Deoxycytidine kinase, reaction catalyzed

Whether there are other kinase activities, perhaps specific for the purine deoxyribonucleosides, remains an open question. The phosphorylation of deoxyguanosine has been demonstrated [for example, in fish milt extracts (Sdeoxycytidine kinase to catalyze this reaction. The apparent Michaelis constant for the phosphorylation of deoxyguanosine by the calf liver kinase is 3.1 X 10 M, about two orders of magnitude higher than that for deoxycytidine 36),... 

There are several points of regulation in the synthesis. For example, dCTP inhibits the salvage reaction catalyzed by deoxycytidine kinase and activates the reaction catalyzed by dCMP deaminase. On the other hand, dTTP inhibits the dCMP deaminase reaction and the enzyme thymidine kinase. 

Nucleoside kinases are a class of enzymes that catalyze the phosphorylation of nucleosides to make nucleoside monophosphates (Figure 22.2) as part of nucleotide biosynthetic salvage pathways. ATP provides the energy and phosphate for the reaction. Example enzymes include thymidine kinase, deoxycytidine kinase, and deoxyguanosine kinase. 

The reversal experiments provide indirect evidence that cytallene (lid) is phos-phorylated by a reaction catalyzed with 2 -deoxycytidine (dCyd) kinase. Thus, the protective effect against HTV-1 infection and cellular toxicity of lid are completely reversed by dCyd. By contrast, neither antiretroviral activity nor toxicity of adenallene (11c) is affected by 2 -deoxyadenosine (dAdo) or dCyd. These results show that a nucleoside kinase different from those using dAdo and dCyd as substrates may be responsible for phosphorylation of 11c in ATH8 cells. 

While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

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