Rate limiting step in pyrimidine synthesis -

Ans. is 'c' i.e.. Aspartate transcarbomoylase o Pyrimidine biosynthesis starts with the formation of carbamoyl phosphate from glutamine, ATP and CO.. The reaction is catalyzed by 'cytosolic' enzymes carbamoyl phosphate synthase-II (CPS-II), an enzymes different form mitochondrial carbamoyl phosphate synthase-1 (CPS-I) required in urea synthesis.o Regulation of pyrimidine nucleotide synthesis occurs at first two reactions. Carbamoyl phosphate synthase-II is feed-back inhibited by UTP and activated by PRPP. Aspartate transcarbamoylase is feed-back inhibited by CTP and activated by ATP.