Carbonic anhydrase activators: The first X-ray crystallographic study of an adduct of isoform I

The X-ray crystallographic structure for the adduct of an activator with human carbonic anhydrase isozyme I (hCA I) is reported. l-Histidine binds deep within the enzyme active site, participating in a network of hydrogen bonds involving its carboxylate moiety and the zinc-bound water molecule, as well as the imidazole of His200, being in van der Waals contacts with Thr199, His200, His64, and His67. This binding is very different from that to the other major cytosolic isozyme hCA II.