Biochem chapter 26

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Question Answer
All of the following statements about nucleotides are true EXCEPT:
a. They are produced by nearly all organisms.
b. The rate of proliferation of a cell can determine the amount of DNA and RNA synthesized per unit of time.
c. Uridine nucleotide derivatives of carbohydrates are common intermediates in cellular transformation of carbohydrates.
d. Many coenzymes are derivatives of nucleotides.
e. The nitrogenous bases serve as energy sources.
Many ________ and _________ are inhibitors of purine and pyrimidine biosynthesis.
a. eicosanoids; aspirin
b. NSAIDs ; antibiotics
c. antibiotics; eicosanoids
d. anticancer drugs; antibiotics
e. anticancer drugs; eicosanoids
What is the limiting substance in the biosynthesis of purines?
a. ribose-5-phosphate
b. 5-phosphoribosyl-β amine
c. formylglycinamidine ribonucleotide
d. 5-phosphoribosyl-α-pyrophosphate (PRPP)
e. α-D-ribose-5-phosphate
Glutamine phosphoribosyl pyrophosphate amidotransferase catalyzes step 2 in the biosynthesis of purines. Which of the following is a true statement regarding the regulation of this enzyme?
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.
e. It is subject to feed-forward activation by ADP.
In purine biosynthesis when glycine adds to 5-phosphoribosyl-β-amine, the bond formed is a(n):a. ester.
b. anhydride.
c. amine.
d. thioester.
e. phosphodiester.
The coenzyme for two single carbon additions in purine biosynthesis is:
a. THF.
b. biotin.
c. SAM.
d. TPP.
e. none are true.
Adenylosuccinase (adenylosuccinate lyase) catalyzes the reaction to remove _______ after _________ has formed an amide with a carbonyl group in purine biosynthesis.
a. succinate; glutamate
b. succinate; aspartate
c. fumarate; alanine
d. fumarate; aspartate
e. α-ketoglutarate; glutamate
All of the following are true concerning the regulation of the purine biosynthetic pathway EXCEPT:
a. ATP is the energy source for AMP synthesis.
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.
e. PRPP competitively inhibits the enzyme Gln:PRPP amidotransferase.
The fate of IMP is regulated by relative levels of ______ and _____; and energy to drive AMP synthesis is provided by _____, and energy for GMP synthesis by ________.
The reaction, base + PRPP → nucleoside-5-phosphate + PPi is catalyzed by the enzyme:
a. nucleotide diphosphate kinase.
b. phosphoribosyltransferase.
c. ribose-5-phosphate pyrophosphokinase.
d. adenylate kinase.
e. GMP synthetase.
Which of the following statements explains why sulfonamides are effective antibiotics?
a. serve as analogs of p-aminobenzoic acid and block bacterial synthesis of folate
b. act as inhibitors of bacterial dihydrofolate reductase, thus preventing production of tetrahydrofolate
c. act as suicide substrates for the enzyme dihydropteroate synthetase
d. do not affect humans because folate is a vitamin
e. both a and d are correct
What 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 re-synthesis of nucleotides from bases in salvage reactions.
d. they mediate the degradation of IMP.
e. they are competitive inhibitors employed in the salvage of purine nucleotides.
All are characteristics of Lesch-Nyham Syndrome EXCEPT:
a. gouty arthritis due to excessive uric acid accumulation
b. severe malfunctions of the nervous system
c. complete deficiency in HGPRT activity
d. congenital, recessive, sex-linked trait manifested only in males
e. all are true
About 30% of severe combined immunodeficiency syndrome (SCIDS) patients suffer from a deficiency in the activity of the enzyme ______ that is a ____-dependent enzyme and _____ deficiency can also lead to reduced immune function.
a. HGPRT; Ca2+; Ca2+
b. nucleosidase; Ca2+; Ca2+
c. nucleoside phosphorylase; Pi; Pi
d. adenosine deaminase; Zn2+; Zn2+
e. none are true
What is the common product of purine catabolism?
a. xanthine
b. uric acid
c. inosine
d. hypoxanthine
e. xanthosine
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.
e. It is used in energy metabolism in the liver.
All are characteristics of xanthine oxidase EXCEPT:
a. present in large amounts in liver, intestinal mucosa and milk.
b. uses molecular oxygen and produces H2O2.
c. involved in production of urea.
d. possesses an electron-transport chain with FAD, Fe-S cluster and molybdenum cofactor.
e. all are true.
Allopurinol is an analog of ________ that tightly binds to ________ and prevents ____________ formation.
a. hypoxanthine; xanthine oxidase; uric acid
b. guanine; guanine deaminase; xanthine
c. inosine; xanthine oxidase; xanthine
d. hypoxanthine; guanine deaminase; xanthine
e. none of the above
Which of the following statements explains why allopurinol is able to prevent gout?
a. it binds to uric acid thus preventing the formation of uric acid crystals
b. it acts as a diuretic to increase the elimination of uric acid
c. it inhibits the production of uric acid and allows for the more highly soluble xanthine and hypoxanthine to be excreted from the body
d. it stimulates the purine salvage pathway, thus eliminating the need for production of uric acid
e. none of the above
All are substrates of carbamoyl phosphate synthetase II (CPS-II) EXCEPT:
a. H2O.
b. NH4+.
c. HCO3-.
d. glutamine.
e. 2ATP.
In mammals CPS-II is the regulated step on pyrimidine biosynthesis; however, in bacteria _____________ is the regulated step.
a. dihydroorotase
b. aspartate transcarbamoylase (ATCase)
c. OMP decarboxylase
d. dehydroorotate dehydrogenase
e. none are true
Which of the following serves as an activator of CPS-II in mammals?
a. thyrotropin
b. epidermal growth factor
c. cortisol
d. gonadotropin
e. none of the above
What type of reaction gives UMP from OMP?
a. reduction
b. decarboxylation
c. oxidation
d. condensation
e. hydrolysis
Which of the following is an advantage of metabolic channeling?
a. It allows the product of one reaction to be available for a reaction later in another pathway.
b. It allows the diffusion of substrates.
c. It allows the dilution of substrates.
d. It allows a more diverse interaction of substrates, intermediates, and enzymes.
e. It does not allow for substrate dilution into the milieu and for the accumulation of intermediates.
Which of the following is a true statement regarding pyrimidine biosynthesis in animals?
a. PRPP and ATP are allosteric inhibitors.
b. CTP is a feedback inhibitor.
c. It is allosterically regulated at aspartate transcarbomoylase.
d. UDP and UTP are feedback inhibitors of CPS-II.
e. ACTase catalyzes the committed step in pyrimidine synthesis.
Enzymatic ribonucleotide reduction is accomplished via a free radical mechanism. Which of the following statements is true regarding this process?
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 two Fe atoms that generate the free radicals required for reduction.
c. R2, but not R1, function in the delivery of reducing equivalents.
d. R2 homodimer carries two regulatory sites.
e. A final decarboxylation forms the deoxyribonucleotide product.
The immediate reducing power of ribonucleotide reductase is provided by:
a. thioredoxin.
b. Ferredoxin.
c. NADH.
d. Fe-S complex.
e. Cyt P-450.
All of the following are a result of high ATP levels with respect to the regulation of ribonucleotide reductase EXCEPT:
a. dTTP concentration increases.
b. UDP and CDP are reduced to dUDP and dCDP.
c. a rise in dGTP levels.
d. an increase in the concentration of dADP.
e. increases the overall DNA synthesis.
Thymine nucleotides are not synthesized directly by cells but indirectly from other pyrimidine deoxyribonucleotides. All of the following statements regarding thymine nucleotide synthesis are true EXCEPT:
a. dUMP is the immediate precursor for dTMP.
b. The action of dUTPase allows dUTP to serve as a substrate in DNA synthesis.
c. Thymidylate synthase catalyzes the formation of dTMP from dUMP.
d. The methyl donor in the reaction of dTMP is N5, N10-methylene-THF.
e. THF is used as a cofactor.
Unlike hydrogen, which is often abstracted from substrates as H+, electronegative ______ cannot be readily eliminated as ______, thus enzyme inhibitors can be fashioned in which ______ replaces H at positions where catalysis involves H-removal as H+.
a. sodium; Na+; Na
b. potassium; K+; K
c. deuterium; D+; D
d. fluorine; F+; F
e. all are true
Thymidylate synthase synthesizes dTMP from _____ by _____ utilizing the coenzyme __________.
a. dCTP; carboxylation; biotin
b. dUMP; methylation; THF
c. dCMP; methylation; THF
d. dGMP; phosphorylation; ATP
e. dUMP; phosphorylation; ATP
Which of the following is the mechanism of action of 5-fluorouracil?
a. competitive inhibitor of dihydrofolate reductase
b. suicide substrate for thymidylate synthase
c. competitive inhibitor of ribonucleotide reductase
d. non-competitive inhibitor of thioredoxin reductase
e. none of the above

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