Maug Catalysis: A Tale of Ferryl Iron, Radicals and Long Distance Hopping

Methylamine dehydrogenase (MADH) enables some methylotrophic/autotrophic bacteria to grow on methylamine as a sole source of carbon and energy. MADH catalysis depends on the cofactor tryptophan tryptophylquinone (TTQ) that is a posttranslational modification of two Trp residues in the MADH β-subunit. The maturation of MADH depends on four gene products located in the methylamine utilization (mau) gene cluster. One of these, mauG, encodes a c-type di-heme enzyme that completes synthesis of the TTQ cofactor. The potent oxidant is an unusual bis-Fe(IV) MauG species composed of a ferryl heme (Fe(IV)=O) with an oxidizing equivalent stored as Fe(IV) at the second heme, which has an unusual His, Tyr axial ligation. The bis-Fe(IV) oxidant is formally Fe(V) and equivalent to Compound I. Completion of TTQ to generate active MADH involves long-range electron transfer and a radical hopping mechanism to effect catalysis over a 40 Å distance. The MauG catalyzed reaction occurs in three discrete 2-electron events in a H₂O₂ or O₂-dependent process. A crystal structure of MauG in complex with its protein substrate, a precursor form of MADH known as preMADH, has been solved. The crystals are catalytically active. The order of the 2-electron chemistry catalyzed by MauG was determined through a series of structures from crystals harvested after different amounts of time following crystallization. H₂O₂ to initiate the reaction was generated by the slow breakdown of polyethylene glycol used in crystallization. These in crystallo data are corroborated by mass spectrometry in solution experiments.