Mechanism of Sulfoxidation and C–S Bond Formation Involved in the Biosynthesis of Ergothioneine Catalyzed by Ergothioneine Synthase (EgtB)

Ergothioneine synthase (EgtB) is a unique non-heme mononuclear iron enzyme that catalyzes the sulfoxidation and C–S bond formation between γ-glutamyl cysteine (γGC) and N-α-trimethyl histidine (TMH) as a pivotal step in the ergothioneine biosynthesis. A controversy has arisen regarding the sequence of sulfoxidation and C–S bond formation in the catalytic cycle. To clarify this issue, the QM/MM approach has been employed to investigate the detailed mechanism of EgtB. Two binding modes of O2 to Fe(II) (“end-on” and “side-on”) have been identified. Within the present computational model, the end-on binding mode of O2 is preferred. The open-shell singlet is calculated to be the ground state, whereas the quintet is the most active state. Moreover, the sulfoxidation is prior to the formation of the C–S bond, and the reaction mainly occurs on the quintet state surface. Due to the electron transfer from the γGC to the ferric superoxide, the sulfur atom of γGC has partial radical characteristics, which facilitates...