Design, synthesis and analysis of new synthetic substrates for the aspartic proteinases.

certainty although its general position is reasonably well defined. In the energy refinement, this part of the structure contracts and attempts to fold back onto the N-terminal domain, leading not only to a large movement as found in other external loops but also a large conformational change in the main chain. Re-examination of this loop has shown that an alternative local conformation can be modelled which is consistent to that found in other P-hairpin loops including the penicillopepsin ‘flap’. The use of this lessstrained conformation will not prevent the contraction of the loop but hopefully will prevent the large conformational change. However, in general, the energy minimization does not lead itself to new insights into the structure and usually finds a local minimum of reasonable energy. As observed by Novotnf et al. (1984), it is not a discrimination of the overall correctness ot the model but, given a good starting conformation, it will optimize the local interactions. In this study a comparison of the refined structures of endothiapepsin and its complex with H-142 will be of particular interest as the energy-minimization procedure should be able to identify local changes in conformation in the native structure due to the presence of the H-142 inhibitor. Only small changes have been indicated between the conformations of endothiapepsin and its complex with H-142 by using energy minimization and this is consistent with changes defined by the high-resolution X-ray studies. The detail of these changes will be reported elsewhere. At present, it appears that the conformational changes even in the active site aspartic acid residues and the ‘flap’ region are smaller than might have been expected and much less extensive than those reported in the X-ray analysis of inhibitor complexes involving penicillopepsin (James et al., 1982); this result appears to have been mainly a consequence of the intermolecular hydrogen bonding of the ‘flap’ in the crystals of penicillopepsin.