Final answer:
Thymine is biosynthesized through the pyrimidine nucleotide biosynthetic pathway, involving a sequence of enzymatically catalyzed reactions beginning with carbamoyl phosphate and ending with the conversion of uridylate to thymidine monophosphate.
Step-by-step explanation:
Thymine biosynthesis occurs through the pyrimidine nucleotide biosynthetic pathway. Initially, carbon dioxide and ammonia combine to form carbamoyl phosphate in a reaction catalyzed by carbamoyl phosphate synthetase-II. Carbamoyl phosphate then reacts with aspartate, in a process facilitated by aspartate transcarbamoylase, to result in carbamoyl aspartate. This product then forms dihydroorotate, catalyzed by dihydroorotase, which then converts to orotic acid via oxidation involving NAD as an electron acceptor. Orotic acid subsequently attaches to ribose phosphate to form orotidylic acid, which is decarboxylated to yield uridylate (UMP). Finally, UMP is converted into other pyrimidine nucleotides, including thymidine monophosphate (TMP), through further enzymatic reactions.Regulation of this pathway is achieved through feedback inhibition, primarily at the committed step catalyzed by aspartate transcarbamoylase, which is inhibited by the end product, CTP. Additionally, carbamoyl phosphate synthetase-II is feedback inhibited by UMP.