CRYSTAL-STRUCTURE AT 1.9-ANGSTROM RESOLUTION OF HUMAN-IMMUNODEFICIENCY-VIRUS (HIV)-II PROTEASE COMPLEXED WITH L-735,524, AN ORALLY BIOAVAILABLE INHIBITOR OF THE HIV PROTEASES

L-735,524 is a potent, orally bioavailable inhibitor of human immunode ficiency virus (HIV) protease currently in a Phase II clinical trial. We report here the three-dimensional structure of L-735,524 complexed to HIV-2 protease at 1.9-Angstrom resolution, as well as the structure of the native HIV-2 protease at 2.5-Angstrom resolution. The structur e of HIV-2 protease is found to be essentially identical to that of HI V-1 protease. In the crystal lattice of the HIV-2 protease complexed w ith L-735,524, the inhibitor is chelated to the active site of the hom odimeric enzyme in one orientation. This feature allows an unambiguous assignment of protein-ligand interactions from the electron density m ap. Both Fourier and difference Fourier maps reveal clearly the closur e of the flap domains of the protease upon L-735,524 binding. Specific interactions between the enzyme and the inhibitor include the hydroxy group of the hydroxyaminopentane amide moiety of L-735,524 ligating t o the carboxyl groups of the essential Asp-25 and Asp-25' enzymic resi dues and the amide oxygens of the inhibitor hydrogen bonding to the ba ckbone amide nitrogen of Ile-50 and Ile-50' via an intervening water m olecule. A second bridging water molecule is found between the amide n itrogen N2 of L-735,524 and the carboxyl oxygen of Asp 29'. Although o ther hydrogen bonds also add to binding, an equally significant contri bution to affinity arises from hydrophobic interactions between the pr otease and the inhibitor throughout the pseudo-symmetric S1/S1', S2/S2 ', and S3/S3' regions of the enzyme. Except for its pyridine ring, all lipophilic moieties (t-butyl, indanyl, benzyl, and piperidyl) of L-73 5,524 are rigidly defined in the active site.