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

Radical SAM (RS) enzymes use -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] cluster to bound SAM in the active site of the hydrogenase maturase RS enzyme, HydG, results in specific homolytic cleavage of the S-CH 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 bond cleavage in a RS enzyme. Photoinduced ET in HydG generates a trapped CH radical, as well as a small population of an organometallic species with an Fe-CH bond, denoted . The CH 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 quenches with a half-time of ∼2 min at this temperature. The rapid quenching and rotational/translational freedom of CH 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.