.
4. Starting with glucose, the synthesis of which one of the following would require the
participation of isocitrate dehydrogenase?
(A) Serine
(B) Alanine
(C) Aspartate
(D) Glutamate
(E) Cysteine
5. The answer is A. Glutamate cannot produce histidine because histidine is an
essential amino acid in humans. Glutamate can fix ammonia to form glutamine in
a reaction catalyzed by glutamine synthetase. Glutamate can be synthesized from
