The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis-Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis-Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 Å, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis-Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis-Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues.
Keywords: charge transfer; electron hopping; high-valent iron; metalloprotein; tryptophan radical.