Crystallization and preliminary X-ray diffraction studies of a Bowman-Birk inhibitor from Vigna unguiculata seeds

A Bowman-Birk type trypsin/chymotrypsin inhibitor isolated from Vigna unguiculata seeds has been crystallized. Crystals were grown using the vapour-diffusion method at pH 4.0 using citrate/phosphate as a buffer and 30% saturated ammonium sulfate as precipitant. The crystals belonged to the monoclinic space group P2₁, with unit-cell parameters a = 32.4, b = 61.8, c = 32.9 Å, β = 114.5°. The Matthews coefficient calculated assuming two molecules in the asymmetric unit was 1.95 ų Da^-1, which corresponds to a 37% solvent content. X-ray data were collected to 2.5 Å resolution from a flash-frozen crystal. The structure was solved using the molecular-replacement method using tracy soybean inhibitor structure (PDB entry 1pi2) as a model.