The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst because
i. the imidazole group has a pka in the physiological ph range.
ii. in the physiological ph range, the sulfur in the ring can be easily protonated/deprotonated. (verified with instructor that sulfur was intended, not nitrogen)
iii. one guanidino group is protonated, and the other is deprotonated at physiological ph.
iv. the imidazole group is a strong reducing agent at physiological ph
a. i, ii
b. ii, iv
c. iv
d. i
e. ii, iii

The reason why the imidazole side chain of histidine can function as either acid or base catalyst is: I and II: Option A.

Explanation:

The imidazole group of the histidine has a pKa of 6.0, which is closer to the pKa of the physiological environment. Thus, at every reaction, either acidic or basic, the side chain act as a buffering catalyst, which can be useful irrespective of the pH of the reactants. Also, since the reaction of a catalyst is influenced by the pH of the environment, at physiological pH, the sulfur in the ring can be protonated by acidic reaction or deprotonated by the primary reaction within the physiological environment.

the answer would be b i believe