Nucleotide salvage

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.

Substrates

The salvage pathway requires distinct substrates:

Pyrimidines

Uridine phosphorylase adds ribose-1-phospate to the free base uracil, forming uridine monophosphate. Uridine kinase then phosphorylates this nucleoside into its diphosphate and triphosphate forms. Deoxythymidine phosphorylase adds deoxyribose-1-phosphate to thymine, forming deoxythymidine monophosphate. Thymidine kinase can then phosphorylate this compound to deoxythymidine diphosphate and triphosphate.

The salvage of pyrimidine ribonucleotides.

Purines

Phosphoribosyltransferases add activated ribose-5-phosphate (called phosphoribosyl pyrophosphate or PRPP) to bases, creating nucleotide monophosphates. There are two types of phosphoribosyltransferases: adenosine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Lesch-Nyhan syndrome is associated with a deficiency of HGPRT.

Nucleoside	Enzyme	Nucleotide
hypoxanthine	hypoxanthine/guanine phosphoribosyl transferase (HGPRT)	IMP
guanine	hypoxanthine/guanine phosphoribosyl transferase (HGPRT)	GMP
adenine	adenine phosphoribosyltransferase (APRT)	AMP

External links

• "Enzymes in the methionine salvage pathway: structure and function" at Brandeis University