Fig. 7.
The active site chemistry of carbonic anhydrase–mediated reversible catalysis of carbon dioxide (CO2) hydration and bicarbonate (HCO3–) dehydration. A divalent zinc ion is activated by coordination to three histidine (His) molecules in a tetrahedral arrangement that permits binding of water, its hydrolysis to a zinc-hydroxide (reaction 1), nucleophilic attack on the carbonyl bond of carbon dioxide (reaction 2) to produce bicarbonate in the active site (reaction 3), and release of bicarbonate and regeneration of zinc-bound water (reaction 4). Reaction 1 is facilitated by rapid shuttling of the proton produced in reaction 1 to bulk water by hydrogen ion–titratable amino acid side chains leading out of the active site. Zn2+ = divalent positively charged zinc ion. Figure supplied by David Silverman, Ph.D., University of Florida, Gainesville, Florida.