THE FIRST STRUCTURE AT 1.8-ANGSTROM RESOLUTION OF AN ACTIVE AUTOLYSATE FORM OF PORCINE ALPHA-TRYPSIN

The first crystal structure of an active autolysate form of porcine al pha-trypsin (APT), a two-chain molecule obtained from the limited auto lysis of porcine beta-trypsin at position Lys145-Ser146, has been dete rmined. APT crystallizes in space group P2(1)2(1)2(1) With one protein molecule in the asymmetric unit. The structure was solved by molecula r replacement followed by refinement using X-PLOR to an R factor of 0. 200 and an R-free of 0.285 for 8.0-1.8 Angstrom data with r.m.s. devia tions from ideal values of 0.01 Angstrom and 1.7 degrees for bond leng ths and bond angles, respectively. Comparison with inactive autolysate porcine epsilon-trypsin (EPT) and porcine beta-trypsin in complex wit h bittergourd trypsin inhibitor (MCT) revealed a small but systematic directional chain shift around the active-site residues from APT to EP T to MCT.