CRYSTAL-STRUCTURE OF HUMAN PEPSIN AND ITS COMPLEX WITH PEPSTATIN

The three-dimensional crystal structure of human pepsin and that of it s complex with pepstatin have been solved by X-ray crystallographic me thods. The native pepsin structure has been refined with data collecte d to 2.2 Angstrom resolution to an R-factor of 19.7%. The pepsin:pepst atin structure has been refined with data to 2.0 Angstrom resolution t o an R-factor of 18.5%. The hydrogen bonding interactions and the conf ormation adopted by pepstatin are very similar to those found in compl exes of pepstatin with other aspartic proteinases. The enzyme undergoe s a conformational change upon inhibitor binding to enclose the inhibi tor more tightly. The analysis of the binding sites indicates that the y form an extended tube without distinct binding pockets. By comparing the residues on the binding surface with those of the other human asp artic proteinases, it has been possible to rationalize some of the exp erimental data concerning the different specificities. At the S1 site, valine at position 120 in renin instead of isoleucine, as in the othe r enzymes, allows for binding of larger hydrophobic residues. The poss ibility of multiple conformations for the P2 residue makes the analysi s of the S2 site difficult. However, it is possible to see that the sp ecific interactions that renin makes with histidine at P2 would not be possible in the case of the other enzymes. At the S3 site, the smalle r volume that is accessible in pepsin compared to the other enzymes is consistent with its preference for smaller residues at the P3 positio n.