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

• Published in: Journal of the American Chemical Society Vol. 143; no. 1; pp. 335 - 348
• Main Authors: Impano, Stella, Yang, Hao, Jodts, Richard J., Pagnier, Adrien, Swimley, Ryan, McDaniel, Elizabeth C., Shepard, Eric M., Broderick, William E., Broderick, Joan B., Hoffman, Brian M.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society (ACS) 13.01.2021
• Subjects: active sites; Bacterial Proteins - chemistry; Bacterial Proteins - radiation effects; Biocatalysis; catalytic activity; Catalytic Domain; Chemistry; Clostridium acetobutylicum - enzymology; Density Functional Theory; electron transfer; enzymes; homolytic cleavage; iron; Iron-Sulfur Proteins - chemistry; Iron-Sulfur Proteins - radiation effects; Light; Models, Chemical; Molecular Structure; Oxidation-Reduction - radiation effects; Oxidoreductases Acting on Sulfur Group Donors - chemistry; Oxidoreductases Acting on Sulfur Group Donors - radiation effects; Photolysis; regioselectivity; ribose; S-adenosylmethionine; S-Adenosylmethionine - chemistry; S-Adenosylmethionine - radiation effects; sulfur; Thermotoga maritima - enzymology
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.0c10925

Catalysis by canonical radical -adenosyl-l-methionine (SAM) enzymes involves electron transfer (ET) from [4Fe-4S] to SAM, generating an R S 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 , 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 . 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 S radical is a ( ) state with its antibonding unpaired electron in an orbital doublet, which renders R S 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.