Abstract
Catalysis of H2 production and oxidation reactions is critical in renewable energy systems based around H2 as a clean fuel, but the present reliance on platinum-based catalysts is not sustainable. In nature, H2 is oxidized at minimal overpotential and high turnover frequencies at [NiFe] catalytic sites in hydrogenase enzymes. Although an outline mechanism has been established for the [NiFe] hydrogenases involving heterolytic cleavage of H2 followed by a first and then second transfer of a proton and electron away from the active site, details remain vague concerning how the proton transfers are facilitated by the protein environment close to the active site. Furthermore, although [NiFe] hydrogenases from different organisms or cellular environments share a common active site, they exhibit a broad range of catalytic characteristics indicating the importance of subtle changes in the surrounding protein in controlling their behavior. Here we review recent time-resolved infrared (IR) spectroscopic studies and IR spectroelectrochemical studies carried out in situ during electrocatalytic turnover. Additionally, we re-evaluate the significant body of IR spectroscopic data on hydrogenase active site states determined through more conventional solution studies, in order to highlight mechanistic steps that seem to apply generally across the [NiFe] hydrogenases, as well as steps which so far seem limited to specific groups of these enzymes. This analysis is intended to help focus attention on the key open questions where further work is needed to assess important aspects of proton and electron transfer in the mechanism of [NiFe] hydrogenases.
Highlights
Hydrogenases are ancient enzymes that evolved to allow primitive organisms to extract energy from the H2-rich primordial environment
SUMMARY AND OUTLOOK Recent spectroscopic studies have confirmed the involvement of Nia-SI, Nia-R, and Nia-C in the [NiFe] hydrogenase catalytic cycle, and these studies reinforce the likely importance of a range of NiI species, Nia-L, as on-pathway intermediates between Nia-C and Nia-SI
The initial proton acceptor during the transition from Nia-C to Nia-L is thought to be the terminal cysteine thiolate, with a conserved glutamate residue demonstrated to be important for proton transfer beyond the active site
Summary
Hydrogenases are ancient enzymes that evolved to allow primitive organisms to extract energy from the H2-rich primordial environment. This led to the suggestion of a frustrated Lewis pair (FLP)-type mechanism for H2 splitting in which the guanidinium side chain is transiently deprotonated and the resulting guanidine provides the strong base required for H2 cleavage (Figure 7A) This mechanism has obvious similarities to FeFe hydrogenases, in which an azadithiolate bridging ligand positions a basic nitrogen atom above the distal Fe site of the active site H-cluster (Figure 7B),[103] and to enzyme-inspired mimetic Ni pincer complexes, which contain pendant amine groups acting as the initial proton acceptor during H2 oxidation (Figure 7C).[104,56]. Hirota and co-workers demonstrated that it is possible to produce significant quantities of Nia-SI during photolysis of the Nia-C state at low temperature, but only from samples in which the proximal iron−sulfur cluster is oxidized and can accept electrons from the active site (Figure 9A).[60] Figure 9B shows prephotolysis spectra and lightminus-dark difference spectra following photolysis for samples of the O2-sensitive [NiFe] hydrogenase from D. vulgaris MF under a H2 atmosphere. Hirota and co-workers observed pH-dependence of Nia-L formation from Nia-C, with
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