Photoinhibition of FeFe hydrogenase

In the enzyme FeFe hydrogenase, hydrogen oxidation and production occur at the H-cluster, a Fe6S6 active site that bears intrinsic carbonyl and cyanide ligands. This enzyme has been coupled to photosensitizers to design H2 photoproduction systems, and yet, according to earlier reports, the enzyme from Desulfovibrio desulfuricans is "easily destroyed" in "normal laboratory light". Here we report direct electrochemistry measurements of the effect of light on the activity of the enzymes from Chlamydomonas reinhardtii and Clostridium acetobutylicum, together with TDDFT and DFT calculations of the reactivity of the excited states of the H-cluster. We conclude that visible light does not inhibit these enzymes, but absorption of UVB (280-315 nm) irreversibly damages the H-cluster by triggering the release of an intrinsic CO ligand; the resulting unsaturated species rearranges and protonates to form a stable, inactive dead-end. Answering the question of which particular hydrogenase can resist which particular wavelengths is important regarding solar H2 production, and our results show that some but not all FeFe hydrogenases can actually be combined with photosensitizers that utilize the solar spectrum, provided a UV screen is used. We suggest that further investigations of the compatibility of hydrogenases or hydrogenase mimics with lightharvesting systems should also consider the possibility of irreversible photoinhibition.