Crystallization of two cubic forms of soybean trypsin inhibitor E-I, a member of the Bowman-Birk inhibitor family.

The soybean trypsin inhibitor E-I, known to be relatively rich in methionine and cysteine, has been crystallized at room temperature in the presence of polyethylene glycol 4000, sodium chloride, and ammonium sulfate. A pronounced polymorphism in the crystals has been observed and two distinctly different cubic forms have been identified. Of the two, one form crystallizes in a unit cell of symmetry F432 with parameters a = b = c = 128.0 A, and diffracts at least to 2.6-A resolution. Each of the 96 asymmetric units contains one protein molecule, and has a solvent content of 60% by volume. The other form is of space group P4132 or P4332 with the unit-cell edge 87.1 A, and diffracts barely to 3-A spacings. The unit cell is composed of 24 asymmetric units, each of which probably also contains one molecule and has a solvent content of 69% by volume. The former cubic form appears to be more suitable for x-ray study than the latter in terms of stability and the extent of diffraction.