Quantum Coulombic Interactions Mediate Free Radical Control in Radical SAM Viperin/RSAD2

There are thousands of radical S-adenosylmethionine (rSAM) enzymes capable of catalyzing over 80 distinct reactions, yet their use in biotechnological applications is limited, primarily due to a lack of understanding of how these enzymes control highly reactive radical intermediates. Here, we show that little-known quantum Coulombic interactions are, in part, responsible for free radical control in Viperin/RSAD2, one of the few radical SAM enzymes expressed in humans. Using molecular dynamics and high-level multi-state broken-symmetry quantum mechanical/molecular mechanics calculations, we elucidated the mechanistic details of the catalysis, identifying a key step characterized by the formation of an unprecedented metastable deprotonated ribose radical intermediate. This intermediate is thermodynamically stabilized by spin-charge exchange-correlation interactions—a quantum Coulombic effect. The magnitude of this stabilization is such that the radical displays acidity two to six pKa units lower than that of closed-shell ribose. Given the omnipresence of negative charges in biological systems, these interactions potentially represent a universal mechanism for stabilizing and controlling highly reactive radical intermediates across radical enzymes, opening new avenues for enzymatic engineering and biotechnological applications.