Linear Free Energy Relationships for N(7)-Substituted Guanosines as Substrates of Calf Spleen Purine Nucleoside Phosphorylase. Possible Role of N(7)-Protonation as an Intermediary in Phosphorolysis

Quantitative structure-activity relationships (QSAR) for a series of N(7)-substituted guanosines as substrates for calf spleen purine nucleoside phosphorylase (PNP) were developed, and compared with those for acid hydrolysis of these analogues. There is no correlation between the rates for enzymatic phosphorolysis and acid hydrolysis, indicating that for the enzymatic reaction labilization of the glycosidic bond is not the only, nor the predominant, effect of N(7)-substitution. Multiple regression analysis of the enzymatic process revealed that optimal substrate properties (minimal Michaelis constant) are associated with the Taft electronic constant equal zero and a substituent size, parametrized by the Taft steric constant, smaller than that for a methyl group. These results support the hypothesis of protonation of the N(7)-position of the base by the enzyme as a catalytic mechanism for calf spleen PNP. Attention is drawn to the postulated similar mechanism of action of other purine N-glycosidases, including plant antiviral proteins which function as RNAN-glycosidases, and possibly some DNA N-glycosidases which function as repair enzymes