Radical S-adenosylmethionine (SAM) enzymes contain a [4Fe-4S]-cluster cofactor that can catalyze the reductive cleavage of SAM into methionine and the strong oxidant 5’-deoxyadenosyl radical.  The generated 5’-deoxyadenosyl radical can then abstract a target hydrogen from enzyme bound substrates as in the case of the Radical SAM enzyme HydG.  In the case of the Radical SAM enzyme HydE the generated radical reduces a substrate low-spin Fe(II) to a low spin Fe(I) state.  Both HydG and HydE catalyze critical steps in the assembly of the complex metallocofactor H-cluster in the enzyme [FeFe]-hydrogenase.  By using total and selective labeling of HydG and HydE with <sup>57</sup>Fe, important intermediates and products have been characterized through EPR and Mössbauer spectroscopies.  Results have given a better understanding of how the electronic structure changes as the active site H-cluster in [FeFe]-hydrogenase is pieced together enzymatically.

Characterization of the radical S-adenosylmethionine (SAM) enzymes HydG and HydE by EPR and Mössbauer spectroscopies

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