pH is an important factor in the activity of RNase A, because of the active site residues present in the enzyme. Histidine residues are able to be protonated and deprotonated at different pHs, suppressing the enzyme activity when outside the optimum.
Acid-base catalysis is found in the active site, with the histidine residues being important. The protonation of one group leads to the deprotonation of the other, and this can be seen in kinetic studies of the reaction, where a titratable response is observed. One histidine residue must act as a base, while the other acts as an acid. This ‘one or the other’ feature is likely due to the close proximity of the two residues – they can’t both be protonated or deprotonated.
The formation of intermediate molecules in the restriction of ribonucleic acid by RNase A is hindered by pH away from the optimum. Therefore, consideration must be made to the physiological pH of that enzyme.