Current State of [FeFe]-Hydrogenase Research: Biodiversity and Spectroscopic Investigations

Abstract

Hydrogenases are redox enzymes that catalyze the conversion of protons and molecular hydrogen (H2). Based on the composition of the active site cofactor, the monometallic [Fe]hydrogenase is distinguished from the bimetallic [NiFe]-or [FeFe]-hydrogenase. The latter has been reported with particularly high turnover activities for both H2 release and H2 oxidation, notably at neutral pH, ambient temperatures, and negligible electric overpotential. Due to these properties, [FeFe]-hydrogenase represents the 'gold standard' in enzymatic hydrogen turnover.Understanding hydrogenase chemistry is crucial for the design of transition metal complexes that serve as potentially sustainable proton reduction or H2 oxidation catalysts, e.g. in electrolytic devices or fuel cells.Even 20 years after the crystal structures of [FeFe]-hydrogenase have been published, several aspects of biological hydrogen turnover are heatedly discussed. In this perspective, we give an overview on how the diversity of naturally occurring and artificially prepared, semi-synthetic [FeFe]-hydrogenases deepens our understanding of hydrogenase chemistry. In parallel, we cover recent results from biophysical techniques that go beyond the scope of conventional Xray diffraction, EPR, and FTIR spectroscopy. Taking into account both proton transfer and electron transfer as well as the notorious sensitivity of [FeFe]-hydrogenases towards carbon monoxide, the discussion further touches upon the molecular proceedings of biological hydrogen turnover.

Characterization of a putative sensory [FeFe]-hydrogenase provides new insight into the role of the active site architecture

“…A similar downshi of the H ox -signal upon formation of H ox H has recently been reported for Cr HydA1. 9 As comparing spectra of samples prepared at mildly basic and acidic conditions did not reveal signicant changes in the relative amplitudes of the rhombic signals their interconversion appears to be more complicated than an acid-base equilibrium. The structural details of this H ox H-like state in Tam HydS remains to be fully elucidated.…”

“…3,4 Phylogenetically, [FeFe]-hydrogenases can be broadly divided into four main groups, denoted group A, B, C, and D, which in turn contain numerous subclasses. [5][6][7][8][9] Considering the wellconserved nature of the auxiliary proteins involved in cofactor assembly (HydEFG), 6 they all arguably share a dependence on the same hexanuclear iron cofactor, the "H-cluster". This biologically unique cofactor consists of a canonical [4Fe-4S] cluster ([4Fe-4S] H ) connected to a low valent dinuclear iron complex ([2Fe] H ).…”

“…It will also serve to strengthen efforts related to biotechnological energy applications and potentially facilitate the development of selective antibiotics. 8,9 We recently reported the whole-cell characterization of an [FeFe]-hydrogenase from the thermophilic rmicute Thermoanaerobacter mathranii in E. coli. 40 The enzyme belongs to the hitherto uncharacterised M2e subclass, which displays a number of well-conserved differences in amino acid sequence as compared to the prototypical group A hydrogenases; namely in the active site cavity and the proton transfer pathway (Fig.…”

Biosynthesis of the catalytic H-cluster of [FeFe] hydrogenase: the roles of the Fe–S maturase proteins HydE, HydF, and HydG

“…3B), where a simple C(2)H 2 replaces the C a H-NH 2 of tyrosine. A strong new HPPAderived radical EPR signal was observed, with majority spin density on the C2 carbon: when the HPPA C2 carbon was specifically 13 C-labeled, a strong 13 C-hyperne splitting was observed. Moreover, mass spectrometry of dAdoH with the HPPA reactions run in buffer with natural abundance water versus 2 H-enriched water showed that the abstracted H-atom is not exchangeable with HPPA as a substrate, eliminating the H of the HPPA phenol OH group as the abstracted hydrogen.…”

“…1). The [2Fe] H unit, thought to be the site where the H + and H 2 substrates bind and react, [10][11][12][13] harbors the organometallic elements of the H-cluster, with the two irons exhibiting CO and CN terminal ligands, and with two bridges linking these two irons in the form of a third CO along with a unique SCH 2 NHCH 2 S azadithiolate (adt) species. Unlike the [4MnCa] oxygen evolving complex of photosystem II, which self-assembles under illumination in a process termed photoactivation, 14 the complex catalytic Fe-S clusters of enzymes such as these hydrogenases and the nitrogenases must be synthesized enzymatically.…”