The imidazole side chain of histidine can function as either a general acid catalyst or a general base catalyst becausea. the imidazole group has a pka in the physiological ph range. b. in the physiological ph range, the nitrogen in the ring can be easily protonated/deprotonated. c. one guanidino group is protonated, and the other is deprotonated at physiological ph. d. the imidazole group is a strong reducing agent at physiological ph.