A Study of the Interactions between Flavoprotein and Quasi-substratesCircular Dichroism Spectra of D-Amino Acid Oxidase Complexes

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Abstract
Visible circular dichroism spectra (CD spectra) of D-amino acid oxidase complexes with ring-substituted benzoate derivatives (quasi-substrates) were measured and compared with CD spectra of the enzyme complex with benzoate (“benzoate complex”) to study the effects of substituted groups on the interactions between D-amino acid oxidase and quasi-substrates. CD spectra of the enzyme complexes with o-, m-, and p-fluorobenzoate and o-, m-, and p-chlorobenzoate (“o-, m-, and p-fluorobenzoate complexes” and “o-, m-, and p-chlorobenzoate complexes”) showed patterns similar to that of the benzoate complex. However, the CD spectra of the chlorobenzoate complexes, particularly o- and m-chlorobenzoate complexes, differed more from that of the benzoate complex than did those of the fluorobenzoate complexes. This might be due to a stronger steric effect at the active site of the enzyme caused by the larger chlorine atom. The CD spectra of the m- and p-methylbenzoate complexes and the o-, m-, and p-hydroxybenzoate complexes were different from that of the benzoate complex. They showed negative ellipticity around 500 nm and had crossover points which did not exist for the benzoate complex, at 482 nm for p- and 475 nm for m-methylbenzoate complexes and 470 nm for o-, 455 nm for m- and 492 nm for p-hydroxybenzoate complexes. Furthermore, these CD spectra were similar to those of the “dehydrogenase type” of flavoenzymes, although D-amino acid oxidase belongs to the “oxidase type.” The patterns of CD spectra of m- and p-nitrobenzoate complexes differed from those of all the other complexes mentioned above. We concluded that the gross modifications of CD spectra by substituted groups were determined by the group species with further minor modifications due to its o-, m- or, p-posision. These findings suggest that each substituted group interacts with the enzyme at the active site in a specific mode. Furthermore, the drastic differences were largely in the spectral regions corresponding to the lowest transition of absorption spectra; this was consistent with the hypothesis that this band is affected by the “through-space” interaction.

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