MauG-dependent tryptophan tryptophylquinone (TTQ) from a precursor protein of methylamine dehydrogenase (preMADH) is a six-electron oxidation process and a typical long-range electron transfer (ET) reaction in proteins.What is electron transfer mechanism in this biosynthesis processes and which pathway do these six electrons go? These questions are of fundamental importance to unravel the key biological processes and also are the main difficulties of life science encountered today.