Super-exchange and Exchange-Enhanced Reactivity in Fe₄S₄-Mediated Activation of SAM by Radical SAM Enzymes^†

4Fe-4S-dependent radical S-adenosylmethionine (SAM) proteins are a superfamily of oxidoreductases that can catalyze a series of challenging transformations using the common 5-dAdo radical intermediate. Although the structures and functions of radical SAM enzymes have been extensively studied, the electronic state-dependent reactions of the 4Fe-4S clusters in these enzymes are still elusive. Herein we performed QM/MM calculations to elucidate the electronic state-dependent reactivity of the 4Fe-4S cluster in pyruvate-formate lyase activating enzyme. Our calculations show that the electronic state-dependent SAM activation by the 4Fe-4S clusters in radical SAM enzyme is determined by both the super-exchange and exchange-enhanced reactivities. The super-exchange coupling in the 4Fe-4S cluster favors the antiferromagnetic coupling between two neighbouring pairs, which results in the \alpha-electron rather than the \beta-electron donation from the 4Fe-4S^1+ cluster toward the SAM activation. Meanwhile, in the most favorable electronic state for the reductive cleavage of S-C5', Fe4 would donate its \alpha-electron to gain the maximum exchange interactions in the Fe4-block. Such super-exchange and exchange-enhanced reactivity could be the general principles for reactivities of 4Fe-4S cluster in RS enzymes.