The cofactor of several flavoenzymes is autocatalytically bound to the polypeptide via a histidyl(N3)-(8α)-FAD linkage which makes the generation of apoenzyme difficult. We introduced an alternative covalent protein-FAD bond at the active site of 6-hydroxy-n-nicotine oxidase (6HDNO) by replacing the FAD-binding histidine with cysteine. The resulting mutant enzyme was expressed with noncovalently attached cofactor. Incubation with 8-(methylsulfonyl)FAD, and less efficiently with 8-chloro-FAD, resulted in the spontaneous replacement of the noncovalently bound FAD by the flavin derivative and the formation of an 8-(N-acetylcysteinyl)FAD linkage. The flavinylated 6HDNO.cys exhibited close to wild-type activity levels. This strategy may be generally applicable to the attachment of artificially designed flavin derivatives to the active site of covalently flavinylated enzymes.
