Identification of 7-hydroxyindole as an alternative substrate of MauG by in silico and in vitro analysis

MauG catalyzes the six-electron oxidation of pre-tryptophan tryptophylquinone (preTTQ) cofactor in methylamine dehydrogenase (MADH) to form mature tryptophan tryptophylquinone (TTQ) via long-range electron transfer. To identify alternative substrates for MauG, docking models for 10 tryptophan-like compounds were constructed using Autodock Vina. These demonstrated spontaneous binding to the preTTQ binding site of MauG, with hydroxyindoles most frequently sharing the natural substrate binding site of MauG. To confirm the result of in silico analysis, 7-hydroxyindole was reacted with bis -Fe IV of MauG. The spectroscopic change, representing the reactivity of MauG, revealed the highly increased reaction rate ( k 3 ) toward 7-hydroxyindole, suggesting that bis -Fe IV MauG extracted an electron from the 7-hydroxyindole and then oxidized to di-ferric MauG.