Figure 2.

Pyrimidine metabolism and synthesis of DNA precursors.

Synthesis of DNA uses NDPs as precursors. Pyrimidine biosynthesis does not proceed de novo via CDP, producing directly CTP instead. ATP large excess over ADP does not allow nucleoside diphosphokinase to make CDP so that CDP can only come from RNA degradation, via RNAses and polynucleotide phosphorylase. The maintenance pyrophosphate hydrolase (NudG) would produce CMP from CTP, but it is a moonlighting activity derived from its major substrates which are CTP derivatives modified at position 5 of cytosine.