The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst becausei. 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 pha. i, iib. ii, ivc. ivd. ie. ii, iii