The aspartic proteinases are an important family of enzymes associated with several pathological conditions such as hypertension (renin), gastric ulcers (pepsin), neoplastic disease (cathepsins D and E), and AIDS (HIV proteinase). Studies of inhibitor binding are therefore of great importance for design of novel inhibitors for potential therapeutic applications. Numerous X-ray analyses have shown that transition-state isostere inhibitors of aspartic proteinases bind in similar extended conformations in the active-site cleft of the target enzyme. Upon comparison of 21 endothiapepsin inhibitor complexes, the hydrogen bond lengths were found to be shortest where the isostere (Pl-P;) interacts with the enzyme's catalytic aspartate pair. Hydrogen bonds with good geometry also occur at P;, and more so at P,, where a conserved water molecule is involved in the interactions. Weaker interactions also occur at P2, where the side-chain conformations of the inhibitors appear to be more variable than at the more tightly held positions. At Pz and, to a lesser extent, P,, the side-chain conformations depend intriguingly on interactions with spatially adjacent inhibitor side chains, namely P; and P, , respectively. The tight binding at PI-P;, P,, and Pi is also reflected in the larger number of van der Waals contacts and the large decreases in solvent-accessible area at these positions, as well as their low temperature factors. Our analysis substantiates earlier proposals for the locations of protons in the transition-state complex. Aspartate 32 is probably ionized in the complexes, its charge being stabilized by 1, or sometimes 2, hydrogen bonds from the transition-state analogues at P I . The detailed comparison also indicates that the P, and P2 residues of substrate in the ES complex may be strained by the extensive binding interactions at P3, Pi, and Pi in a manner that would facilitate hydrolysis of the scissile peptide bond.Keywords: aspartic proteinase; inhibitor complexes; transition-state analogues Aspartic proteinases provide excellent subjects for studying molecular recognition because they bind a range of peptide substrates and inhibitors with varying degrees of specificity. Catalysis stems from 2 invariant aspartate residues, at positions 32 and 215 in porcine pepsin, which are thought to activate a bound water molecule to hydrolyze the scissile bond of the substrate. X-ray analyses have shown that these enzymes have deep and extended active-site clefts in which there are binding subsites[S&] for 9 amino acid residues of the ligand (see Davies, 1990, for review). Inhibitors bind in an extended conformation making P-sheet hydrogen bonds with the enzyme (James et al., 1982;Foundling et al., 1987;Suguna et al., 1992). This class of enzymes includes not only the pepsin-like enzymes with approxi-