Structural Studies on the PQQ-Like Cofactor of Nitroalkane Oxidase from Fusarium Oxysporum

Nitroalkane oxidase from Fusarium oxysporum exhibits absorption maxima at 274, 340 and 450 nm, and shoulders at 380 and 470 nm. The absorption and fluorescence spectra of the enzyme suggest the presence of both FAD and a PQQ-like compound as cofactors. The bound FAD is easily resolved from the enzyme by dialysis against 1 M KBr. The KBr-treated enzyme has no absorption peak in the 380–500 nm region, but still exhibits the absorption peak at 340 nm. The 340-nm chromophore can be isolated from the enzyme protein only by treatment with protein denaturants. Although the isolated compound shows spectrophotometric properties similar to PQQ, it does not activate apoglucose dehydrogenase. The compound also differs from PQQ in chromatographic properties. However, mass spectroscopic analysis indicates that it is composed of mass fragments very close to PQQ. These results suggest that nitroalkane oxidase contains a new cofactor, which is not identical with PQQ but structurally similar to it.