X‐ray crystal structure of the serine proteinase inhibitor eglin c at 1.95 Å resolution

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Abstract
The crystal structure eglin c. naturally occurring in the leech Hirudo medicinalis, is known from its complexes with various serine proteinases, but the crystallization of free eglin c has not yet been reported. A method is described for growing well-diffracing crystals of free eglic c from highly concentrated protein solutions (≈2OO mg/ml). The space group of the orthorhombic crystals was determined to be P212121 with unit cell parameters a = 32.6. b = 42.0. c = 44.1 Å. The structure or free eglin c was resolved at 1.95 Å resolution by Patterson search methods. The final model contains all 70 amino acids of eglin c and 125 water molecules. In comparison to the eglin structure known from its complexes with proteinases, only small differences have been observed in free eglin c. However, the reactive site-binding loop and a few residues on the surface of eglin have been found in different conformations due to crystal contacts. In contrast to the complex structures, the first seven amino acids of the highly flexible amino terminus can be located. Crystallographic refinement comprised molecular dynamics refinement, classical retrained least-squares refinement and individual isotropic atomic temperature refinement. The final R-factor is 15.8%.