Deck 18: Nucleotide Metabolism

Question 1

The purine ring system is shown below. What is the source of the C-6 atom during de novo biosynthesis of purines?

A) carbon dioxide
B) tetrahydrofolate
C) carbamoyl phosphate
D) glycine

Answer 1

carbon dioxide

Question 2

The first step in the de novo biosynthesis of IMP is a reaction between glutamine and PRPP. Which statement is not true about this reaction?

A) An amine group is attached to the anomeric carbon atom of the ribose ring.
B) The configuration of the anomeric carbon atom is inverted from β to α.
C) A pyrophosphate is lost from PRPP.
D) The reaction is catalyzed by glutamine-PRPP aminotransferase.

Answer 2

The configuration of the anomeric carbon atom is inverted from β to α.

Question 3

A ribose sugar is added to rings after their synthesis and to rings during their synthesis.

A) purine; pyrimidine
B) pyrimidine; purine
C) purine; purine
D) pyrimidine; pyrimidine

Answer 3

pyrimidine; purine

Question 4

The purine ring system is shown below. What is the source of the N-1 atom during de novo biosynthesis of purines?

A) glycine
B) glutamine
C) aspartate
D) arginine

Question 5

Which is a common precursor in the de novo synthesis of purine nucleotides?

A) inosine 5ʹ-monophosphate
B) xanthosine monophosphate
C) orotate
D) adenylosuccinate

Question 6

The purine ring system is shown below. What is the source of the atoms 5 and 7 during de novo biosynthesis of purines?

A) glycine
B) glutamine
C) aspartate
D) arginine

Question 7

Which is the source of the ribose ring in purine nucleotides?

A) ATP
B) AMP
C) PRPP
D) cGDP

Question 8

Which enzyme catalyzes the step in purine nucleotide biosynthesis that is the principal site of regulation for the pathway?

A) adenylsuccinate lyase
B) OPM decarboxylase
C) IMP cyclohydrolase
D) glutamine-PRPP amidotransferase

Question 9

The pyrimidine ring system is shown below. What is the source of the C-2 during the de novo biosynthesis of pyrimidines?

A) aspartate
B) PRPP
C) bicarbonate
D) tetrahydrofolate

Question 10

The pyrimidine ring system is shown below. What is the source of the N-3 during the de novo biosynthesis of pyrimidines?

A) ammonia
B) glutamine
C) arginine
D) urea

Question 11

Which structure below is PRPP?

A) I
B) II
C) III
D) IV

Question 12

The purine ring system is shown below. What is the source of the nitrogen atoms 3 and 9 during the de novo biosynthesis of purines?

A) aspartate
B) glycine
C) arginine
D) glutamine

Question 13

is a precursor for the de novo synthesis of AMP and GMP.

A) OMP
B) XMP
C) Adenylosuccinate
D) IMP

Question 14

Which is a method by which the synthesis of AMP or GMP is regulated?

A) covalent modification
B) feedback inhibition
C) feed-forward activation
D) competitive inhibition

Question 15

Which are purines?

A) adenine and thymine
B) guanine and adenine
C) cytosine and thymine
D) uracil and guanine

Question 16

The first nucleotide product in the de novo biosynthetic pathway of purines is

A) AMP.
B) GMP.
C) IMP.
D) XMP.

Question 17

The purine ring system is shown below. What is the source of the carbon atoms 2 and 8 during de novo biosynthesis of purines?

A) carbon dioxide
B) aspartate
C) 10-formyltetrahydrofolate
D) glutamine

Question 18

Which amino acid is not directly involved in the de novo biosynthesis of purines?

A) glycine
B) glutamine
C) aspartate
D) arginine

Question 19

The pyrimidine ring system is shown below. What is the source of the N-1 during the de novo biosynthesis of pyrimidines?

A) aspartate
B) urea
C) asparagine
D) IMP

Question 20

Four of the ten enzymes required for the biosynthesis of IMP are listed below. In which order do the reactions these enzymes catalyze occur during the synthesis of IMP? 1. IMP cyclohydrolase
2. GAR synthase
3. glutamine-PRPP amidotransferase
4. AIR carboxylase

A) 1 → 2 → 3 → 4
B) 4 → 3 → 2 → 1
C) 3 → 2 → 4 → 1
D) 2 → 3 → 4 → 1

Question 21

Which catalytic site is not contained in the eukaryotic multifunctional protein dihydroorotate synthase?

A) orotate phosphoribosyltransferase
B) ATCase
C) carbamoyl phosphate synthetase II
D) dihydroorotase

Question 22

Which is not a direct precursor of pyrimidines in their de novo biosynthesis?

A) HCO₃^-
B) glutamine
C) glycine
D) aspartate

Question 23

The conversion of UTP to CTP involves .

A) hydrolysis of a phosphate group
B) oxidation of an OH group
C) transfer of an NH₂ group
D) formation of a carbon-carbon double bond

Question 24

The pyrimidine ring system is shown below. What is the source of the carbons 4,5 and 6 during the de novo biosynthesis of pyrimidines?

A) glutamine
B) aspartate
C) 3 bicarbonate ions
D) pyruvate

Question 25

Carbamoyl phosphate synthetase I is located in the in eukaryotes and uses as a nitrogen source.

A) mitochondria; glutamine
B) mitochondria; ammonia
C) cytosol; glutamine
D) cytosol; ammonia

Question 26

Inhibition of ATCase by a nucleotide and activation of ATCase by a nucleotide keep the balance of purine and pyrimidine pools in E. coli.

A) purine ATP); purine ATP)
B) pyrimidine; pyrimidine
C) pyrimidine; purine ATP)
D) purine ATP); pyrimidine

Question 27

The one-carbon group needed to convert dUMP into dTMP is derived from

A) methanol.
B) 5,10-methylenetetrahydrofolate.
C) glycine.
D) serine.
E) biotin.

Question 28

The source of the hydrogen to form water during the reduction of nucleoside triphosphates is .

A) thioredoxin
B) NADP
C) NADH
D) NADPH
E) FADH₂

Question 29

The ribonucleotides that are reduced to deoxyribonucleotides in most organisms are .

A) nucleosides
B) mononucleotides
C) dinucleotides
D) trinucleotides

Question 30

Which is the correct order for the synthesis of dTMP?

A) dUDP → dUTP→ dTTP → dTDP→ dTMP
B) UMP → UDP→ dUDP → dUMP→ dTMP
C) UMP → TMP→ dTMP
D) UMP → dUMP→ dTMP

Question 31

DNA synthesis is often measured by using radioactive

A) biotin.
B) thymine.
C) thiamine.
D) thymidine.
E) All of the above

Question 32

Eukaryotic cells maintain the correct balance of deoxyribonucleotides needed in a species DNA by .

A) controlling the activity of ribonucleotide reductase
B) controlling the specificity of ribonucleotide reductase
C) controlling both the specificity and activity of ribonucleotide reductase at the same time
D) feedback inhibition
E) controlling the balance of NADPH formed

Question 33

What is unusual about the reaction catalyzed by ribonucleotide reductase?

A) It involves the conversion of -SH groups to -OH groups.
B) It uses a rare derivative of tetrahydrofolate.
C) It has a free radical mechanism.
D) This enzyme and its reaction occur only in a few species of archaebacteria.

Question 34

dUTP is not a direct precursor of dTTP because .

A) there are no enzymes for this reaction
B) dUTP controls DNA synthesis
C) it could possibly be incorporated into DNA
D) All of the above

Question 35

Substrates for reduction to form deoxyribonucleotides are

A) mononucleotides.
B) dinucleotides.
C) trinucleotides.
D) B and C
E) All of the above

Question 36

High levels of will almost totally inhibit ATCase.

A) CTP
B) ATP
C) CTP and UTP
D) ADP and UDP

Question 37

Which is the correct order for the synthesis of CTP?

A) UMP → UDP→ UTP → CTP
B) GMP → GDP→ GTP → CTP
C) UTP → TTP→ CTP
D) UMP → CMP→ CDP → CTP

Question 38

Which is the precursor of all pyrimidine ribo- and deoxyribonucleotides?

A) dTMP
B) CTP
C) IMP
D) UMP

Question 39

The reduction of ribonucleotides by ribonucleotide reductase requires the formation of a/an .

A) covalent intermediate
B) free radical protein
C) intermediate methyl group
D) acid-base catalyst

Question 40

Which type of enzymes catalyze the phosphorylation of dADP, dGDP and dCDP?

A) nucleotide phosphorylases
B) nucleoside diphosphate kinases
C) triphosphatases
D) deoxyribosylphosphatases

Question 41

Allopurinol may be used to treat gout due to an excess of uric acid production because it can be converted to oxypurinol by , and oxypurinol strongly inhibits .

A) xanthine oxidase; urease
B) urease; xanthine oxidase
C) xanthine oxidase; xanthine oxidase
D) urease; urease
E) xanthine oxidase; sodium urate

Question 42

During de novo biosynthesis of the purine nucleotides, the purine ring is first formed as a free base before being attached to the ribose 5-phosphate moiety.

Question 43

Lesch-Nyhan syndrome, a result of hypoxanthine-guanine phosphoribosyltransferase deficiency, there is a/an .

A) accumulation of hypoxanthine
B) accumulation of GMP
C) complete breakdown of purine biosynthesis
D) excess of degradation of IMP and GMP
E) accumulation of IMP

Question 44

When phosphoribosyl pyrophosphate PRPP) is used to form AMP from adenine as well as in numerous other cell reactions), the reaction is rendered irreversible because

A) PRPP has such a high affinity for the enzyme involved.
B) the rate of the reaction favors formation of the products).
C) this reaction is thermodynamically feasible.
D) the presence of pyrophosphatase hydrolyzes pyrophosphate formed.
E) All of the above

Question 45

The excretory products of uric acid catabolism vary among organisms depending on their ability to .

A) tolerate the end-product
B) remove the end-product to less toxic products
C) solubilize the end-product
D) remove an end-product like ammonia to the environment
E) All of the above

Question 46

Nucleotides have a β configuration about the anomeric carbon atom of the ribose or deoxyribose ring.

Question 47

Dihydrofolate reductase and thymidylate synthetase are major targets for anticancer drugs because

A) these enzymes are unique in cancer cells.
B) cancer cell grow rapidly and are very dependent upon the activities of these enzymes.
C) they donate one-carbon groups.
D) cancer cells lack sufficient amounts of these enzymes.
E) All of the above

Question 48

Channeling is an important process in the biosynthesis of pyrimidine nucleotides in mammals.

Question 49

In birds and reptiles, purine nucleotides and surplus nitrogen from amino acid catabolism is disposed of in the form of while humans form that, as well as in nitrogenous waste.

A) xanthine, uric acid
B) uric acid; xanthine
C) uric acid; urea
D) urea; uric acid
E) urea; guanine

Question 50

In eukaryotes all enzymes for the biosynthesis of the pyrimidine nucleotides occur exclusively in the cytosol.

Question 51

Xanthine oxidase catalyzes the formation of .

A) xanthine
B) uric acid
C) hypoxanthine
D) xanthine and uric acid
E) All of the above

Question 52

Pyrimidines are catabolized to ammonia, bicarbonate, and either from cytosine or uracil) or from thymine).

A) succinyl CoA; acetyl CoA
B) β-alanine; β-aminobutyrate
C) fumarate; succinyl CoA
D) acetyl CoA; succinyl CoA
E) acetyl CoA; acetyl CoA

Question 53

The pathway for the synthesis of IMP releases considerable energy and is an important source of ATP.

Question 54

Pigs excrete guanine rather that further catabolizing it because .

A) deamination leads to a product toxic to pigs
B) guanase is produced in extremely small amounts and is only activated under conditions of starvation
C) guanine is a component of antibiotics fed to pigs to enhance their growth
D) pigs lack the enzyme guanase

Question 55

In the reactions catalyzed by xanthine oxidase, electrons are transferred to to form .

A) O₂; H₂O₂
B) H₂O₂; O₂
C) H₂O; O₂
D) O₂; H₂O

Question 56

In pyrimidine nucleotide biosynthesis orotate is the molecule that displaces the pyrophosphate group on PRPP.

Question 57

The coenzyme biotin is required for the carboxylation of aminoimidazole ribonucleotide (AIR) during the de novo synthesis of IMP by the enzyme AIR carboxylase.

Question 58

The purine nucleotide cycle serves to form for the citric acid cycle while recycling formed in contracting muscle cells.

A) ATP; AMP
B) fumarate; AMP
C) citrate; AMP
D) fumarate; ADP
E) ATP; fumarate

Question 59

In prokaryotes, carbamoyl phosphate is an intermediate in the de novo biosynthesis of pyrimidine nucleotides.

Question 60

Virtually all organisms and tissues can synthesize nucleotides.

Question 61

When a purine or a purine triphosphate is bound to the specificity site of ribonucleotide reductase, the activity of the catalytic site of the enzyme is specific for a pyrimidine or a pyrimidine triphosphate.

Question 62

The cofactor, methylenetetrahydrofolate, can donate a variety of one-carbon groups as well as act as a reducing agent in biochemical reactions.

Question 63

dCMP, dGMP, dAMP and dTMP are substrates for a reductase that phosphorylates them to the triphosphate levels for incorporation into DNA.

Question 64

In the de novo synthesis of the deoxyribonucleotides, the deoxy form of PRPP is used in place of the PRPP used in the synthesis of the ribonucleotides.

Question 65

Ribonucleotides are produced from oxidation of the deoxyribonucleotides at the 2ʹ carbon atom.