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

The [4Fe-4S]-dependent radical S-adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry a panoply of challenging transformations. Despite the extensive studies of structure-function relationships of radical SAM (RS) enzymes, the electronic state-dependent reactivity of the [4Fe-4S] cluster in these enzymes remains elusive. Using combined MD simulations and QM/MM calculations, we deciphered the electronic state-dependent reactivity of the [4Fe-4S] cluster in Dph2, a key enzyme involved in the biosynthesis of diphthamide. Our calculations show that the reductive cleavage of the S-C-(gamma) bond is highly dependent on the electronic structure of [4Fe-4S]. Interestingly, the six electronic states can be classified into a low-energy and a high-energy groups, which are correlated with the net spin of Fe4 atom ligated to SAM. Due to the driving force of Fe4-C-(gamma) bonding, the net spin on the Fe4 moiety dictate the shift of the opposite spin electron from the Fe1-Fe2-Fe3 block to SAM. Such spin-regulated electron transfer results in the exchange-enhanced reactivity in the lower-energy group compared with those in the higher-energy group. This reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe-4S] cluster in RS enzymes.

Automated summary

By using MD simulations and QM/MM calculations, the electronic state-dependent reactivity of the [4Fe-4S] cluster in the enzyme Dph2 involved in diphthamide biosynthesis was unveiled. The reactivity of the S-C-(gamma) bond cleavage is highly influenced by the net spin of Fe4 atom ligated to SAM, resulting in exchange-enhanced reaction in the lower-energy group compared to the higher-energy group. This spin-regulated electron transfer provides mechanistic insights into the reactivities of [4Fe-4S] cluster in RS enzymes.