MCQ Biosynthesis AA, Nucleotides

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 α-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

n 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

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 9

D) proline.

Question 10

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 10

E) tripeptide of glycine, glutamate, and cysteine.

Question 11

The hormones epinephrine and norepinephrine are derived biosynthetically from:
A) arginine. B) histidine. C) isoleucine. D) tryptophan. E) tyrosine.

Answer 11

E) tyrosine.

Question 12

One amino acid directly involved in the purine biosynthetic pathway is:
A) alanine. B) aspartate. C) glutamate. D) leucine.
E) tryptophan

Answer 12

B) aspartate.

Question 13

5-Phosphoribosyl-α-pyrophosphate (PRPP) is a synthetic precursor for all of the following except:
A) AMP.
B) arginine. C) histidine. D) tryptophan. E) UMP.

Answer 13

B) arginine.

Question 14

Glutamine is a nitrogen donor in the synthesis of:
A) CTP.
B) dTTP.
C) inosinic acid (IMP).
D) orotate.
E) UMP.

Answer 14

C) inosinic acid (IMP).

Question 15

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 15

B) incorporation of CO2.

Question 16

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 16

A) 5-phosphoribosyl 1-pyrophosphate.

Question 17

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 17

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

Question 18

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 18

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

Question 19

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 19

E) pyrimidine synthesis.

Question 20

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 20

A) carbamoyl phosphate and aspartate.

Question 21

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 21

A) aspartate and carbamoyl phosphate.

Question 22

CMP, UMP, and TMP all have \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ as a common precursor.
A) adenosine
B) aspartate
C) glutamine
D) inosine
E) S-adenosyl methionine

Answer 22

B) aspartate

Question 23

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 23

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

Question 24

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 24

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

Question 25

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 25

D) thymidylate (TMP).

Question 26

An intermediate of purine degradation in humans is:
A) glutamate.
B) NH4+.
C) succinate.
D) urea.
E) uric acid.

Answer 26

E) uric acid.

Question 27

Give the overall reaction that results from the combined action of glutamate synthase and glutamine synthetase.

Answer 27

α-Ketoglutarate + NH4+ + NADPH + ATP → glutamate + NADP+ + ADP + Pi.