MCQs on chapter 14- 1st year

(1)

In purines , the carbon atom which is obtained from respiratory CO2 is.................
a) C1
b) C5
c) C6
d) C2






2- Non-hepatic tissues obtain their purines from...............
a) Liver
b) De novo synthesis
c) Salvage pathway
d) all the above
e) both a & c







3- .................... is the active form of ribose . It is used in de novo synthesis as well as salvage pathway for both purines & pyrimidines
a) glutamine
b) PRPP
c) ATP
d) GTP







4- In de novo synthesis of purines , the rate limiting enzyme (Glutamine-PRPP transferase) is activated by ............... & inhibited by ................
a) IMP
b) AMP
c) GMP
d) IMP , AMP & GMP
e) PRPP








5-IN de novo synthesis of purines , the first synthesized nucleotide is.............,with consumption of...............ATP
a) AMP
b) IMP
c) GMP
d) 2 ATP
e) 4 ATP
f) 6 ATP








6-IMP is converted into AMP by addition of amino group at .............of purine ring , the non-ring "N" is derived from ................ & consumes ...............
a) C4
b) C5
c) C6
d) glutamine
e) aspartate
f) 1 ATP
g) 2 ATP
h) 1 GTP
i) 2 GTP








7- IMP is converted into GMP by addition of amino group at .............of purine ring , the non-ring "N" is derived from ................ & consumes ...............
a) C8
b) C2
c) C5
d) glutamine
e) aspartate
f) 1 ATP
g) 2 ATP
h) 1 GTP
i) 2 GTP







8-PRPP synthetase is allosterically inhibited by ..............
a) AMP
b) GMP
c) UMP
d) TMP
e) all the above
f) both a & b






9-which of the following inhibitors of de novo synthesis of purines is an effective inhibitor of enzymes that utilize glutamine as amide donor:-
a) PABA analogs ( sulfonamide)
b) azaserine
c) antimetabolites (5-flurouracil)
d) folic acid analogs ( methotrexate & trimethoprim)






10- which of the following inhibitors of de novo synthesis of purines are structural analogs of purine (& pyrimidines) which competitively inhibit specific reactions:-
a) PABA analgs ( sulfonamide)
b) azaserine
c) antimetabolites (5-flurouracil)
d) folic acid analogs ( methotrexate & trimethoprim)






11-which of the following inhibitors of de novo synthesis of purines prevent folic acid synthesis by bacteria:-
a) PABA analgs ( sulfonamide)
b) azaserine
c) antimetabolites (5-flurouracil)
d) folic acid analogs ( methotrexate & trimethoprim)







12- which of the following inhibitors of de novo synthesis of purines competitively inhibit dihydrofolate reductase:-
a) PABA analgs ( sulfonamide)
b) azaserine
c) antimetabolites (5-flurouracil)
d) folic acid analogs ( methotrexate & trimethoprim)







13-the IMP dehydrogenase enzyme is used in the conversion of IMP into .................
a) AMP
b) GMP
c) xanthosine-5-p
d) adenylate succinate





14- the adenylo succinase enzyme is used in the conversion of ............... into .................
a) IMP into adenylo succinate
b) adenylo succinate into AMP
c) IMP into hypoxanthine
d) adenylo succinate into GMP





15- ADP is converted into ATP by :-
a) Adenylate kinase
b) oxidative& substrate - level phosphorylation
c) adenosine diphospjate kinase
d) ribonucleotide reductase





16-Lesh-Nyhan syndrome is due to a virtuall deficiency of .................
a) adenine phosphribosyl transferase (APRT)
b) Hypoxanthine-guanine phosphribosyl transferase (HGPRT)
c) IMP dehydrogenase
d) adenylo succinase






17- the reactions catalyzed by the ribonucleotide reductase enzyme needs the presence of ...............which is derived from ................ with the utilize of ................
a) oxidized thioredoxin(S-S) , reduced thioredoxin(2SH) & NADPH + H+
b) reduced thioredoxin(2SH) , oxidized thioredoxin(S-S) & NADPH + H+
c) reduced thioredoxin(2SH) , oxidized thioredoxin(S-S) & NADP+
d) oxidized thioredoxin(S-S) , reduced thioredoxin(2SH) & NADP+






18- all the following are the causes of secondary gout except :-
a) chronic renal insufficiency , due to an underexcretion of uric acid
b) cancer & leukemia & those undergoing chemotherapy due to excessive cellular breakdown
c) overproduction of purines due to an inherited abnormality in the enzyme of purine metabolism
d) consumption of excessive amounts of alcohol or purine-rich foods
e) Lesh-Nyhan syndrome & Von Gierke disease (glucose-6-phosphate deficiency)





19-Acute attacks of gout cannot be treated by the following except :-
a) allopurinol
b) alkalization of urine
c) colchicine & anti-inflammatory drugs(such as aspirin)
d) uricosuric agents(probenecid)






20- xanthine oxidase enzyme catalyzes the conversion of ................into...............
a) xanthine into hypoxanthine
b) hypoxanthine into xanthine
c) hypoxanthine into uric acid
d) xanthine into uric acid
e) both a & d
f) both b & d






21- B-alanine can serve as a precursor for ................ while B-aminoisobutyrate can serve as a precursor for................
a) Acetyl coA
b) PRPP
c) succinyl coA
d) none of the above






22-hydroxyurea can be used in treatment of cancer because it can inhibit:-
a) dihydrofolate reductase
b) ribonucleotide reductase
c) thymidylate synthase
d) xanthine oxidase





23- A-one year old female patient lethargic , weak & anemic , her height & weight are both low for her age . her purine contains an elevated level of orotic acid . the administation of which of the following is most likelyto alleviate her symptoms:-
a) Aenine
b) Guanine
c) Thymidine
d) uridine





24- In de novo synthesis of purine , how many of the C & N atoms of the purine ring system would be provided by glycine & glutamine together:-
a) 2
b) 3
c) 4
d) 5
e) 6





25- A defect in the enzyme that catalyzes the attacks of guanine on PRPP was discovered in a biopsy sample from a patient with hyperuricemia . which metabolic process can you conclude was defective in the patient:-
a) purine salvage
b)De novo synthesis of IMP
c) synthesis of GMP from IMP
d) regulation of PRPP amidotransferase
e) feed back inhibition of PRPP synthetase





26- in the biosynthesis of pyrimidines , how is the ribose sugar covalently linked to pyrimidine :-
a) the pyrimidine ring is built atom by atom on PRPP
b) the ribose sugar is built atom by atom on a completed pyrimidine ring
c) the pyrimidine ring is completed prior to covalent linkage with ribose sugar
d) an intermediate in the pyrimidine biosynthesis pathway is linked to PRPP before ring closure





27- in lesh-nyhan patients , lack of Hypoxanthine-guanine phosphribosyl transferase (HGPRT) leads to hyperuricemia due to an inability to convert purine bases to the corresponding nucleotides . the resulting decrease in the pool nucleotides further stimulates the overproduction of uric acid by :-
a)inhibition of xanthine oxidase
b) inhibition of inosinicase
c) inhibition of PRPP synthetase
d) activation of PRPP amido-transferase
e) activation of ribonucleotide reductase





28- products of highly regulated , commited step in de novo purine biosynthesis are :-
a) PRPP+AMP
b) PRPP+glutamine
c) AMP+PPi
d) XMP+NADH
e) uric acid+ H2O2
f) none of the above





29- all of the following are precursors for biosynthesis of both purine & pyrimidine nucleotides except :-
a) glutamine
b) aspartate
c) CO2
d) PRPP
e) glycine






30-the dihydrofolate reductase inhibitor methotrexate is an effective anti-cancer drug because :-
a) high levels of N10-formyl tetrahydrofolate are toxic to tumpr cells
b) conversion of tetrahydrofolate to N5,N10-methylene-tetrahydrofolate is blocked
c) conversion of N5,N10-methylene-tetrahydrofolate to dihydrofolate is blocked
d) cell replication is blocked in the absence of thymidine triphosphate
e) conversion of ribonucleoside diphosphates (NDPs) to deoxyribonucleoside diphosphates (dNDPs) is blocked














-model answers:-
1- c
2-e
3-b
4-e,d
5-b,f
6-c,e,h
7-b,d,f
8-f
9-b
10-c
11-a
12-d
13-c
14-b
15-b
16-b
17-b
18-c
19-c
20-f
21-a,c
22-b
23-d
24-d
25-a
26-c
27-d
28-f
29-e
30-d