Salvage Pathways of Purine Nucleotide Biosynthesis

The net formation of purine nucleotides is performed by the de novo pathway, but rapid turnover of nucleic acids, especially RNA, is required for nucleotide production by the salvage pathways. In plant cells, purine bases and nucleosides originate from the intercellular breakdown of nucleic acids and nucleotides, as well as other reactions which release purine bases and nucleosides. Nucleoside kinases are enzymes which catalyse the transfer of γ-phosphate from adenosine triphosphate to nucleosides leading to formation of nucleoside-5' -monophosphates. Deoxyribonucleoside kinases salvage deoxyribonucleosides by transfer of a phosphate group to the 5' position of a deoxyribonucleoside. This salvage pathway is well characterized in mammals, but little is known about deoxyribonucleoside salvage enzymes in plants. In contrast to nucleoside kinases, nucleoside phosphotransferase use nucleoside monophosphates as phosphate donors. The non-specific nucleoside phosphotransferase catalyses the conversion of nucleosides to nucleoside monophosphate using 5'-nucleoside monophosphate.