The X-ray crystal structure for the adduct of human carbonic anhydrase (hCA) II with a topically acting antiglaucoma sulfonamide (the 2-N,N-diethylaminoethylamide of 5-(4-carboxybenzenesulfonamido-1,3,4- thiadiazole-2-sulfonamide), has been resolved at a resolution of 1.6Å. This compound is a very potent inhibitor of the physiologically most relevant isozyme hCA II for the secretion of aqueous humor within the eye (KI of 1.4nM), and in animal models of glaucoma showed very effective intraocular pressure (IOP) lowering after topical administration. Surprisingly, the inhibitor bound within the enzyme active site is in the sulfonylimido-4H- δ2-1,3,4-thiadiazoline tautomeric form. The inhibitor is directly bound to the Zn(II) ion of the enzyme through the deprotonated primary sulfonamide moiety, participating to the classical hydrogen bond network involving residues of the zinc-binding function and Thr 199 and Glu 106. The 1,3,4-thiadiazoline fragment of the inhibitor makes two hydrogen bonds with the active site residue Thr 200, the secondary sulfonamide moiety makes two hydrogen bonds involving a water molecule and the residue Gln 92, whereas the phenyl ring of the inhibitor participates to an edge-to-face interaction with the phenyl ring of Phe 131, the two cycles being almost perfectly perpendicular to each other. The tertiary amine fragment of the carboxamido tail and the carboxamido moiety itself make hydrogen bonds with water molecules present at the rim of the active site entrance and van der Waals contacts with His 4, Trp 5, and Phe 20. All these multiple interactions never evidenced previously in CA-sulfonamide complexes, explain the very high affinity of this inhibitor for the hCA II active site and may allow further optimization of this class of inhibitors. © 2004 Elsevier Ltd. All rights reserved.