2. Ribonuclease catalyzes the hydrolysis of ribonucleic acid (RNA). The active s

Question

2. 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? ov Oo-p=0 Pyrimidine H/ His-12 TO-- A His-119 -:N NH o- = 0 milo HNO,NH CH, 0, Pyrimidine or purine H H OF OH

Explanation / Answer

From the mechanism, it can be concluded that there are two histidines which behave differently in the mechanism. His 119 behave as an electrophile in the reaction mechanism whereas His 12 behave as a nucleophile in the reaction mechanism, so they will behave differently at different pH conditions. The pI value of His lies between 6.00 to 9.17, i.e., 6.2. So the His 12 will show inflection point at 6.2 pH, whereas the electrophilic His 119 will show inflection point at slightly lower pH of 5.8.

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