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Q:
Which form of the folate coenzyme is directly involved in sythesis of the purine ring?
a. N5,N10-methenyl THF
b. N10-formyl THF
c. N5-formyl THF
d. tetrahydrofolate
e. dihydrofolate
Q:
Which of the following would cause an increase in the conversion of IMP to GMP?
a. high levels of CMP
b. high levels of dTTP
c. high levels of UMP
d. high levels of AMP
e. none of the above
Q:
Which of the following serves as one of the nitrogen sources for the pyrimidine ring?
a. glutamate
b. carbamoyl phosphate
c. asparagine
d. glycine
e. none of the above
Q:
Which of the following is a source of 2 carbon atoms of the purine ring system?
a. aspartate
b. carbon dioxide
c. glycine
d. glutamine
e. glutamic acid
Q:
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
Q:
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
Q:
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
Q:
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.
Q:
Which of the following statements regarding the activity of ribonucleotide reductase is correct?
a. the overall activity of the enzyme is inhibited by high levels of ATP
b. binding of ATP to the specificity site increases the use of GDP as a substrate
c. binding of dGTP to the specificity site increases the use of CDP as a substrate
d. binding of dTTP to the specificity site inhibits the use of UDP as a substrate
e. none of the above
Q:
The immediate reducing power of ribonucleotide reductase is provided by:
a. thioredoxin.
b. Ferredoxin.
c. NADH.
d. Feï€S complex.
e. Cyt P-450.
Q:
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.
Q:
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.
Q:
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.
Q:
What type of reaction gives UMP from OMP?
a. reduction
b. decarboxylation
c. oxidation
d. condensation
e. hydrolysis
Q:
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
Q:
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
Q:
All are substrates of carbamoyl phosphate synthetase II (CPS-II) EXCEPT:a. H2O.b. NH4+.c. HCO3-.d. glutamine.e. 2ATP.
Q:
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
Q:
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
Q:
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, Feï€S cluster and molybdenum cofactor.
e. all are true.
Q:
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.
Q:
What is the common product of purine catabolism?
a. xanthine
b. uric acid
c. inosine
d. hypoxanthine
e. xanthosine
Q:
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
Q:
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
Q:
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.
Q:
Which of the following statements explains why methotrexate is effective as a cancer chemotherapeutic?
a. it serves as an analog of p-aminobenzoic acid and block bacterial synthesis of folate
b. acts as an inhibitor of dihydrofolate reductase, thus preventing production of tetrahydrofolate
c. it acts as a suicide substrates for the enzyme dihydropteroate synthetase
d. prevents the conversion of N5,N10-methenyl THF to N10-formyl THF which is required in 2 steps of the purine synthesis pathway
e. both b and d are correct
Q:
The reaction, base + PRPP ï‚®nucleoside-5-phosphate + PPiis catalyzed by the enzyme:
a. nucleotide diphosphate kinase.
b. phosphoribosyltransferase.
c. ribose-5-phosphate pyrophosphokinase.
d. adenylate kinase.
e. GMP synthetase.
Q:
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 ____.
a. IMP; PRPP; ATP; GTP
b. PRPP; ATP; ATP; GTP
c. AMP; ATP; GTP; ATP
d. AMP; GMP; GTP; ATP
e. IMP; GMP; ATP; GTP
Q:
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.
Q:
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. ï¡-ketoglutarate; glutamate
Q:
The coenzyme for two single carbon additions in purine biosynthesis is:
a. THF.
b. biotin.
c. SAM.
d. TPP.
e. none are true.
Q:
In purine biosynthesis when glycine adds to 5-phosphoribosyl--amine, the bond formed is a(n):a. ester.b. imine.c. amine.d. amide.e. lactam.
Q:
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.
Q:
What is the limiting substance in the biosynthesis of purines?a. ribose-5-phosphateb. 5-phosphoribosyl- aminec. formylglycinamidine ribonucleotided. 5-phosphoribosyl--pyrophosphate (PRPP)e. -D-ribose-5-phosphate
Q:
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
Q:
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.
Q:
Glutamine synthetase is an important enzyme in the regulation of ammonia concentrations. Which of the following would be an activator of Gln synthetase?a. -ketoglutarateb. glutaminec. histidined. alaninee. none of the above
Q:
Which of the following amino acids plays the most central role in nitrogen and amino acid metabolism?
a. Asn
b. Glu
c. Gly
d. Cys
e. none of the above
Q:
Which of the following sequence of events best explains the mechanism of carbamoyl phosphate synthetase?
a. CO2 and ATP form a carbonic-phosphoric anhydride (CPA), then NH3 performs a nucleophilic attack on the CPA to form carbamate
b. Activated CO2 is transferred to biotin, then reacts with NH3 to produce carbamate
c. NH3 reacts with ATP, which then reacts with CO2 to produce carbamate
d. NH3 reacts with CO2 to produce carbamate followed by reaction with ATP
e. None of the above
Q:
Microorganisms that fix nitrogen have a symbiotic relationship with certain plants. What molecule is produced by plants to prevent oxygen poisoning of the nitrogenase enzyme?a. -ketoglutarateb. leghemoglobinc. glutamic acidd. glucosee. none of the above
Q:
Glutamine synthetase plays an important role in the incorporation of ammonia (or ammonium ions) into glutamine. Which of the following mechanisms of covalent modification controls the activity of glutamine synthetase?
a. Phosphorylation/dephosphorylation
b. Adenylation/deadenylation
c. Acetylation/deacetylation
d. Methylation/demethylation
e. None of the above
Q:
All of the following are characteristics of alkaptonuria EXCEPT:
a. excretion of homogentisate
b. air oxidation causes urine to turn dark on standing
c. results in mental retardation due to accumulation of Phe metabolites
d. homogentisate accumulation in joints may cause arthritis
e. all are true
Q:
All of the following are characteristics of phenylketonuria EXCEPT:
a. excretion of phenylpyruvate.
b. air oxidation causes urine to turn dark on standing.
c. treated by putting patient on a diet low in phenylalanine.
d. untreated patients suffer severe mental retardation.
e. deficiency or defect in phenylalanine hydroxylase.
Q:
All are characteristics of Maple Syrup Urine Disease EXCEPT:a. hereditary enzyme defect.b. elevated levels of val, leu, and ile and their corresponding -keto acids in blood and urine.c. restrictive intake of proteins with val, leu and ile is required.d. fatal without detection and treatment.e. elevated blood levels of acetoacetate and -hydroxybutyrate.
Q:
Degradation of valine, isoleucine and methionine leads to succinyl-CoA via the sequence of:
A. carboxylation using biotin and ATP
B. mutase
C. methylmalonyl-CoA
D. epimerase
E. Propionyl-CoA
a. E, B, A, C, D, C
b. E, A, C, D, C, B
c. A, C, D, C, E, B
d. C, A, D, B, C, E
e. D, A, C, B, C, E
Q:
The carbon skeletons of all of the amino acids below converge to pyruvate EXCEPT:
a. alanine.
b. serine.
c. cysteine.
d. glycine.
e. glutamate.
Q:
The term ketogenic amino acids refers to amino acids:a. that are precursors for glucose synthesis.b. degraded to yield acetyl CoA or acetoacetate.c. that can not be converted to fatty acids or ketone bodies.d. degraded to yield succinyl-CoA, pyruvate, -ketoglutarate, fumarate and oxaloacetate.e. none of the above.
Q:
All are convergent metabolic intermediates of -amino acid carbon skeletons EXCEPT:a. Citrate.b. -ketoglutarate.c. Oxaloacetate.d. succinyl-CoA.e. Fumarate.
Q:
Herbicides (e.g., "Roundup") that inhibit biosynthesis of "essential" amino acids are supposedly safe for animal exposure because animals do not have enzymes for:
a. synthesis of the members of the 3-phosphoglycerate family
b. the glyoxylate cycle.
c. the glutamine synthesis pathway.
d. Phe, Val, Leu, and Ile biosynthetic pathways.
e. none are correct.
Q:
Chorismate biosynthesis occurs via the shikimate pathway and is an important precursor in the synthesis of all of the following EXCEPT:
a. vitamin K.
b. folic acid.
c. threonine.
d. tyrosine.
e. tryptophan.
Q:
All of the following are true statements regarding the 3-Phosphoglycerate (3-PG) family EXCEPT:
a. 3-PG is diverted from glycolysis by phosphoglycerate mutase.
b. Glycine can be derived from serine.
c. Serine, glycine and cysteine are all nonessential amino acids.
d. Production of glycine also produces N5, N10-methylene-THF which is important in the biosynthesis of purines.
e. Serine production is regulated by a direct feedback mechanism.
Q:
Transamination of pyruvate with glutamate as amino donor gives:
a. alanine.
b. serine.
c. cysteine.
d. aspartate.
e. valine.
Q:
Elevated levels of homocysteine in the blood is related to all EXCEPT:
a. homocysteinuria at very high levels.
b. folic acid deficiency.
c. folic acid-dependent conversion of homocysteine to methionine.
d. higher risk of heart attack and stroke.
e. all of the above are correct.
Q:
Amino acids biosynthesized from aspartate include all EXCEPT:
a. asparagine.
b. threonine.
c. methionine.
d. lysine.
e. glutamate.
Q:
Homoserine and homocysteine are related to serine and cysteine by having:
a. an extra amino group.
b. an additional methylene group.
c. an additional carboxyl group.
d. an aldehyde group.
e. a phosphoryl group.
Q:
Aspartate is formed from transamination of:a. asparagine.b. aspartame.c. oxaloacetate.d. citrate.e. -ketoglutarate.
Q:
Which of the following directly serves as one of the nitrogen atoms of a urea molecule?
a. the amide N of Asn
b. the N of Asp
c. the R-group N of Lys
d. the N of glucosamine
e. none of the above
Q:
All of the following are true of the urea cycle EXCEPT:
a. It helps in the excretion of excess nitrogen.
b. It is mainly confined to the liver.
c. It is linked to the citric acid cycle through fumarate.
d. Stimulation of carbamoyl-phosphate synthetase I (CPS-I) decreases the activity of the urea cycle.
e. It is completed by the regeneration of ornithine from arginine.
Q:
Ornithine serves three metabolically important roles, but is not found in proteins. What is one of the important roles of ornithine?
a. amine donor for many transamination reactions
b. an intermediate in the urea cycle
c. involved in the synthesis of serine, which serves as a precursor for glycine and cysteine
d. its reaction with methenyl tetrahydrofolate produces lysine
e. none of the above
Q:
All of the carbons and nitrogens of ornithine come biosynthetically from:
a. proline.
b. valine.
c. alanine.
d. glutamate.
e. glycine.
Q:
____ is an intermediate in biosynthesis of ornithine and a regulator of carbamoyl-phosphate synthetase-Ia. -Glutamyl-phosphateb. -Ketoglutaratec. N-Acetylglutamated. Glutaminee. Glutamine-5-phosphate
Q:
Which of the following is NOT a commonly used transamination pair?a. Phe / phenylpyruvateb. Asp / oxaloacetatec. Glu / -ketoglutarated. Ala / pyruvatee. all are correct
Q:
Which of the following is both an essential amino acid (for humans) and also an exclusively ketogenic amino acid?
a. proline
b. valine
c. glutamine
d. leucine
e. phenylalanine
Q:
All of the following are true of transamination EXCEPT:a. It is characterized by the transfer of an -amino group from an amino acid to the -keto position of an -keto acid.b. The amino donor becomes an -keto acid.c. The coenzyme needed is thiamin pyrophosphate (TPP).d. Humans are capable of synthesizing the -keto acid analog of nonessential amino acids and using transamination to form the amino acids, but are not able to construct carbon skeletons of the essential amino acids.e. The -keto acid acceptor becomes an -amino acid.
Q:
In the process of amino acid biosynthesis, how are glutamic acid, glutamine, proline, and arginine all related?a. They are all derived from -KG.b. They are all derivatives of acetyl CoA.c. They are all derivatives of pyruvate.d. They are all derived from aspartate.e. They are all derivatives of 3-phosphoglycerate.
Q:
In plants and microorganisms, amino acid biosynthesis is a matter of synthesizing the appropriate ____ followed by transamination with ____.a. acetyl-CoA derivatives; glutamineb. -ketoacid; glutamatec. phospho-carbon skeleton; alanined. nitrogenous base; glutamatee. none are true
Q:
What would be a result of a high [Gln]/[-KG] ratio?a. Increase in deadenylation of glutamine synthetase.b. An increase in activity of glutamine synthetase.c. An increase in the degree of adenylation of glutamine synthetase.d. Inhibition of AT:PIIA and stimulation of AT:PIID.e. A need for ammonium fixation by glutamine synthetase.
Q:
All of the following are characteristic features of ATP:GS:adenylyl transferase (AT) EXCEPT:a. It catalyzes the adenylation of glutamine synthetase.b. It exists in two forms, PIIA and PIID. PIIA causes the deadenylation of glutamine synthetase and its activity increases with an increase in glutamine.c. AT:PIIA is inhibited by -ketoglutarate.d. AT:PIIA needs ATP to complete the reaction.e. It is an enzyme that causes the adenylation/deadenylation of a specific tyrosine residue.
Q:
All of the following act in feedback inhibition of glutamine synthetase in prokaryotes EXCEPT:
a. AMP.
b. glucosamine-6-phosphate.
c. histidine.
d. proline.
e. CTP.
Q:
Bacterial glutamine synthetase monomers are inactive because they must be:
a. phosphorylated for activity.
b. activated by binding glutamine.
c. stacked for interface active site development.
d. combined for allosteric regulation.
e. none are true.
Q:
Glutamate synthase catalyzes the reaction ____ + ____ + ____ ->2 glutamate + NADP+a. -ketoglutarate; NADPH; glutamineb. glutamate; -ketoglutarate; NADHc. glutamyl-phosphate; glutamine; NADPHd. N-acetylglutamate; -ketoglutarate; NADHe. none are true
Q:
The reaction, 2 NH4++ -ketoglutarate + NADPH + ATP ->glutamine + NADP++ ADP + Pi+ H2O, is the combined result of what two enzymes?a. first, glutamate synthase, then glutamate dehydrogenaseb. first, glutamate dehydrogenase ,then glutamine synthetasec. first, glutamine synthetase, then glutamate synthased. first, glutamine synthetase, then glutamate dehydrogenasee. none of the above
Q:
Glutamine synthetase is activated by which of the following molecules?
a. glycine
b. AMP
c. CTP
d. carbamoyl phosphate
e. none of the above
Q:
The reaction, glutamate + NAD(P)++ H2O -> NH4++ -ketoglutarate + NAD(P)H + H+, is catalyzed by:a. Nitrogenase.b. carbamoyl-phosphate synthetase (CPS-I).c. glutamate dehydrogenase (GDH).d. glutamine synthetase (GS).e. none are true.
Q:
What reaction does glutamate dehydrogenase (GDH) catalyze?a. The reductive amination of -ketoglutarate to yield glutamate.b. Phosphorylation of carbamate to yield carbamoyl-phosphate.c. The amidation of the -carboxyl group of glutamate to form glutamine.d. The deadenylation of glutamine synthetase.e. The adenylation of glutamine synthetase.
Q:
N-acetylglutamate functions in ammonium incorporation into metabolic intermediates as:a. a coenzyme for glutamine synthetase (GS).b. a competitive inhibitor for glutamine synthetase (GS).c. an allosteric activator for carbamoyl-phosphate synthetase I (CPS-I).d. the energy source needed for the reductive amination of to yield glutamate.e. the substrate for the amination of the -glutamylphosphate.
Q:
Control over nitrogenase activity is accomplished by which of the following?
a. ATP serves as an allosteric activator
b. AMP serves as an allosteric inhibitor
c. NH4+ prevents expression of the genes that encode nitrogenase
d. phosphorylation prevents activity
e. none of the above
Q:
The involvement of ATP in the nitrogenase complex is because ATP is needed to:
a. provide electrons.
b. phosphorylate the enzyme.
c. break the N2 triple bond.
d. transfer electrons from ferredoxin.
e. all are true.
Q:
All N2-fixing systems are nearly identical and have all of the requirements EXCEPT:
a. ATP.
b. O2-free conditions.
c. the enzyme nitrogenase.
d. strong reductant such as ferredoxin.
e. all are true.
Q:
Nitrogen fixation involves the enzyme ____ catalyzing the reaction:N2+ 10 ____ + ____ e- -> 2NH4++ H2a. nitrate reductase; H2O; 6b. nitrogenase; H+; 8c. nitrogenase; H+; 2d. nitrogen reductase; H2O; 6e. none are true