Comparison of six structurally equivalent residues with side chains between FIV PR and HIV-1 PR. The roles of these residues in the determination of substrate and inhibitor specificity were analyzed extensively in this study. (A) I37, N55, M56, V59, I98, and Q99 of FIV PR. (B) The structurally equivalent V32, M46, I47, I50, P81, and V82 of HIV-1 PR, respectively.