Biochemistry Nucleotide Chemistry and Metabolism 2

Question 1

Which enzyme catalyzes the committed (first unique) step of pyrimidine biosynthesis?

Accepted Answer

Aspartate transcarbamoylase (ATCase)

Answer

PRPP synthetase

Answer

Carbamoyl phosphate synthetase II (CPS II)

Answer

Dihydroorotase

Question 2

Which nucleotide intermediate serves as the branch point from which both AMP and GMP are synthesized in the purine de novo pathway?

Accepted Answer

IMP (inosine monophosphate)

Answer

XMP

Answer

PRPP

Answer

Adenylosuccinate

Question 3

PRPP (5-phosphoribosyl-1-pyrophosphate) is best described as which of the following?

Accepted Answer

An activated ribose donor used in both de novo synthesis and salvage pathways

Answer

The final catabolic product of both purines and pyrimidines

Answer

A substrate exclusive to pyrimidine de novo synthesis

Answer

An allosteric inhibitor of purine salvage enzymes

Question 4

Orotic aciduria type I is caused by a deficiency of which bifunctional enzyme?

Accepted Answer

UMP synthase (UMPS)

Answer

CPS II

Answer

ATCase

Answer

Dihydroorotate dehydrogenase

Question 5

5-Fluorouracil (5-FU) exerts its anticancer activity primarily by inhibiting which enzyme?

Accepted Answer

Thymidylate synthase

Answer

Ribonucleotide reductase

Answer

Dihydrofolate reductase

Answer

CTP synthetase

Question 6

PRPP amidotransferase, which catalyzes the first committed step of de novo purine synthesis, is allosterically inhibited by which molecules?

Accepted Answer

AMP, ADP, ATP, GMP, GDP, and GTP

Answer

Carbamoyl phosphate and aspartate

Answer

Orotate and UMP

Answer

IMP and XMP only

Question 7

In the pyrimidine salvage pathway, uridine is converted directly to UMP by which enzyme?

Accepted Answer

Uridine kinase

Answer

UMP synthase

Answer

Uridine phosphorylase

Answer

CTP synthetase