Discussion
Various atoms of pyrimidine ring are derived from :
Answer: A
The atoms come from aspartate (N1, C4, C5, C6), bicarbonate (C2) and glutamine (N3).
In de novo pyrimidine synthesis, carbamoyl phosphate synthetase II makes carbamoyl phosphate from bicarbonate and the amide nitrogen of glutamine; that molecule supplies C2 and N3. Aspartate transcarbamoylase then adds aspartate, which supplies N1, C4, C5 and C6, and the ring closes to form orotate.
The option swapping glutamine and aspartate has the sources reversed. Glutamate is what is left after glutamine gives up its amide nitrogen, so it is not the N3 donor, and asparagine is not used at all.
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