Spectroscopic studies of pepsin and its complex with Streptomyces pepsin inhibitor

Binding of Streptomyces pepsin (EC 3.4.23.1) inhibitor to the active site of pepsin causes a characteristic ultraviolet difference spectrum having a trough around 298 nm which suggests that tryptophan residue(s) are involved in a decreased refractive index or different charge density. The fluoreschat of the pepsin-inhibitor complex. Relatively large circular dichroism (CD) spectrum change at 280--310 nm was observed upon binding of the inhibitor. Solvent perturbation difference spectra of pepsin alone and the pepsin-inhibitor. Solvent perturbation difference spectra of pepsin alone and the pepsin-inhibitor complex obtained with 20% ethylene glycol as perturbant showed that the exposed 2.5 tryptophan residues were not buried upon binding of the inhibitor, whereas 1.5 tyrosine residues were buried. It is speculated that the microenvironmental change around tryptophan residue(s) which are not located at the inhibitor binding site is induced upon binding of the inhibitor.