Flavinylation in wild-type trimethylamine dehydrogenase and differentially charged mutant enzymes: a study of the protein environment around the N1 of the flavin isoalloxazine

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Abstract
In wild-type trimethylamine dehydrogenase, residue Arg-222 is positioned close to the isoalloxazine N1/C2 positions of the 6S-cysteinyl FMN. The positively charged guanidino group of Arg-222 is thought to stabilize negative charge as it develops at the N1 position of the flavin during flavinylation of the enzyme. Three mutant trimethylamine dehydrogenases were constructed to alter the nature of the charge at residue 222. The amount of active flavinylated enzyme produced in Escherichia coli is reduced when Arg-222 is replaced by lysine (mutant R222K). Removal or reversal of the charge at residue 222 (mutants R222V and R222E, respectively) leads to the production of inactive enzymes that are totally devoid of flavin. A comparison of the CD spectra for the wild-type and mutant enzymes revealed no major structural change following mutagenesis. Like the wild-type protein, each mutant enzyme contained stoichiometric amounts of the 4Fe-4S cluster and ADP. Electrospray MS also indicated that the native and recombinant wild-type enzymes were isolated as a mixture of deflavo and holo enzyme, but that each of the mutant enzymes have masses expected for deflavo trimethylamine dehydrogenase. The MS data indicate that the lack of assembly of the mutant proteins with FMN is not due to detectable levels of post-translational modification of significant mass. The experiments reported here indicate that simple mutagenic changes in the FMN-binding site can reduce the proportion of flavinylated enzyme isolated from Escherichia coli and that positive charge is required at residue 222 if flavinylation is to proceed.