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

Stella Impano, Hao Yang, Richard J. Jodts, Adrien Pagnier, Ryan Swimley, Elizabeth C. McDaniel, Eric M. Shepard, William E. Broderick, Joan B. Broderick*, Brian M. Hoffman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


Catalysis by canonical radical S-adenosyl-l-methionine (SAM) enzymes involves electron transfer (ET) from [4Fe-4S]+ to SAM, generating an R3S0 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 ·CH3, 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 ·CH3. 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 R3S0 radical is a (2E) state with its antibonding unpaired electron in an orbital doublet, which renders R3S0 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.

Original languageEnglish (US)
Pages (from-to)335-348
Number of pages14
JournalJournal of the American Chemical Society
Issue number1
StatePublished - Jan 13 2021

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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