Ribonuclease catalyzes the hydrolysis of ribonucleic acid (RNA). The active site of the enzyme contains two histidine residues that participate in the first step of hydrolysis. (see reaction mechanism) The pH rate profile for the ribonuclease-catalyzed reaction is bell shaped with inflection points at approximately pH 5.8 and 6.2. How is the pH-rate profile related to the mechanism for ribonuclease? His-119 Pyrimidine H H His-12 H NH HN ON. CH,O. Pyrimidine or purine H H H mo. OH