<prompt> Rate limiting step in pyrimidine synthesis -Dihydro-orotaseOrnithine transcarbomoylaseAspartate transcarbomoylaseCarbamoyl phosphate synthase-I  <response>: 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. 