Solvent accessibility and microenvironment in a bacterial protein proteinase inhibitor SSI (Streptomyces subtilisin inhibitor).

Solvent accessibility (Lee, B. & Richards, F.M. (1971) J. Mol. Biol. 55, 379-400) was calculated for each atom of a bacterial protein proteinase inhibitor SSI (Streptomyces subtilisin inhibitor) based on crystallographic coordinates. Mainly based on this information, various chemical and spectroscopic (UV, Raman, NMR) observations made on the microenvironments of cystines, methionines, tryptophan, histidines, and tyrosines of SSI in solution were evaluated. Crystallographic data and the latter two sets of data were mainly at least qualitatively consistent with each other. These data include (1) the conformation of the two disulfide bridges, (2) the flexibility of the three methionyl side chains, (3) the extent of exposure of the indole ring of a tryptophan, (4) the environment of the two histidines, (5) the environment of the tyrosines, and (6) the hydrogen-deuterium exchangeability of peptide NH's. However, the extents of exposure of tyrosines deduced by solvent perturbation UV difference spectroscopy were significantly larger than those based on solvent accessibility calculations. Possible reasons for this discrepancy are discussed.