Spin‐Regulated Electron Transfer and Exchange‐Enhanced Reactivity in Fe₄S₄‐Mediated Redox Reaction of the Dph2 Enzyme During the Biosynthesis of Diphthamide

Abstract

The [4Fe-4S]-dependent radical S-adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry apanoply of challenging transformations. Despite the extensive studies of structure-function relationships of radical SAM (RS) enzymes,t he electronic statedependent reactivity of the [4Fe-4S] cluster in these enzymes remains elusive.U sing combined MD simulations and QM/ MM calculations,wedeciphered the electronic state-dependent reactivity of the [4Fe-4S] cluster in Dph2, ak ey enzyme involved in the biosynthesis of diphthamide.O ur calculations show that the reductive cleavage of the S À C (g) bond is highly dependent on the electronic structure of [4Fe-4S].Interestingly, the six electronic states can be classified into alow-energy and ahigh-energy groups,whichare correlated with the net spin of Fe4atom ligated to SAM. Due to the driving force of Fe4ÀC (g) bonding,the net spin on the Fe4moiety dictate the shift of the opposite spin electron from the Fe1-Fe2-Fe3 blockt oS AM. Such spin-regulated electron transfer results in the exchangeenhanced reactivity in the lower-energy group compared with those in the higher-energy group.T his reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe-4S] cluster in RS enzymes.