Purine salvage pathway

A salvage pathway is a pathway in which nucleotides (purine and pyrimidine) are synthesized from intermediates in the degradative pathway for nucleotides.

Salvage pathways are used to recover bases and nucleosides that are formed during degradation of RNA and DNA. This is important in some organs because some tissues cannot undergo de novo synthesis.

The salvaged bases and nucleosides can then be converted back into nucleotides.

The salvage pathway requires distinct substrates:

Uridine phosphorylase or pyrimidine-nucleoside phosphorylase adds ribose 1-phosphate to the free base uracil, forming uridine. Uridine kinase (aka uridine–cytidine kinase) can then phosphorylate this nucleoside into uridine monophosphate (UMP). UMP/CMP kinase (EC 2.7.4.14) can phosphorylate UMP into uridine diphosphate, which nucleoside diphosphate kinase can phosphorylate into uridine triphosphate.

Thymidine phosphorylase or pyrimidine-nucleoside phosphorylase adds 2-deoxy-alpha-D-ribose 1-phosphate to thymine, forming thymidine. Thymidine kinase can then phosphorylate this compound into thymidine monophosphate (TMP). Thymidylate kinase can phosphorylate TMP into thymidine diphosphate, which nucleoside diphosphate kinase can phosphorylate into thymidine triphosphate.

...
Wikipedia