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 22.214.171.124) 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.
The nucleosides cytidine and deoxycytidine can be salvaged along the uracil pathway by cytidine deaminase, which converts them to uridine and deoxyuridine, respectively. Alternatively, uridine–cytidine kinase can phosphorylate them into cytidine monophosphate (CMP) or deoxycytidine monophosphate (dCMP). UMP/CMP kinase can phosphorylate (d)CMP into cytidine diphosphate or deoxycytidine diphosphate, which nucleoside diphosphate kinase can phosphorylate into cytidine triphosphate or deoxycytidine triphosphate.
Phosphoribosyltransferases add activated ribose-5-phosphate (Phosphoribosyl pyrophosphate, PRPP) to bases, creating nucleoside monophosphates. There are two types of phosphoribosyltransferases: adenine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT). It is an important enzyme in Purine pathway metabolism. It is also involved in Lesch-Nyhan syndrome associated with a deficiency of HGPRT.
|hypoxanthine||hypoxanthine/guanine phosphoribosyl transferase (HGPRT)||IMP|
|guanine||hypoxanthine/guanine phosphoribosyl transferase (HGPRT)||GMP|
|adenine||adenine phosphoribosyltransferase (APRT)||AMP|
- Ansari MY, Equbal A, Dikhit MR, Mansuri R, Rana S, Ali V, Sahoo GC, Das P (Nov 2015). "Establishment of Correlation between In-Silico &In-Vitro Test Analysis against Leishmania HGPRT to inhibitors". International Journal of Biological Macromolecules. doi:10.1016/j.ijbiomac.2015.11.051. PMID 26616453.