Chapter 22

Question 1

Which of the following statements about the fixation of atmospheric nitrogen (N2) into NH3 by living
cells is false?

A) It involves the transfer of 8 electrons per mol of N2.
B) It occurs in certain microorganisms, but not in humans.
C) It requires a source of electrons, normally ferredoxin.
D) It requires one ATP per mol of N2 fixed.
E) It requires two key protein components, each containing iron.

Answer 1

D) It requires one ATP per mol of N2 fixed.

Question 2

Which of the following enzymes is not involved in the assimilation of inorganic nitrogen into an
organic molecule?

A) Arginase
B) Glutamate dehydrogenase
C) Glutamate synthase
D) Glutamine synthetase
E) Nitrogenase

Answer 2

A) Arginase

Question 3

The enzymatic machinery to fix atmospheric N2 into NH4+ is:

A) a means of producing ATP when excess N2 is available.
B) composed of two key proteins, each containing iron.
C) relatively stable when exposed to O2.
D) specific to plant cells.
E) unaffected by the supply of electrons.

Answer 3

B) composed of two key proteins, each containing iron.

Question 4

Erythrose 4-phosphate is a precursor of:

A) aspartate.
B) cysteine.
C) phenylalanine.
D) serine.
E) threonine.

Answer 4

C) phenylalanine.

Question 5

Nonessential amino acids:

A) are amino acids other than those required for protein synthesis.
B) are not utilized in mammalian proteins.
C) are synthesized by plants and bacteria, but not by humans.
D) can be synthesized in humans as well as in bacteria.
E) may be substituted with other amino acids in proteins.

Answer 5

D) can be synthesized in humans as well as in bacteria.

Question 6

An amino acid that does not derive its carbon skeleton, at least in part, from a-ketoglutarate is:

A) arginine.
B) glutamate.
C) glutamine.
D) proline.
E) threonine

Answer 6

E) threonine

Question 7

Glutamine, arginine, and proline:

A) do not have a common precursor.
B) may all be derived from a citric acid cycle intermediate.
C) may all be derived from a Cori cycle intermediate.
D) may all be derived from a glycolytic intermediate.
E) may all be derived from a urea cycle intermediate.

Answer 7

B) may all be derived from a citric acid cycle intermediate.

Question 8

In which group are all the amino acids closely interrelated metabolically?

A) Arginine, hydroxyproline, and histidine
B) Arginine, tyrosine, and glutamate
C) Glycine, valine, glutamine, and aspartate
D) Ornithine, alanine, glycine, and valine
E) Ornithine, proline, arginine, and glutamate

Answer 8

E) Ornithine, proline, arginine, and glutamate

Question 9

If glucose labeled with 14C at C-1 were the starting material for amino acid biosynthesis, the
product(s) that would be readily formed is (are):

A) serine labeled at the carboxyl carbon.
B) serine labeled at alpha carbon.
C) serine labeled at the R-group carbon.
D) all of the above.
E) none of the above

Answer 9

D) all of the above.

Question 10

An amino acid that does not derive its carbon skeleton, at least in part, from oxaloacetate is:

A) aspartate.
B) lysine.
C) methionine.
D) proline.
E) threonine.

Answer 10

D) proline.

Question 11

Homoserine is:

A) a precursor of both methionine and threonine.
B) a precursor of serine.
C) derived from homocysteine.
D) derived from serine.
E) derived from threonine.

Answer 11

A) a precursor of both methionine and threonine.

Question 12

If a cell were unable to synthesize or obtain tetrahydrofolic acid (H4 folate), it would probably be
deficient in the biosynthesis of:

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

Answer 12

D) methionine.

Question 13

The nitrogen atom in the side chain of lysine is derived from which amino acid?

A) aspartic acid.
B) glutamic acid.
C) glutamine.
D) asparagine.
E) arginine.

Answer 13

B) glutamic acid

Question 14

The nitrogen atom in the indole ring of tryptophan is derived from which amino acid?

A) aspartic acid.
B) glutamic acid.
C) glutamine.
D) asparagine.
E) arginine.

Answer 14

C) glutamine.

Question 15

An important intermediate in the biosynthetic pathway to aromatic amino acids is:

A) benzoic acid.
B) lactate.
C) orotate.
D) shikimate.
E) a-ketoglutarate.

Answer 15

D) shikimate.

Question 16

d-Aminolevulinic acid is formed from succinyl-CoA and __________ and is an intermediate in the
biosynthesis of _________.

A) acetyl-CoA; long chain fatty acids
B) glycine; heme
C) serine; heme
D) serine; sphingosine
E) a-ketoglutarate; glutamate and proline

Answer 16

B) glycine; heme

Question 17

Bile pigments are:

A) formed in the degradation of heme.
B) generated by oxidation of sterols.
C) responsible for light reception in the vertebrate eye.
D) secreted from the pancreas
E) the products of purine degradation.

Answer 17

A) formed in the degradation of heme.

Question 18

Glutathione is a(n):

A) enzyme essential in the synthesis of glutamate.
B) isomer of oxidized glutamic acid.
C) methyl-group donor in many biosynthetic pathways.
D) product of glutamate and methionine.
E) tripeptide of glycine, glutamate, and cysteine.

Answer 18

E) tripeptide of glycine, glutamate, and cysteine.

Question 19

The plant hormone indole-3-acetate (auxin) is formed from:

A) arginine.
B) histidine.
C) phenylalanine.
D) threonine.
E) tryptophan.

Answer 19

E) tryptophan.

Question 20

l-Dopa is an intermediate in the conversion of:

A) phenylalanine to homogentisic acid.
B) phenylalanine to tyrosine.
C) tyrosine to epinephrine.
D) tyrosine to phenylalanine.
E) tyrosine to phenylpyruvate.

Answer 20

C) tyrosine to epinephrine.

Question 21

The hormones epinephrine and norepinephrine are derived biosynthetically from:

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

Answer 21

E) tyrosine.

Question 22

One amino acid directly involved in the purine biosynthetic pathway is:

A) alanine.
B) aspartate.
C) glutamate.
D) leucine.
E) tryptophan

Answer 22

B) aspartate.

Question 23

5-Phosphoribosyl-a-pyrophosphate (PRPP) is a synthetic precursor for all of the following except:

A) AMP.
B) arginine.
C) histidine.
D) tryptophan.
E) UMP.

Answer 23

B) arginine.

Question 24

Glutamine is a nitrogen donor in the synthesis of:

A) CTP.
B) dTTP.
C) inosinic acid (IMP).
D) orotate.
E) UMP.

Answer 24

C) inosinic acid (IMP).

Question 25

De novo purine biosynthesis is distinguished from de novo pyrimidine biosynthesis by:

A) condensation of the completed purine ring with ribose phosphate
B) incorporation of CO2.
C) inhibition by azaserine (a glutamine analog).
D) participation of aspartate.
E) participation of PRPP (phosphoribosyl pyrophosphate).

Answer 25

B) incorporation of CO2.

Question 26

The ribosyl phosphate moiety needed for the synthesis of orotidylate, inosinate, and guanylate is
provided most directly by:

A) 5-phosphoribosyl 1-pyrophosphate.
B) adenosine 5’-phosphate.
C) guanosine 5’-phosphate.
D) ribose 5-phosphate.
E) ribulose 5-phosphate.

Answer 26

A) 5-phosphoribosyl 1-pyrophosphate.

Question 27

The synthesis of purine and pyrimidine nucleotides differ in that:

A) ATP is required in the synthesis of purines but not in the synthesis of pyrimidines.
B) purine biosynthesis starts with the formation of PRPP, whereas pyrimidines incorporate the
PRPP near the end of the pathway.
C) purine formation requires a THF derivative, whereas pyrimidine formation does not.
D) pyrimidine biosynthesis is tightly regulated in the cell, whereas purine biosynthesis is not.
E) pyrimidines go through many steps, adding a single carbon or nitrogen each time, whereas the
basic skeleton for purines is formed by two main precursors.

Answer 27

B) purine biosynthesis starts with the formation of PRPP, whereas pyrimidines incorporate the
PRPP near the end of the pathway.

Question 28

Which one of the following statements is true of the biosynthetic pathway for purine nucleotides?

A) CO2 does not participate in any of the steps in this pathway.
B) Deoxyribonucleotides are formed from 5-phosphodeoxyribosyl 1-pyrophosphate.
C) Inosinate is the purine nucleotide that is the precursor of both adenylate and guanylate.
D) Orotic acid is an essential precursor for purine nucleotides.
E) The amino acid valine is one of the precursors contributing to purine nucleotides

Answer 28

C) Inosinate is the purine nucleotide that is the precursor of both adenylate and guanylate.

Question 29

Orotic aciduria is an inherited metabolic disease in which orotic acid (orotate) accumulates in the
tissues, blood, and urine. The metabolic pathway in which the enzyme defect occurs is:

A) epinephrine synthesis.
B) purine breakdown.
C) purine synthesis.
D) pyrimidine breakdown.
E) pyrimidine synthesis.

Answer 29

E) pyrimidine synthesis.

Question 30

Precursors for the biosynthesis of the pyrimidine ring system include:
A) carbamoyl phosphate and aspartate.
B) glutamate, NH3, and CO2.
C) glycine and succinyl-CoA.
D) glycine, glutamine, CO2, and aspartate.
E) inosine and aspartate.

Answer 30

A) carbamoyl phosphate and aspartate.

Question 31

The most direct precursors of the nitrogens of UMP are:

A) aspartate and carbamoyl phosphate.
B) glutamate and aspartate.
C) glutamate and carbamoyl phosphate.
D) glutamine and aspartate.
E) glutamine and carbamoyl phosphate.

Answer 31

A) aspartate and carbamoyl phosphate.

Question 32

CMP, UMP, and TMP all have ________________ as a common precursor.

A) adenosine
B) aspartate
C) glutamine
D) inosine
E) S-adenosyl methionine

Answer 32

B) aspartate

Question 33

Which of the following is not true of the reaction catalyzed by ribonucleotide reductase?

A) Glutathione is part of the path of electron transfer.
B) It acts on nucleoside diphosphates.
C) Its mechanism involves formation of a free radical.
D) There is a separate enzyme for each nucleotide (ADP, CDP, GDP, UDP).
E) Thioredoxin acts as an essential electron carrier.

Answer 33

D) There is a separate enzyme for each nucleotide (ADP, CDP, GDP, UDP).

Question 34

Which one of the following statements correctly describes the biosynthetic pathway for purine
nucleotides?

A) Purine deoxynucleotides are made by the same path as ribonucleotides, followed by reduction of
the ribose moiety.
B) The first enzyme in the path is aspartate transcarbamoylase (ATCase).
C) The nitrogen in the purine base that is bonded to ribose in the nucleotide is derived originally
from glycine.
D) The pathway occurs only in plants and bacteria, not in animals.
E) The purine rings are first synthesized, then condensed with ribose phosphate.

Answer 34

A) Purine deoxynucleotides are made by the same path as ribonucleotides, followed by reduction of
the ribose moiety.

Question 35

A cell that is unable to synthesize or obtain tetrahydrofolic acid (H4 folate) would probably be
deficient in the biosynthesis of:

A) CMP.
B) GMP.
C) orotate.
D) thymidylate (TMP).
E) UMP.

Answer 35

D) thymidylate (TMP).

Question 36

An intermediate of purine degradation in humans is:

A) glutamate.
B) NH4+
C) succinate.
D) urea.
E) uric acid.

Answer 36

E) uric acid.