Fig. 25.17 The catalytic site in α-carbonic anhydrases is found at the end of a funnel-shaped binding pocket. There an OH− ion which is coordinated to the Zn2+ ion nucleophilically attacks a CO2 molecule. Bicarbonate forms, which is held in place by Thr199 (left). A sulfonamide, deprotonated at nitrogen, fits at the site of the carbonate in the very narrow binding pocket (right). Because of the tetravalency of the sulfur, this site can be fitted with another substituent, as is shown in the case above with a p-fluorophenyl group.