Cryocrystallography of a Kunitz-type serine protease inhibitor: the 90 K structure of winged bean chymotrypsin inhibitor (WCI) at 2.13 Å resolution

The crystal structure of a Kunitz-type double-headed α-­chymotrypsin inhibitor from winged bean seeds has been refined at 2.13 Å resolution using data collected from cryo-cooled (90 K) crystals which belong to the hexagonal space group P6₁22 with unit-cell parameters a = b = 60.84, c = 207.91 Å. The volume of the unit cell is reduced by 5.3% on cooling. The refinement converged to an R value of 20.0% (R_free = 25.8%) for 11100 unique reflections and the model shows good stereochemistry, with r.m.s. deviations from ideal values for bond lengths and bond angles of 0.011 Å and 1.4°, respectively. The structural architecture of the protein consists of 12 antiparallel β-strands joined in the form of a characteristic β-trefoil fold, with the two reactive-site regions, Asn38–Leu43 and Gln63–Phe68, situated on two external loops. Although the overall protein fold is the same as that of the room-temperature model, some conformational changes are observed in the loop regions and in the side chains of a few surface residues. A total of 176 ordered water molecules and five sulfate ions are included in the model.