1H NMR Spectroscopy of [FeFe] Hydrogenase: Insight into the Electronic Structure of the Active Site

Oxygen sensitivity of [FeFe]-hydrogenase: a comparative study of active site mimics inside vs. outside the enzyme

[FeFe]-hydrogenase is nature's most efficient proton reducing and H2-oxidizing enzyme. However, biotechnological applications are hampered by the O2 sensitivity of this metalloenzyme, and the mechanism of aerobic deactivation is not well understood. Here, we explore the oxygen sensitivity of four mimics of the organometallic active site cofactor of [FeFe]-hydrogenase, [Fe2(adt)(CO)6-x(CN)x]x- and [Fe2(pdt)(CO)6-x(CN)x]x- (x = 1, 2) as well as the corresponding cofactor variants of the enzyme by means of infrared, Mössbauer, and NMR spectroscopy. Additionally, we describe a straightforward synthetic recipe for the active site precursor complex Fe2(adt)(CO)6. Our data indicate that the aminodithiolate (adt) complex, which is the synthetic precursor of the natural active site cofactor, is most oxygen sensitive. This observation highlights the significance of proton transfer in aerobic deactivation, and supported by DFT calculations facilitates an identification of the responsible reactive oxygen species (ROS). Moreover, we show that the ligand environment of the iron ions critically influences the reactivity with O2 and ROS like superoxide and H2O2 as the oxygen sensitivity increases with the exchange of ligands from CO to CN-. The trends in aerobic deactivation observed for the model complexes are in line with the respective enzyme variants. Based on experimental and computational data, a model for the initial reaction of [FeFe]-hydrogenase with O2 is developed. Our study underscores the relevance of model systems in understanding biocatalysis and validates their potential as important tools for elucidating the chemistry of oxygen-induced deactivation of [FeFe]-hydrogenase.

Spin Polarization Reveals the Coordination Geometry of the [FeFe] Hydrogenase Active Site in Its CO-Inhibited State

The active site of [FeFe] hydrogenase features a binuclear iron cofactor Fe₂ADT(CO)₃(CN)₂, where ADT represents the bridging ligand aza-propane-dithiolate. The terminal diatomic ligands all coordinate in a basal configuration, and one CO bridges the two irons leaving an open coordination site at which the hydrogen species and the competitive inhibitor CO bind. Externally supplied CO is expected to coordinate in an apical configuration. However, an alternative configuration has been proposed in which, due to ligand rotation, the CN^- bound to the distal Fe becomes apical. Using selective ¹³C isotope labeling of the CN^- and CO_ext ligands in combination with pulsed ¹³C electron-nuclear-nuclear triple resonance spectroscopy, spin polarization effects are revealed that, according to density functional theory calculations, are consistent with only the "unrotated" apical CO_ext configuration.

Asymmetry in the Ligand Coordination Sphere of the [FeFe] Hydrogenase Active Site Is Reflected in the Magnetic Spin Interactions of the Aza-propanedithiolate Ligand

[FeFe] hydrogenases are very active enzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. Their active site, the H-cluster, contains a unique binuclear iron complex, [2Fe]_H, with CN^- and CO ligands as well as an aza-propane-dithiolate (ADT) moiety featuring a central amine functionality that mediates proton transfer during catalysis. We present a pulsed ¹³C-ENDOR investigation of the H-cluster in which the two methylene carbons of ADT are isotope labeled with ¹³C. We observed that the corresponding two ¹³C hyperfine interactions are of opposite sign and corroborated this finding using density functional theory calculations. The spin polarization in the ADT ligand is shown to be linked to the asymmetric coordination of the distal iron site with its terminal CN^- and CO ligands. We propose that this asymmetry is relevant for the enzyme reactivity and is related to the (optimal) stabilization of the iron-hydride intermediate in the catalytic cycle.

Electronic Structure of Two Catalytic States of the [FeFe] Hydrogenase H-Cluster As Probed by Pulse Electron Paramagnetic Resonance Spectroscopy

The active site of the [FeFe] hydrogenase (HydA1), the H-cluster, is a 6-Fe cofactor that contains CO and CN^- ligands. It undergoes several different oxidation and protonation state changes in its catalytic cycle to metabolize H₂. Among them, the well-known H_ox state and the recently identified H_hyd state are thought to be directly involved in H₂ activation and evolution, and they are both EPR active with net spin S = 1/2. Herein, we report the pulse electronic paramagnetic spectroscopic investigation of these two catalytic states in Chlamydomonas reinhardtii HydA1 ( CrHydA1). Using an in vitro biosynthetic maturation approach, we site-specifically installed ¹³C into the CO or CN^- ligands and ⁵⁷Fe into the [2Fe]_H subcluster of the H-cluster in order to measure the hyperfine couplings to these magnetic nuclei. For H_ox, we measured ¹³C hyperfine couplings (¹³CO a_iso of 25.5, 5.8, and 4.5 MHz) corresponding to all three CO ligands in the H-cluster. We also observed two ⁵⁷Fe hyperfine couplings (⁵⁷Fe a_iso of ∼17 and 5.7 MHz) arising from the two Fe atoms in the [2Fe]_H subcluster. For H_hyd, we only observed two distinct ¹³CO hyperfine interactions (¹³CO a_iso of 0.16 and 0.08 MHz) and only one for ¹³CN^- (¹³CN a_iso of 0.16 MHz); the couplings to the ¹³CO/¹³CN^- on the distal Fe of [2Fe]_H may be too small to detect. We also observed a very small (<0.3 MHz) ⁵⁷Fe HFI from the labeled [2Fe]_H subcluster and four ⁵⁷Fe HFI from the labeled [4Fe-4S]_H subcluster (⁵⁷Fe a_iso of 7.2, 16.6, 28.2, and 35.3 MHz). These hyperfine coupling constants are consistent with the previously proposed electronic structure of the H-cluster at both H_ox and H_hyd states and provide a basis for more detailed analysis.

Spectroscopic investigations of a semi-synthetic [FeFe] hydrogenase with propane di-selenol as bridging ligand in the binuclear subsite: comparison to the wild type and propane di-thiol variants

[FeFe] Hydrogenases catalyze the reversible conversion of H₂ into electrons and protons. Their catalytic site, the H-cluster, contains a generic [4Fe–4S]_H cluster coupled to a [2Fe]_H subsite [Fe₂(ADT)(CO)₃(CN)₂]²⁻, ADT = µ(SCH₂)₂NH. Heterologously expressed [FeFe] hydrogenases (apo-hydrogenase) lack the [2Fe]_H unit, but this can be incorporated through artificial maturation with a synthetic precursor [Fe₂(ADT)(CO)₄(CN)₂]²⁻. Maturation with a [2Fe] complex in which the essential ADT amine moiety has been replaced by CH₂ (PDT = propane-dithiolate) results in a low activity enzyme with structural and spectroscopic properties similar to those of the native enzyme, but with simplified redox behavior. Here, we study the effect of sulfur-to-selenium (S-to-Se) substitution in the bridging PDT ligand incorporated in the [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii using magnetic resonance (EPR, NMR), FTIR and spectroelectrochemistry. The resulting HydA1-PDSe enzyme shows the same redox behavior as the parent HydA1-PDT. In addition, a state is observed in which extraneous CO is bound to the open coordination site of the [2Fe]_H unit. This state was previously observed only in the native enzyme HydA1-ADT and not in HydA1-PDT. The spectroscopic features and redox behavior of HydA1-PDSe, resulting from maturation with [Fe₂(PDSe)(CO)₄(CN)₂]²⁻, are discussed in terms of spin and charge density shifts and provide interesting insight into the electronic structure of the H-cluster. We also studied the effect of S-to-Se substitution in the [4Fe–4S] subcluster. The reduced form of HydA1 containing only the [4Fe–4Se]_H cluster shows a characteristic S = 7/2 spin state which converts back into the S = 1/2 spin state upon maturation with a [2Fe]–PDT/ADT complex.

Direct Detection of the Terminal Hydride Intermediate in [FeFe] Hydrogenase by NMR Spectroscopy

Hydride state intermediates are known to occur in various hydrogen conversion enzymes, including the highly efficient [FeFe] hydrogenases. The intermediate state involving a terminal iron-bound hydride has been recognized as crucial for the catalytic mechanism, but its occurrence has up to now eluded unequivocal proof under (near) physiological conditions. Here we show that the terminal hydride in the [FeFe] hydrogenase from Chlamydomonas reinhardtii can be directly detected using solution ¹H NMR spectroscopy at room temperature, opening new avenues for detailed in situ investigations under catalytic conditions.