Methylamine dehydrogenase (MADH) requires the cofactor tryptophan tryptophylquinone (TTQ) for activity. TTQ is a posttranslational modification that results from an 8-electron oxidation of two specific tryptophans in the MADH β-subunit. The final 6-electron oxidation is catalyzed by an unusual c-type di-heme enzyme, MauG. The di-ferric enzyme can react with H 2O2, but atypically for c-type hemes the di-ferrous enzyme can react with O2 as well. In both cases, an unprecedented bis-Fe(iv) redox state is formed, composed of a ferryl heme (Fe(iv)O) with the second heme as Fe(iv) stabilized by His-Tyr axial ligation. Bis-Fe(iv) MauG acts as a potent 2-electron oxidant. Catalysis is long-range and requires a hole hopping electron transfer mechanism. This review highlights the current knowledge and focus of research into this fascinating system.
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