[The pH dependence of kinetic parameters of Penicillium brevicompactum RNAase]

Abstract

The effect of pH on the kinetic parameters (Km and Ki) for extracellular acid Penicillium brevicompactum RNAse (pH max 4.7+/-0.1), non-specific to the chemical nature of nucleic bases, was studied. The pKm--pH dependence curve showed bends within the following intervals of pH: 3.5--4.0 and 5.6--6.0 (upward side concavity) and 6.2--6.8 (downward side concavity). The pKi--pH dependence for adenosine-3'-monophosphate as an inhibitor is identical to the pH dependence on pKm for the substrate. On the other hand, the pKi--pH dependence curves obtained for the base-free inhibitors (ribose-5'-monophosphate, or phosphate (adenosine) show no bends within the pH intervals of 3.0--4.0 and 5.6--7.0 respectively. A possibility is discussed of the presence of a carboxylic (pK 3.58+/-0.1) and two imidazole groups (pK 6.42+/-0.1--a weakly protonated and 5.8+/-+/-0.1--a strongly protonated group) in the RNAse active site and their participation in the formation of the RNAse-nucleotide (RNAse-substrate) complex.