Crystallization of an enzyme-inhibitor complex: proteinase K with its protein inhibitor, PK13

Crystals of a complex of proteinase K (molecular mass, 28,790 Da) with its naturally occurring protein inhibitor PK13 (19,641 Da), have been prepared by a microdialysis technique and modified by hanging drop vapor diffusion against 25% ammonium sulfate in 50 mM Tris-HCl, pH 7.8. The crystals are long prisms with diamond-shaped cross sections of 0.2 x 0.4 x 1.5 mm3 and they diffract X-rays to a resolution of 2.5 A. They belong to the orthorhombic space group P2(1)2(1)2(1) with cell dimensions a = 64.1 A, b = 66.8 A, and c = 133.8 A. Assuming one whole complex in the asymmetric unit, one obtains VM = 2.95 A3/Da and the solvent content, Vsolv = 58.3%.