The active site of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans.: II.: Redox properties, light sensitivity and CO-ligand exchange as observed by infrared spectroscopy

In [FeFe]-hydrogenases, the H cluster (hydrogen-activating cluster) contains a di-iron centre ([2Fe](H) subcluster, a (L)(CO)(CN)Fe(mu-RS2)(mu-CO)Fe (CysS)(CO)(CN) group) covalently attached to a cubane iron-sulphur cluster ([4Fe-4S](H) subcluster). The Cysthiol functions as the link between one iron (called Fe1) of the [2Fe]H subcluster and one iron of the cubane subcluster. The other iron in the [2Fe]H subcluster is called Fe2. The light sensitivity of the Desulfovibrio desulfuricans enzyme in a variety of states has been studied with infrared (IR) spectroscopy. The aerobic inactive enzyme (H-inact state) and the CO-inhibited active form (H-ox-CO state) were stable in light. Illumination of the H-ox state led to a kind of cannibalization; in some enzyme molecules the H cluster was destroyed and the released CO was captured by the H clusters in other molecules to form the light-stable H-ox-CO state. Illumination of active enzyme under (CO)-C-13 resulted in the complete exchange of the two intrinsic COs bound to Fe2. At cryogenic temperatures, light induced the photodissociation of the extrinsic CO and the bridging CO of the enzyme in the H-ox-CO state. Electrochemical redox titrations showed that the enzyme in the H-inact state converts to the transition state (H-trans) in a reversible one-electron redox step (E-m,E- pH 7 = -75 mV). IR spectra demonstrate that the added redox equivalent not only affects the [4Fe-4S](H) subcluster, but also the di-iron centre. Enzyme in the H-trans state reacts with extrinsic CO, which binds to Fe2. The H-trans state converts irreversibly into the H-ox state in a redox-dependent reaction most likely involving two electrons (E-m, pH (7) = -261 mV). These electrons do not end up on any of the six Fe atoms of the H cluster; the possible destiny of the two redox equivalents is discussed. An additional reversible one-electron redox reaction leads to the H-red state (E-m, (pH) (7)= -354 mV), where both Fe atoms of the [2Fe](H) subcluster have the same formal oxidation state. The possible oxidation states of Fe1 and Fe2 in the various enzyme states are discussed. Low redox potentials (below -500 mV) lead to destruction of the [2Fe](H) subcluster.