The Molecular Proceedings of Biological Hydrogen Turnover

Abstract

Over the past two decades, the bioinorganic chemistry of hydrogenases has attracted much interest from basic and applied research. Hydrogenases are highly efficient metalloenzymes that catalyze the reversible reduction of protons to molecular hydrogen (H) in all domains of life. Their iron- and nickel-based cofactors represent promising blueprints for the design of biomimetic, synthetic catalysts. In this Account, we address the molecular proceedings of hydrogen turnover with [FeFe]-hydrogenases. The active site cofactor of [FeFe]-hydrogenases ("H-cluster") comprises a unique diiron complex linked to a [4Fe-4S] cluster via a single cysteine. Since it was discovered that a synthetic analogue of the diiron site can be incorporated into apoprotein in vitro to yield active enzyme, significant progress has been made toward a comprehensive understanding of hydrogenase catalysis. The diiron site carries three to four carbon monoxide (CO) and two cyanide (CN) ligands that give rise to intense infrared (IR) absorption bands. These bands are sensitive reporters of the electron density across the H-cluster, which can be addressed by infrared spectroscopy to follow redox and protonation changes at the cofactor. Synthetic variation of the metal-bridging dithiolate ligand at the diiron site, as well as site-directed mutagenesis of amino acids, provides access to the proton pathways toward the cofactor. Quantum chemical calculations are employed to specifically assign IR bands to vibrational modes of the diatomic ligands and yield refined H-cluster geometries. Here, we provide an overview of recent research on [FeFe]-hydrogenases with emphasis on experimental and computational IR studies. We describe advances in attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) and protein film electrochemistry, as well as density functional theory (DFT) calculations. Key cofactor species are discussed in terms of molecular geometry, redox state, and protonation. Isotope editing is introduced as a tool to evaluate the cofactor geometry beyond the limits of protein crystallography. In particular, the role of proton-coupled electron transfer (PCET) in the generation of catalytically relevant redox species is addressed. We propose that site-selective protonation of the H-cluster biases surplus electrons either to the [4Fe-4S] cluster or to the diiron site. Protonation of the [4Fe-4S] cluster prevents premature reduction at the diiron site and stabilizes a reactive, terminal hydride. The observed H-cluster species are assigned to rapid H conversion or to reactions possibly involved in activity regulation and cellular H sensing. In the catalytic cycle of [FeFe]-hydrogenases, an H-cluster geometry is preserved that features a bridging CO ligand. PCET levels the redox potential for two steps of sequential cofactor reduction. The concept of consecutive PCET at a geometrically inert cofactor with tight control of electron and proton localization may inspire the design of a novel generation of biomimetic catalysts for the pro...

H2 Activation in [FeFe]‐Hydrogenase Cofactor Versus Diiron Dithiolate Models: Factors Underlying the Catalytic Success of Nature and Implications for an Improved Biomimicry

“…The possibility of as ubstrate (H + ,i nt his case) binding in the FeÀFe region has recently been postulated also in relation to the enzyme, although the authors clearly stated that pathways featuring m-H formation are incompatible with rapid proton reduction catalysis. [52,53] This is due to the easy isomerization to theu nrotated form undergone by H ox -like species, as previously predicted in the few cases investigated. [33,54] The present results, however,o nly refer to H 2 binding occurring at the rotatedi ron unit because 1) although unrotated structures of Fe 2 -dithiolates in the mixed-valence Fe II Fe I state are energetically not that distant from rotated isomers, the latter are generally more stable, and 2) H 2 bindingt ou nrotated isomers is predicted to be unfavorable compared with that occurring at rotated structures (see Scheme S2 in the Supporting Information).…”

Spectroscopic investigations under whole-cell conditions provide new insight into the metal hydride chemistry of [FeFe]-hydrogenase

“…[7][8][9] Consequently, intense efforts have been invested in elucidating the structure and catalytic mechanism of these enzymes. 10,11 The reactivity of [FeFe]-hydrogenases is enabled by a hexanuclear iron complex, referred to as the hydrogen-forming cluster or simply the "H-cluster" (Fig. 1).…”

Research advances towards large-scale solar hydrogen production from water

“…In microalgae the enzyme responsible for HER, [FeFe] hydrogenase, is the most active type with turnover rates of up to 1000 s −1 and near zero overpotential. 149 However, due to extreme oxygen sensitivity of [FeFe] hydrogenase, the HER is quenched within minutes by the oxygen produced on PS2. [150][151] As the deactivation is irreversible and regeneration of [FeFe] hydrogenase requires strict anaerobic induction, 152 the process has limited practicality.…”