Crystallographic and FTIR spectroscopic evidence of changes in Fe coordination upon reduction of the active site of the Fe-only hydrogenase from Desulfovibrio desulfuricans

Fe-only hydrogenases, as well as their NiFe counterparts, display unusual i ntrinsic high-frequency LR bands that have been assigned;to CO and CN- liga tion to iron in their active sites. FTIR experiments performed on the Fe-on ly hydrogenase from Desulfovibrio desulfuricans indicate that upon reductio n of the active oxidized form, there is a major shift of one of these bands that is provoked, most likely, by the change of a CO ligand from a bridgin g position to a terminal one. Indeed, the crystal structure of the reduced active site of this enzyme shows that the previously bridging CO is now ter minally bound to the iron ion that most likely corresponds to the primary h ydrogen binding site (Fe2). The CO binding change may result from changes i n the coordination sphere of Fe2 or its reduction. Superposition of this re duced active site with the equivalent region of a NiFe hydrogenase shows a remarkable coincidence between the coordination of Fe2 and that of the Fe i on in the NiFe cluster. Both stereochemical and mechanistic considerations suggest that the small organic molecule found at the Fe-only hydrogenase ac tive site and previously modeled as 1,3-propanedithiolate may, in fact, be di-(thiomethyl)-amine.