Resonance Raman spectroscopic studies of the interactions between trypsin and a competitive inhibitor

Raman spectroscopy was used to study the interactions between bovine trypsin and a competitive inhibitor. For this purpose, a chromophoric substrate analogue, 4-amidino-4'-dimethylamine azobenzene, was synthesized. This compound competitively inhibits the enzyme with a 1:1 stoichiometry and an inhibition constant Ki of 2.3 muM at pH 6.08 and 15 degrees. Resonance Raman spectra in aqueous solution of free or enzyme-bound inhibitor were analyzed. The main spectral changes observed upon enzyme-inhibitor complex formation were changes in the relative intensities of four bands (1171, 1206, 1315, 1608 cm-1) while no large frequency shifts occurred. The binding of the inhibitor molecule to the enzyme did not induce a twisting of the phenyl groups around the N=N bond. Some modifications of the band widths are interpreted in terms of a restriction of rotational motions in the inhibitor molecule. The possible involvement of specific interactions between trypsin and the benzamidinium ion part of the inhibitor molecule is discussed.