Question

What is the net equation for pyrimidine biosynthesis?

Answer 1

Pyrimidine are the heterocyclic nitrogenous base like uracil, thymine, cytosine and orotic acid which are synthesized from carbamyl phosphate and aspartate. There are several steps in the biosynthesis of pyrimidine which are as follows:

(i) Synthesis of carbamoyl phosphate- It is the first step of pyrimidine biosynthesis where carbamoyl phosphate is synthesized from bicarbonate ion and amide nitrogen of glutamine in the presence of enzyme carbamoyl phosphate synthetase II (CPS II).

(ii) Synthesis of carbamoyl aspartate- Condensation of carbamoyl phosphate with aspartate to yield carbamoyl aspartate catalyzed by enzyme aspartate transcarbamylase (ATCase).

(iii) Ring closure to form dihydroorotate- This step involved the formation of pyrimidine ring yielding dihydroorotate by intramolecular condensation catalyzed by enzyme dihydroorotase with zinc as cofactor.

(iv) Oxidation of dihydroorotate- Dihydroorotate is reversibly oxidised to orotate by dihydroorotate dehydrogenase.

(v) Transfer of orotate to Ribose-5-monophosphate- Orotate reacts with 5-phospho-d-ribosyl-1-pyrophosphate to produce orotidine-5-monophosphate (OMP) catalyzed by orotate phosphoribosyltransferase.

(vi) decarboxylation to form UMP- It is the final step of the pathway, where decarboxylation of OMP to form UMP catalyzed by enzyme OMP decarboxylase.

The overall process of the pyrimidine biosynthesis is as follows: