Deck 26: Synthesis and Degradation of Nucleotides

Question 1

Which of the following is the limiting substance in the biosynthesis of purines?

A)ribose-5-phosphate
B)5-phosphoribosyl-b-amine
C)formylglycinamidine ribonucleotide
D)5-phosphoribosyl-a-pyrophosphate (PRPP)

Answer 1

D

Question 2

Which of the following enzyme activities is deficient in about 30% of severe combined immunodeficiency syndrome (SCIDS)patients?

A)HGPRT
B)nucleosidase
C)nucleoside phosphorylase
D)adenosine deaminase

Answer 2

D

Question 3

Which of the following compounds is the common product of purine catabolism?

A)xanthine
B)uric acid
C)inosine
D)hypoxanthine

Answer 3

B

Question 4

What is the importance of the purine nucleoside cycle?

A)It participates in the conversion of purines into pyrimidines.
B)It generates fumarate,which inhibits the formation of AMP from IMP.
C)It provides fumarate as an anaplerotic enhancement of the citric acid cycle.
D)It inhibits the citric acid cycle in skeletal muscles.

Question 5

Glutamine phosphoribosyl pyrophosphate amidotransferase catalyzes step 2 in the biosynthesis of purines.Which of the following regarding the regulation of this enzyme is correct?

A)It is accelerated by an input of ATP or GTP.
B)It is sensitive to activation by azaserine.
C)It is accelerated by an input of AMP or GMP.
D)The G series of nucleotides as well as the adenine nucleotides act as competitive inhibitors.

Question 6

Which of the following statements is NOT correct concerning the regulation of the purine biosynthetic pathway?

A)PRPP competitively inhibits the enzyme Gln:PRPP amidotransferase.
B)ADP and GDP are feedback inhibitors of ribose-5-phosphate pyrophosphokinase.
C)"A" and "C" nucleotides allosterically inhibit glutamine phosphoribosyl pyrophosphate amidotransferase (Gln:PRPP amidotransferase).
D)The rate of IMP formation is dependent on the levels of the adenine and guanine nucleotides produced.

Question 7

Which of the following enzymes catalyzes this reaction?
Base + PRPP à nucleoside-5-phosphate + PPᵢ

A)nucleotide diphosphate kinase
B)phosphoribosyltransferase
C)ribose-5-phosphate pyrophosphokinase
D)adenylate kinase

Question 8

Which of the following is NOT a characteristic of Lesch-Nyhan syndrome?

A)gouty arthritis due to excessive uric acid degradation
B)severe malfunctions of the nervous system
C)complete deficiency in HGPRT activity
D)congenital,recessive,sex-linked trait manifested only in males

Question 9

Which of the following regarding nucleotides is correct?

A)Uridine nucleotide derivatives of carbohydrates are common intermediates in cellular transformation of carbohydrates.
B)The nitrogenous bases serve as energy sources.
C)Many coenzymes are derivatives of amino acids.
D)They are produced only in eukaroytes.

Question 10

Which of the following compounds are inhibitors of purine and pyrimidine biosynthesis?

A)eicosanoids and aspirin
B)NSAIDs and antibiotics
C)anticancer drugs and antibiotics
D)antibiotics and eicosanoids

Question 11

Which of the following types of bonds is formed in purine biosynthesis when glycine adds to 5-phosphoribosyl-b-amine?

A)glycosidic
B)anhydride
C)amine
D)thioester

Question 12

Which of the following is NOT a substrate of carbamoyl phosphate synthetase II (CPS-II)?

A)H₂O
B)NH₄⁺
C)HCO₃-
D)glutamine

Question 13

Which of the following is the function of phosphoribosyltransferases?

A)They are involved in the reversible reactions of purine salvage.
B)They mediate the degradation of AMP.
C)They are central to the resynthesis of nucleotides from bases in salvage reactions.
D)They mediate the degradation of IMP.

Question 14

Which of the following factors regulates the fate of IMP?

A)the relative levels of PRPP and GTP
B)the relative levels of PRPP and ATP
C)the relative levels of ATP and GTP
D)the relative levels of AMP and GMP

Question 15

Which of the following enzymes catalyzes the reaction to remove fumarate after aspartate has formed an amide with a carbonyl group in purine biosynthesis?

A)malate dehydrogenase
B)citrate synthase
C)fumarase
D)adenylosuccinase

Question 16

Which of the following is characteristic characteristic of xanthine oxidase?

A)present in small amounts in liver,intestinal mucosa,and milk
B)uses H₂O₂ to produce molecular oxygen
C)possesses an electron-transport chain with FAD,Fe-S cluster,and molybdenum cofactor
D)involved in production of urea

Question 17

Which of the following coenzymes responsible for 2 single-carbon additions in purine biosynthesis?

A)THF
B)biotin
C)SAM
D)TPP

Question 18

Which of the following statements explains why sulfonamides are effective antibiotics?

A)They serve as analogs of p-aminobenzoic acid and block bacterial synthesis of folate.
B)They act as inhibitors of bacterial dihydrofolate reductase,thus preventing production of tetrahydrofolate.
C)They act as suicide substrates for the enzyme dihydropteroate synthetase.
D)They do not affect humans because folate is a vitamin.

Question 19

Which of the following statements explains why allopurinol is able to prevent gout?

A)Allopurinol binds to uric acid,thus preventing the formation of uric acid crystals.
B)Allopurinol acts as a diuretic to increase the elimination of uric acid.
C)Allopurinol inhibits the production of uric acid and allows for the more highly soluble xanthine and hypoxanthine to be excreted from the body.
D)Allopurinol stimulates the purine salvage pathway,thus eliminating the need for production of uric acid.

Question 20

Which of the following characterizes the compound allopurinol?

A)It is an analog of hypoxanthine that binds tightly to xanthine oxidase and prevents uric acid formation.
B)It is an analog of guanine that binds tightly to guanine deaminase and prevents xanthine formation.
C)It is an analog of inosine that binds tightly to xanthine oxidase and prevents xanthine formation.
D)It is an analog of hypoxanthine that binds tightly to guanine deaminase and prevents uric acid formation.

Question 21

Which of the following is an advantage of metabolic channelling?

A)It does not allow for substrate dilution into the milieu or the accumulation of intermediates.
B)It allows the product of one reaction to be available for a reaction later in another pathway.
C)It allows the diffusion of substrates.
D)It allows a more diverse interaction of substrates,intermediates,and enzymes.

Question 22

Hydrogen is often abstracted from substrates as H⁺; however,electronegative fluorine cannot be readily eliminated.Which of the following explains how enzyme inhibitors be fashioned to overcome this?

A)by replacing H with F-F at positions where catalysis involves H removal as H⁺
B)by replacing H with F- at positions where catalysis involves H removal as H⁺
C)by replacing H with F+ at positions where catalysis involves H removal as H⁺
D)by replacing H with F at positions where catalysis involves H removal as H⁺

Question 23

Which of the following provides the immediate reducing power of ribonucleotide reductase?

A)thioredoxin
B)ferredoxin
C)NADH
D)Fe-S complex

Question 24

Enzymatic ribonucleotide reduction is accomplished via a free radical mechanism.Which of the following statements regarding this process is correct?

A)The substrate specificity site on the R1 homodimer determines if the enzyme is active or inactive.
B)R2 has a single active site containing 2 Fe atoms that generate the free radicals.
C)R2,but not R1,functions in the delivery of reducing equivalents.
D)R2 homodimer carries 2 regulatory sites.

Question 25

Which of the following hormones serves as an activator of CPS-II in mammals?

A)thyrotropin
B)epidermal growth factor
C)cortisol
D)gonadotropin

Question 26

In mammals,CPS-II is the regulated step in pyrimidine biosynthesis.Which of the following steps in pyrimidine biosynthesis is regulated in bacteria?

A)dihydroorotase
B)aspartate transcarbamoylase (ATCase)
C)OMP decarboxylase
D)dihydroorotate dehydrogenase

Question 27

Which of the following mechanisms explains how thymidylate synthase synthesizes dTMP from dUMP?

A)by methylation utilizing the coenzyme biotin
B)by carboxylation utilizing the coenzyme THF
C)by phosphorylation utilizing the coenzyme ATP
D)by reduction utilizing the coenzyme ATP

Question 28

Which of the following is a result of high ATP levels with respect to the regulation of ribonucleotide reductase?

A)an increase in the concentration of dADP
B)decreased dTTP concentration
C)oxidation of UDP and CDP to dUDP and dCDP
D)a rise in dGTP levels

Question 29

Which of the following types of reactions gives UMP from OMP?

A)reduction
B)decarboxylation
C)oxidation
D)condensation

Question 30

Thymine nucleotides are not synthesized directly by cells but indirectly from other pyrimidine deoxyribonucleotides.Which of these statements is NOT correct regarding thymine nucleotide synthesis?

A)dUMP is the immediate precursor for dTMP.
B)The action of dUTPase allows dUTP to serve as a substrate in DNA synthesis.
C)The methyl donor in the reaction of dTMP is N5,N10-methylene-THF.
D)THF is used as a cofactor.

Question 31

Which of the following is the mechanism of action of 5-fluorouracil?

A)competitive inhibitor of dihydrofolate reductase
B)competitive inhibitor of ribonucleotide reductase
C)suicide substrate for thymidylate synthase
D)non-competitive inhibitor of thioredoxin reductase

Question 32

Which of the following regarding pyrimidine biosynthesis in animals is correct?

A)PRPP and ATP are allosteric inhibitors.
B)CTP is a feedback inhibitor.
C)ATCase catalyzes the committed step in pyrimidine synthesis.
D)UDP and UTP are feedback inhibitors of CPS-II.