1044 Journal of Medicinal Chemistry.1994.Vol 37.No 8 Perspective ns (dashed lines) n A-77003 and HIV PR. s.It isthis sed on lobuand com D-50 residu with one zine at sheets being 15 and des 16Atothezincatom.Theactive-sitecav ty is dra His 96.The iy 19 ith both tanding thi adife the trophobitendhe oord h hene ay between the hyd and hydro for the de s in fat d the alky nthe less f with xial ori ntati Ab initio calcula ns at the 6-31 dbyabout/mol.The potential fo n ided drogen yge of the SOz and the eh ers which differed 100- old in of the SOz on athe of Phe red in a competition Both the (Tabl a。and the 2dmdeHoaee,wosigaiheantdiierenceswerefound1044 Journal of Medicinal Chemistry, 1994, Vol. 37, No. S Perspective t Figure 9. Hydrogen-bonding interactions (dashed lines) between A-77003 and HIV PR. The aspartic acids are on top, the flaps on the bottom, and the inhibitor and buried water molecule (both in red) are sandwiched between them in this view. The @)-OH of A-77003 is shown interacting with both carboxylates; the (5’)-OH points behind the plane of the figure, away from the aspartates. Numbers refer to hydrogen-bond distances, in angstroms, between heavy atoms (the third decimal place should be ignored). Reprinted with permission.22 lographic ana lyse^.^^^^^ It is this isozyme that served as the model for understanding differences in inhibitory activity based on the structure of enzyme-ligand com￾plexes. Human CA I1 (HCA 11) is a globular enzyme containing 260 amino acid residues with one zinc atom per molecule. The active-site cavity is composed of twisted @ sheets being 15 across at the surface and descending 16 A to the zinc atom. The active-site cavity is cone shaped and becomes very narrow at the catalytic hydrophobic pocket where the zinc atom is coordinated in a tetrahedral fashion to three histidine residues: His 94, His 119, and His 96. The active-site cavity is amphiphilic; one wall is dominated by hydrophobic and the other by hydrophilic residues. Both surfaces are important in defining the interaction between ligand and enzyme.65-70 Structural Basis for Differences in Inhibitory Potency. The general strategy for the design of a topically effective HCA I1 inhibitor is illustrated with 20, a structure which allowed for manipulation of both solubility and lipophilicity. The prototype 21, MK-927, is water soluble, rapidly penetrates ocular tissue, and lowers intraocular pressure (IOP) in animal models. Resolution provided two enantiomers which differed 100-fold in potency, as determined in a functional enzymatic assay (IC~O), and affinity, as measured in a competition assay versus dansyl amide (Ki) (Table 4). Both the more active S- and the less active R-enanti￾omers were cocrystallized with HCA I1 (Figures 11 and 20 HN ,< @SO*NH2 21 12). The X-ray crystallographic results allowed the structure of the bound ligands to be compared with the aim of understanding this 100-fold difference in affinity. With both enantiomers, the sulfonamide group was coordinated to the zinc at the catalytic site through the presumably deprotonated sulfonamide nitrogen while the thiophene ring lay between the hydrophobic and hydro￾philic walls of the active-site cavity. Both enantiomers placed the alkyl amino group in the less favored pseudo￾axial orientation. Ab initio calculations at the 6-31 G* level suggest that the pseudoequatorial conformer would be preferred by about 1 kcal/mol. The potential for hydrogen bonding between both enantiomers and the enzyme involved one oxygen of the SO2 and the side chain of Glu 92, a polar interaction also was possible between the second oxygen of the SO2 and the aromatic ring of Phe 131. The overall geometry of the two inhibitors was similar and analogous to the thiadiazole sulfonamide of aceta￾zolamide. However, two significant differences were found