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

ATCase (aspartate transcarbamoylase) catalyzes the condensation of carbamoyl phosphate with aspartate, forming carbamoyl aspartate, which is the committed step unique to pyrimidine biosynthesis.

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

IMP is the first complete purine nucleotide formed in de novo synthesis and serves as the branch-point intermediate from which AMP and GMP diverge via separate enzymatic routes.

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

PRPP donates the activated ribose-5-phosphate moiety in both de novo nucleotide synthesis and salvage pathways for purines (HGPRT, APRT) and pyrimidines (OPRT).

Question 4

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

Accepted Answer

UMP synthase (UMPS)

Answer

UMP synthase possesses both orotate phosphoribosyltransferase and OMP decarboxylase activities; its deficiency blocks the last two steps of pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia.

Question 5

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

Accepted Answer

Thymidylate synthase

Answer

5-FU is converted to 5-FdUMP, which forms a covalent ternary complex with thymidylate synthase and N5,N10-methyleneTHF, irreversibly blocking dTMP synthesis and depleting the thymidine pool for DNA replication.

Biochemistry Practice Test

Biochemistry Practice Test

Biochemistry Nucleotide Chemistry and Metabolism 2