In proteins, the imidazole groups of histidine play key roles in catalysis involving reversible protonation/deprotonation events. The

Question:

In proteins, the imidazole groups of histidine play key roles in catalysis involving reversible protonation/deprotonation events. The pK of the imidazole group is influenced by surrounding amino acids and in many enzymes is near 7.
(a) What is the significance of this apparent pK in catalysis?
(b) Some unprotonated imidazole groups participate in hydrogen bonding a substrate to an enzyme. Do you expect the pK of these imidazole groups also to be near 7?