This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of Purines and Pyrimidines, Subject name: Biochemistry
Rate limiting step in pyrimidine synthesis - 
Select and explain. 
A. Dihydro-orotase, B. Ornithine transcarbomoylase, C. Aspartate transcarbomoylase, D. Carbamoyl phosphate synthase-I
C: Aspartate transcarbomoylase
Answer: C: Aspartate transcarbomoylase 
Explanation: 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.