試題詳解

56. Which of the following descriptions about pyrimidine synthesis is NOT correct?
(A) The pathway commences with the conversion of bicarbonate and glutamine into carbamoyl phosphate. In mammalian cells, carbamoyl phosphate synthetases- II is the major enzyme responsible for the reaction.
(B) Carbamoyl phosphate reacts with aspartate to produce carbamoyl aspartate. The enzyme aspartate transcarbamoylase catalyzes this reaction.
(C) The enzyme dihydroorotase converts carbamoyl aspartate into dihydroorotate. This step does not involve the incorporation of any atoms from the other substrates.
(D) Dihydroorotate is combined with 5-phosphoribosyl-1-pyrophosphate to form UMP. This step is catalyzed by orotate phosphoribosyltransferase.
(E) Unlike purines, pyrimidines are not synthesized as nucleotide derivatives. The synthesis of the pyrimidine ring is completed before a ribose-5-phosphate moiety is attached.