The First Structure at 1.8 Å Resolution of an Active Autolysate Form of Porcine α-Trysoin

The first crystal structure of an active autolysate form of porcine α-trypsin (APT), a two-chain molecule obtained from the limited autolysis of porcine β-trypsin at position Lys145–Ser146, has been determined. APT crystallizes in space group P2₁2₁2₁ with one protein molecule in the asymmetric unit. The structure was solved by molecular 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 Å data with r.m.s deviations from ideal values of 0.01 Å and 1.7° for bond lengths and bond angles, respectively. Comparison with inactive autolysate porcine [epsilon]-trypsin (EPT) and porcine β-trypsin in complex with bittergourd trypsin inhibitor (MCT) revealed a small but systematic directional chain shift around the active-site residues from APT to EPT to MCT.