A histidine was determined to be the critical residue involved in an enzyme-catalyzed reaction.If the pKa of the histidine is known to be 6.5 in the active site and the pH of maximum catalytic activity is 7.2,what is likely the primary role of histidine in the catalytic reaction?
A)Acts as a proton donor or acceptor.
B)Forms a covalent bond with the substrate.
C)Stabilizes a charged intermediate.
D)Reduces the entropy of the substrate.

Question

Quizplus · Accepted Answer

The fact that the pKa of the histidine is 6.5 in the active site and the pH of maximum catalytic activity is 7.2 suggests that the histidine is likely acting as a proton donor or acceptor in the catalytic reaction. This is because at pH 7.2, the histidine will be partially deprotonated (i.e. have lost a proton) and therefore able to donate a proton to or accept a proton from the substrate or other molecules involved in the reaction. This ability to donate or accept protons is important for many enzyme-catalyzed reactions.

A Histidine Was Determined to Be the Critical Residue Involved