In Vivo EPR Characterization of Semi-Synthetic [FeFe] Hydrogenases

EPR spectroscopy reveals the formation of two different semi-synthetic hydrogenases in vivo. [FeFe] hydrogenases are metalloenzymes that catalyze the interconversion of molecular hydrogen and protons. The reaction is catalyzed by the H-cluster, consisting of a canonical iron-sulfur cluster and an organometallic [2Fe] subsite. It was recently shown that the enzyme can be reconstituted with synthetic cofactors mimicking the composition of the [2Fe] subsite, resulting in semi-synthetic hydrogenases. Herein, we employ EPR spectroscopy to monitor the formation of two such semi-synthetic enzymes in whole cells. The study provides the first spectroscopic characterization of semi-synthetic hydrogenases in vivo, and the observation of two different oxidized states of the H-cluster under intracellular conditions. Moreover, these findings underscore how synthetic chemistry can be a powerful tool for manipulation and examination of the hydrogenase enzyme under in vivo conditions.

Control of the transition between Ni-C and Ni-SI(a) states by the redox state of the proximal Fe-S cluster in the catalytic cycle of [NiFe] hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H2. The electrons produced by the H2 cleavage pass through three Fe-S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni-SI(a), Ni-C, and Ni-R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni-C and Ni-SI(a) states remain unrevealed. In this study, the FT-IR spectra under light irradiation at 138-198 K show that the Ni-L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni-C and Ni-SI(a) states. The transition of the Ni-C state to the Ni-SI(a) state occurred when the proximal [Fe4S4]p(2+/+) cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe4S4]p(2+/+) cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.

Spectroscopic and electrochemical characterization of the [NiFeSe] hydrogenase from Desulfovibrio vulgaris Miyazaki F: reversible redox behavior and interactions between electron transfer centers

Characterizing a new hydrogenase: The newly isolated [NiFeSe] hydrogenase from Desulfovibrio vulgaris Miyazaki F displays catalytic properties distinct from other hydrogenase proteins. Here we apply site-specific spectroscopic and electrochemical techniques to characterize these unique features at the molecular level.

New iron-sulfur clusters help hydrogenases tolerate oxygen

One S less: recent crystallographic studies have revealed a new, oxygen-tolerant kind of iron-sulfide cluster [4Fe-3S], which contains only three rather than four sulfur atoms in its cage (see picture; yellow=S, red=Fe, blue=N, green=cysteine). It is proposed that the cluster's ability to transfer multiple electrons increases the oxygen tolerance by enabling the enzyme to reduce O(2) rapidly, converting the dioxygen into harmless water before it can damage the protein.