CRYSTAL AND MOLECULAR-STRUCTURES OF HUMAN PROGASTRICSIN AT 1.62 ANGSTROM RESOLUTION

The crystal and molecular structures of human progastricsin (hPGC) hav e been determined using multiple isomorphous replacement methods and a nomalous scattering in conjunction with a phased translation function. The structure has been refined to a conventional R-factor (=Sigma par allel to F-o\ - \F-c parallel to/Sigma\F-o\) of 0.179 with data to 1.6 2 Angstrom resolution. The first 37 amino acid residues of the prosegm ent are similar in conformation to the equivalent residues of porcine pepsinogen (pPGN). As in pPGN, the N-zeta atom of Lys37p sits between the active-site carboxylate groups of Asp32 and Asp217, thereby preven ting catalysis. The side-chains of Tyr38p and Tyr9 sit in the S1' and S1 substrate-binding pockets of hPGC, respectively, in an analogous ma nner to what is observed in porcine pepsinogen. There are large confor mational differences centered around the region containing residues Ar g39p to Pro6, relative to the equivalent region in the structure of pP GN. Two surface loops in the vicinity of this segment are also displac ed relative to those in pPGN and in mature aspartic proteinases (Phe71 to Thr81 (the ''flap''), and Tyr125 to Thr131). In hPGC, Tyr75 O-eta does not make its usual hydrogen bond to Trp39 N-epsilon l. Rather, th e ''flap'' containing Tyr75 is excluded from the active site by the po lypeptide segment Arg39p to Pro6. However, the conformation of the inh ibitory segment, Lys37p to Tyr38p, is virtually identical with that ob served in pPGN. Hence the structures of these two proteins indicate th at aspartic proteinase zymogens keep themselves inactive at neutral pH by a very similar mechanism in human progastricsin and porcine pepsin ogen. This similarity likely carries over to all members of both the p epsinogen A and C families of aspartic proteinase zymogens.