Extended Data Fig 6.

A proposed mechanism for cofactor maturation in Fdc^Ubix through oxidation. We propose the prFMN^reduced cofactor produced by UbiX is bound by apo-Fdc and oxidized in a stepwise manner. While the initial radical species resembles that observed during non-physiological oxidation of the prFMN^reduced:UbiX product complex¹⁰, we proposed that in the Fdc enzyme proton abstraction from the prFMN ^radical C1’ leads to a distinct radical species. The latter can either be oxidized further to the corresponding prFMN^iminium or, via a radical based isomerisation process, form the central seven-membered ring ultimately leading to prFMN^ketimine.