Circular dichroism studies on flavoproteins containing covalently bound coenzymes

Absorption and circular dichroism spectra of cholesterol oxidase from Schizophyllum commune and choline oxidase from Alcaligenes sp. were measured and compared. The prosthetic group of cholesterol oxidase is 8 alpha-[N(1)-histidyl]-FAD (1, 2), while that of choline oxidase is 8 alpha-[N(3)-histidyl]-FAD (3). In the CD spectra of the two enzymes in either the oxidized or reduced state, the corresponding bands in the visible region are of approximately the same intensity and shape but of opposite sign. A notable feature in the CD spectra of the two enzymes after light irradiation is the appearance of a CD band in the longer wavelength region (550-650 nm) and the opposite signs of the CD band in this region in the two enzymes. The similarity of the shape and intensity of the CD spectra of the two enzymes suggests that the environments surrounding the flavin moieties are very similar, and the sign reversal of the CD bands suggests that the mutual orientations between the transition moment of flavin and that of its environment differ in the two enzymes.