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doi:10.26434/chemrxiv-2022-jvtgw, [NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H$_2$ into 2 e$^- $ and 2 H$^+$ under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)$_2$(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN$^–$ molecules. Herein, we investigated the sequential assembly of the [NiFe]-cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O$_2$-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)$_2$(CO) fragment, a precursor that contained all cofactor components but remained redox-inactive, and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, X-ray absorption, and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.

[NiFe]‐hydrogenases catalyze the reversible conversion of molecular hydrogen into protons end electrons. This reaction takes place at a NiFe(CN)2(CO) cofactor located in the large subunit of the bipartite hydrogenase module. The corresponding apo‐protein carries usually a C‐terminal extension that is cleaved off by a specific endopeptidase as soon as the cofactor insertion has been accomplished by the maturation machinery. This process triggers complex formation with the small, electron‐transferring subunit of the hydrogenase module, revealing catalytically active enzyme. The role of the C‐terminal extension in cofactor insertion, however, remains elusive. We have addressed this problem by using genetic engineering to remove the entire C‐terminal extension from the apo‐form of the large subunit of the membrane‐bound [NiFe]‐hydrogenase (MBH) from Ralstonia eutropha. Unexpectedly, the MBH holoenzyme derived from this precleaved large subunit was targeted to the cytoplasmic membrane, conferred H2‐dependent growth of the host strain, and the purified protein showed exactly the same catalytic activity as native MBH. The only difference was a reduced hydrogenase content in the cytoplasmic membrane. These results suggest that in the case of the R. eutropha MBH, the C‐terminal extension is dispensable for cofactor insertion and seems to function only as a maturation facilitator., The apo‐forms of [NiFe]‐hydrogenase large subunits are usually synthesized with a C‐terminal peptide extension that is proteolytically cleaved off upon incorporation of the catalytic metal center. Although to a limited amount, an artificially precleaved large subunit, which was deleted for the C‐terminal extension by genetic engineering, still received the active site components delivered by the dedicated maturation machinery. This suggests that the C‐terminal extension optimizes metal center incorporation, but is not essential for the formation of catalytically active [NiFe]‐hydrogenase.

To study metalloenzymes in detail, we developed a new experimental setup allowing the controlled preparation of catalytic intermediates for characterization by various spectroscopic techniques. The in situ monitoring of redox transitions by infrared spectroscopy in enzyme lyophilizate, crystals, and solution during gas exchange in a wide temperature range can be accomplished as well. Two O2-tolerant [NiFe]-hydrogenases were investigated as model systems. First, we utilized our platform to prepare highly concentrated hydrogenase lyophilizate in a paramagnetic state harboring a bridging hydride. This procedure proved beneficial for 57Fe nuclear resonance vibrational spectroscopy and revealed, in combination with density functional theory calculations, the vibrational fingerprint of this catalytic intermediate. The same in situ IR setup, combined with resonance Raman spectroscopy, provided detailed insights into the redox chemistry of enzyme crystals, underlining the general necessity to complement X-ray crystallographic data with spectroscopic analyses.

In this work, we demonstrate that diazonium electrografting of biocompatible interfaces on transparent conducting oxide indium tin oxide (ITO) can be controlled and optimized to achieve low charge transfer resistance, allowing highly efficient electron transfer to an immobilized model enzyme, the oxygen‐tolerant [NiFe]‐hydrogenase from Ralstonia eutropha. The use of a radical scavenger enables control of the interface thickness, and thus facilitates maximization of direct electron transfer processes between the enzyme's active center and the electrode. Using this approach, amine and carboxylic acid functionalities were grafted on ITO, allowing enzyme immobilization both under moderate electrostatic control and covalently via amide bond formation. Despite an initial decrease in catalytic activity, covalent immobilization led to an improvement in current stability compared to just electrostatically immobilized enzyme. Given the superior stability of electrografted interfaces in comparison to adsorbed or self‐assembled interfaces, we propose electrografting as an alternative approach for the functional immobilization of redox‐active enzymes on transparent conducting oxide (TCO) electrodes in bioelectronic devices.

Scientific reports 7, 44628 (2017). doi:10.1038/srep44628, Serial femtosecond crystallography requires reliable and efficient delivery of fresh crystals across the beam of an X-ray free-electron laser over the course of an experiment. We introduce a double-flow focusing nozzle to meet this challenge, with significantly reduced sample consumption, while improving jet stability over previous generations of nozzles. We demonstrate its use to determine the first room-temperature structure of RNA polymerase II at high resolution, revealing new structural details. Moreover, the double flow-focusing nozzles were successfully tested with three other protein samples and the first room temperature structure of an extradiol ring-cleaving dioxygenase was solved by utilizing the improved operation and characteristics of these devices., Published by Springer Nature, London

Scientific reports 7, 46846 (2017). doi:10.1038/srep46846, Serial femtosecond crystallography requires reliable and efficient delivery of fresh crystals across the beam of an X-ray free-electron laser over the course of an experiment. We introduce a double-flow focusing nozzle to meet this challenge, with significantly reduced sample consumption, while improving jet stability over previous generations of nozzles. We demonstrate its use to determine the first room-temperature structure of RNA polymerase II at high resolution, revealing new structural details. Moreover, the double flow-focusing nozzles were successfully tested with three other protein samples and the first room temperature structure of an extradiol ring-cleaving dioxygenase was solved by utilizing the improved operation and characteristics of these devices., Published by Nature Publishing Group, London