HUMAN-IMMUNODEFICIENCY-VIRUS PROTEASE LIGAND SPECIFICITY CONFERRED BYRESIDUES OUTSIDE OF THE ACTIVE-SITE CAVITY

To gain greater understanding of the structural basis of human immunod eficiency virus (HIV) protease ligand specificity, we have crystallize d and determined the structures of the HIV-1 protease (Val32Ile, Ile47 Val, Val82Ile) triple mutant and simian immunodeficiency virus (SIV) p rotease in complex with SB203386, a tripeptide analogue inhibitor cont aining a C-terminal imidazole substituent as an amide bond isostere. S B203386 is a potent inhibitor of HIV-1 protease (K-i = 18 nM) but show s decreased inhibition of the HIV-1 protease (Val32Ile, Ile47Val, Val8 2Ile) triple mutant (K-i = 112 nM) and SIV protease (K-i = 960 nM). Al though SB203386 binds in the active site cavity of the triple mutant i n a similar fashion to its binding to the wild-type HIV-1 protease [Ab del-Meguid et al. (1994) Biochemistry 33, 11671], it binds to SIV prot ease in an unexpected mode showing two inhibitor molecules each bindin g to half of the active site. Comparison of these two structures and t hat of the wild-type HIV-1 protease bound to SB203386 reveals that HIV protease ligand specificity is imparted by residues outside of the ca talytic pocket, which causes subtle changes in its shape. Furthermore, this work illustrates the importance of structural studies in order t o understand the structure-activity relationship (SAR) between related enzymes.