Double-headed protease inhibitors from black-eyed peas. II. Structural studies by optical absorption and circular dichroism.

Two new double-headed protease inhibitors from black-eyed peas have amino acid compositions typical of the low molecular weight protease inhibitors from legume seeds. Black-eyed pea chymotrypsin and trypsin inhibitor (BEPCI) contains no tryptophan, 1 tyrosine, and 14 half-cystines out of 83 amino acid residues per monomer. Black-eyed pea trypsin inhibitor (BEPTI) contains no tryptophan, 1 tyrosine, and 14 half-cystines out of 75 residues per monomer. The molar extinctions at 280 nm are 2770 for BEPCI and 3440 for BEPTI. The single tyrosyl residue is very inaccessible to solvent in native BEPCI and BEPTI at neutral pH and titrates anomalously with an apparent pK = 12. Ionization of tyrosine is complete in 13 hours above pH 12. No heterogeneity of the local environment of the tyrosyl residues in different subunits can be detected spectrophotometrically. The large number of cystine residues leads to an intense and complex near-ultraviolet CD spectrum with cystine contributions in the regions of 248 and 280 nm and tyrosine contributions at 233 and 280 nm. An intact disulfide structure is required for appearance of the tyrosyl CD bands. The inhibitors are unusually resistant to denaturation when compared with similar low molecular weight proteins of high disulfide content. All observations are consistent with a far more rigid structure for BEPCI and BEPTI than for a typical protein.