Crystallization and preliminary X-ray crystallographic analysis of the protease inhibitor ecotin in complex with chymotrypsin

Ecotin, a homodimeric protein composed of 142-residue subunits, is a novel protease inhibitor present in the periplasm of Escherichia coli. It shows a broad inhibitory specificity towards a group of serine proteases and binds two molecules of protease to form a tetrameric complex in a distinct chelation mechanism. The ecotin-chymotrypsin complex has been crystallized in the triclinic space group P1 with unit-cell parameters a = 57.29, b = 57.39, c = 79.75 Å, α = 91.49, β = 88.63 and γ = 112.45°. The asymmetric unit contains the whole tetrameric complex, consisting of two molecules of chymotrypsin bound to the ecotin dimer, with a corresponding crystal volume per protein mass (V(M)) of 2.58 ų Da^-1 and a solvent fraction of 48.9%. The crystals diffract beyond 2.0 Å with Cu Kα X-rays and are very stable in the X-ray beam. Native X-ray data have been collected from a crystal to approximately 2.0 Å Bragg spacing.