Mechanism of activity‐linked ionization in carbonic anhydrase

A new mechanism is proposed to explain the activity-linked ionization in the zinc metalloenzyme carbonic anhydrase. This mechanism identifies the group controlling catalytic activity as an active site histidine. The protonated cationic imidazole ring of this residue reversibly coordinates to zine with increasing pH, losing a proton. Concerted deprotonation and metal ion coordination of imidazole is chemically reasonable and is consistent with most available data on this enzyme.