Catalytic activity of Ntau-carboxymethylhistidine-12 ribonuclease: pH dependence.

The pH-dependence of RNAase A and of Ntau-carboxymethylhistidine-12-RNAase (ribonucleate 3'-pyrimidino-oligonucleotidohydrolase) catalysis was studied. Apparent acid dissociation constants were obtained by least squares analysis of the kinetics data. These dissociation constants were compared with pKa values of model imidazole compounds, and with pKa values of histidine residues 12 and 119 on the protein. The shapes of the kcat versus pH profiles for RNAase A and its carboxymethyl derivative are very similar, from which it is concluded that the mechanism of catalysis is closely similar in the two proteins. Apparent pKa values obtained from the kinetic data are higher for the carboxymethylated protein than for RNAase A, as are the pKa values of residues 12 and 119. The similar shifts are consistent with the conclusions that both these residues are functionally significant in native and modified enzyme, and that an unblocked tau-nitrogen on histidine-12 is not essential for activity. From the enzyme's catalytic dependence on pH, and the NMR determined pKa values we propose that histidine 12 and 119 function catalytically in their basic and acidic forms respectively.