Active-Site Controlled, Jahn-Teller Enabled Regioselectivity in Reductive S-C Bond Cleavage of S -Adenosylmethionine in Radical SAM Enzymes.

Catalysis by canonical radical S -adenosyl-l-methionine (SAM) enzymes involves electron transfer (ET) from [4Fe-4S] ⁺ to SAM, generating an R ₃ S ⁰ radical that undergoes regioselective homolytic reductive cleavage of the S-C5' bond to generate the 5'-dAdo· radical. However, cryogenic photoinduced S-C bond cleavage has regioselectively yielded either 5'-dAdo· or ·CH ₃ , and indeed, each of the three SAM S-C bonds can be regioselectively cleaved in an RS enzyme. This diversity highlights a longstanding central question: what controls regioselective homolytic S-C bond cleavage upon SAM reduction? We here provide an unexpected answer, founded on our observation that photoinduced S-C bond cleavage in multiple canonical RS enzymes reveals two enzyme classes: in one, photolysis forms 5'-dAdo·, and in another it forms ·CH ₃ . The identity of the cleaved S-C bond correlates with SAM ribose conformation but not with positioning and orientation of the sulfonium center relative to the [4Fe-4S] cluster. We have recognized the reduced-SAM R ₃ S ⁰ radical is a ( ² E ) state with its antibonding unpaired electron in an orbital doublet, which renders R ₃ S ⁰ Jahn-Teller (JT)-active and therefore subject to vibronically induced distortion. Active-site forces induce a JT distortion that localizes the odd electron in a single priority S-C antibond, which undergoes regioselective cleavage. In photolytic cleavage those forces act through control of the ribose conformation and are transmitted to the sulfur via the S-C5' bond, but during catalysis thermally induced conformational changes that enable ET from a cluster iron generate dominant additional forces that specifically select S-C5' for cleavage. This motion also can explain how 5'-dAdo· subsequently forms the organometallic intermediate Ω.