Photoinhibition of FeFe Hydrogenase

International audienceIn the enzyme FeFe hydrogenase, hydrogen oxidation and production occur at the H-cluster, a Fe​ 6​ S​ 6 active site that bears intrinsic carbonyl and cyanide ligands. This enzyme has been coupled to photosensitizers to design H​ 2 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 UV-B (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 H​ 2 production, and our results show that some but not all FeFe hydrogenases can actually be combined with photosensitizers that utilise the solar spectrum, provided a UV screen is used. We suggest that further investigations of the compatibility of hydrogenases or hydrogenase mimics with light-harvesting systems should also consider the possibility of irreversible​ ​ photoinhibition