Involvement of a Flavin Iminoquinone Methide in the Formation of 6-Hydroxyflavin Mononucleotide in Trimethylamine Dehydrogenase: A Rationale for the Existence of 8α-Methyl and C6-Linked Covalent Flavoproteins

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Abstract
In trimethylamine dehydrogenase, substrate is bound in the active site via cation−π bonding to three aromatic residues (Tyr-60, Trp-264, and Trp-355). Mutation of one of these residues (Trp-355 → Leu, mutant W355L) influences the chemistry of the flavin mononucleotide in the active site, enabling derivatization to 6-hydroxy-FMN. The W355L mutant is purified as a mixture of deflavo, natural 6-S-cysteinyl-FMN, and inactive 6-hydroxy-FMN forms, and the enzyme is severely compromised in its ability to oxidatively demethylate trimethylamine. Analysis of samples of the native and recombinant wild-type trimethylamine dehydrogenases also revealed the presence of 6-hydroxy-FMN, but at much reduced levels compared with that of the W355L enzyme. Unlike that for a C30A mutant of trimethylamine dehydrogenase, addition of substrate to the W355L trimethylamine dehydrogenase is not required for the production of 6-hydroxy-FMN. A mechanism is proposed for the 6-hydroxylation of FMN in trimethylamine dehydrogenase that involves an electrophilic flavin iminoquinone methide. The proposed mechanism involving the flavin iminoquinone methide could apply to the flavinylation of trimethylamine dehydrogenase at the C6 position but also to the flavinylation of enzymes via the 8α position, thus providing a rationale for the evolution of covalent flavoproteins in general. Covalent linkage at C6 or the 8α-methyl prevents 6-hydroxylation by direct modification at the C6 atom or by preventing formation of the flavin iminoquinone methide, respectively.

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