Microorganisms have long used hydrogen as an energy source. To do this, they rely on hydrogenases that contain metals such as nickel or iron in their catalytic center. In order to use these biocatalysts for hydrogen conversion, researchers around the world are working to understand the details of the catalysis process. A team from three Max Planck Institutes (MPI), the Center for Biostructural Imaging of Neurodegeneration (BIN) at the University Medical Center Göttingen (UMG), the University of Kiel, and the FACCTs GmbH used a chemical peculiarity of hydrogen to amplify the signals of magnetic resonance spectroscopy. In this way, the scientists were able to visualize previously unknown intermediate steps in the conversion of hydrogen.
As a substitute for fossil fuels, energy source, or catalyst in chemical processes – hydrogen is considered a good candidate for a sustainable energy economy. On Earth, the element occurs mainly in bound form, in water, as hydrogen gas, or in fossil raw materials such as natural gas and crude oil. To obtain hydrogen in its pure form, it must be split from the chemical compound using energy. The most common method of producing hydrogen today is the steam methane reforming of natural gas. However, this also produces climate-damaging carbon dioxide (CO₂). In the catalytic production of hydrogen from water, electrodes made of the precious metal platinum have mostly been used up to now. This makes hydrogen production by means of catalysis comparatively expensive.
Figure 1: Middle: [Fe]-hydrogenase catalytic cycle with sensitivity-enhanced species highlighted. Right: Single-scan NMR spectrum showing hydrogen signals obtained for hydrogen (H2) and hydrogen deutride (HD) after release from the enzyme. Left: Overlay of measured and simulated parahydrogen enhanced chemical shift saturation transfer (PHIP-CEST) data, directly probing the hydrogen atoms in an enzyme bound state.
© Lukas Kaltschnee / Max Planck Institute for Multidisciplinary Sciences & Center for Biostructural Imaging of Neurodegeneration
Many microorganisms are a step ahead of these production processes. To split off hydrogen to generate energy, they use three different types of hydrogenases that function without precious metals and do not release CO2: [NiFe] hydrogenases from archaea and bacteria, [FeFe] hydrogenases from bacteria, some algae, and some anaerobic archaea, as well as [Fe] hydrogenases found only in archaea. The latter play a key role in methanogenesis, in which CO2 is reduced to methane (CH4). The homodimeric [Fe] hydrogenase contains one redox-inactive iron (Fe) per subunit, which is bound to a guanylylpyridinol cofactor.
While intermediates in the catalytic cycle of [NiFe] hydrogenases and [FeFe] hydrogenases have already been well studied, the catalytic intermediates of [Fe] hydrogenases were not observable – until now. A research team led by Stefan Glöggler (Max Planck Institute for Multidisciplinary Sciences (MPI-NAT) and the Center for Biostructural Imaging of Neurodegeneration (BIN) at the University Medical Center Göttingen (UMG), Lukas Kaltschnee (MPI-NAT and BIN at UMG, currently at the TU Darmstadt), Christian Griesinger (MPI-NAT), and Seigo Shima (MPI for Terrestrial Microbiology), together with colleagues from the MPI für Kohlenforschung, Kiel University, and the FACCTs GmbH, have now succeeded in detecting the intermediates in the [Fe]-hydrogenases catalysis cycle for the first time.
Figure 2: Geometry-optimized structure of the iron hydride species formed during hydrogen activation by the [Fe]-hydrogenase. The two hydrogen atoms shown in bright gloom were signal-enhanced for indirect intermediate detection by nuclear magnetic resonance spectroscopy during catalysis.
© Lukas Kaltschnee / Max Planck Institute for Multidisciplinary Sciences & Center for Biostructural Imaging of Neurodegeneration
They thereby made use of the fact that hydrogen occurs as so-called parahydrogen and orthohydrogen, depending on its nuclear spin. The researchers showed that nuclear magnetic resonance spectroscopy results in signal amplification when the [Fe] hydrogenase reacts with parahydrogen. This so-called parahydrogen-induced polarization (PHIP) made it possible to identify the reaction intermediates and visualize how the [Fe] hydrogenase binds hydrogen during catalysis. The scientists’ data indicate that a hydride is formed at the iron center during catalysis. The new method also made it possible to study the binding kinetics. Due to its high sensitivity, PHIP is particularly promising for application to living cells and for investigating hydrogen metabolism in vivo. The results could help to develop (bio)catalysts for hydrogen conversion with higher productivity in the future.
Original Publication:
Lukas Kaltschnee, Andrey N. Pravdivtsev, Manuel Gehl, Gangfeng Huang, Georgi L. Stoychev, Christoph Riplinger, Maximilian Keitel, Frank Neese, Jan-Bernd Hövener, Alexander A. Auer, Christian Griesinger, Seigo Shima, Stefan Glöggler:
Parahydrogen-enhanced magnetic resonance identification of intermediates in the active [Fe]-hydrogenase catalysis Nature Catalysis 2024, accepted. DOI: 10.1038/s41929-024-01262-w
