Abstract
The crystal structure of the molecular complex formed by bovine α‐chymotrypsin and the recombinant serine proteinase inhibitor eglin c from Hirudo medicinalis has been solved using monoclinic crystals of the complex, reported previously. Four circle diffractometer data at 3.0 Å resolution were employed to determine the structure by molecular replacement techniques. Bovine α‐chymotrypsin alone was used as the search model; it allowed us to correctly orient and translate the enzyme in the unit cell and to obtain sufficient electron density for positioning the eglin c molecule. After independent rigid body refinement of the two complex components, the molecular model yielded a crystallographic R factor of 0.39. Five iterative cycles of restrained crystallographic refinement and model building were conducted, gradually increasing resolution. The current R factor at 2.6 Å resolution (diffractometer data) is 0.18. The model includes 56 solvent molecules. Eglin c binds to bovine α‐chymotrypsin in a manner consistent with other known serine proteinase/inhibitor complex structures. The reactive site loop shows the expected conformation for productive binding and is in tight contact with bovine α‐chymotrypsin between subsites P~3~ and P′~2~; Leu 45I acts as the P~1~ residue, located in the primary specificity S~1~ site of the enzyme. Hydrogen bonds equivalent to those observed in complexes of trypsin(ogen) with the pancreatic basic‐ and secretory‐ inhibitors are found around the scissile peptide bond.