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Radical SAM (RS) enzymes use S -adenosyl-l-methionine (SAM) and a [4Fe-4S] cluster to initiate a broad spectrum of radical transformations throughout all kingdoms of life. We report here that low-temperature photoinduced electron transfer from the [4Fe-4S] 1+ cluster to bound SAM in the active site of the hydrogenase maturase RS enzyme, HydG, results in specific homolytic cleavage of the S-CH 3 bond of SAM, rather than the S-C5' bond as in the enzyme-catalyzed (thermal) HydG reaction. This result is in stark contrast to a recent report in which photoinduced ET in the RS enzyme pyruvate formate-lyase activating enzyme cleaved the S-C5' bond to generate a 5'-deoxyadenosyl radical, and provides the first direct evidence for homolytic S-CH 3 bond cleavage in a RS enzyme. Photoinduced ET in HydG generates a trapped • CH 3 radical, as well as a small population of an organometallic species with an Fe-CH 3 bond, denoted Ω   M  . The • CH 3 radical is surprisingly found to exhibit rotational diffusion in the HydG active site at temperatures as low as 40 K, and is rapidly quenched: whereas 5'-dAdo • is stable indefinitely at 77 K, • CH 3 quenches with a half-time of ∼2 min at this temperature. The rapid quenching and rotational/translational freedom of • CH 3 shows that enzymes would be unable to harness this radical as a regio- and stereospecific H atom abstractor during catalysis, in contrast to the exquisite control achieved with the enzymatically generated 5'-dAdo • .

Photoinduced Electron Transfer in a Radical SAM Enzyme Generates an S-Adenosylmethionine Derived Methyl Radical