Bärbel Friedrich

Hydrogenases are nature’s catalysts designed to extract protons from molecular hydrogen (H2) thereby forming a reactive hydride species (H−). Those enzymes capable of catalyzing the complete reaction, H2 ↔ 2 e− + 2 H+, come in two different flavors, namely [NiFe]- and [FeFe]-hydrogenases [1, 2, 3]. They are assigned according to the metal content in their active sites. [FeFe]-hydrogenases, which are described in detail in Chapters 3 and 7, are usually highly active in the direction of proton reduction and display turnover rates of up to 20,000 hydrogen molecules per second [4]. However, [FeFe]-hydrogenases are exclusively synthesized under strictly anoxic conditions in their host organisms, and are immediately, and in most cases irreversibly, inactivated by traces of O2. The proposed mechanism of O2-mediated inactivation is described in Chapters 3 and 5. This property makes it extremely difficult to use these enzymes for applied purposes under aerobic conditions.

In this consensus paper resulting from a meeting that involved representatives from more than 20 European partners, we recommend the foundation of an expert group (European Steering Committee) to assess the potential benefits and draw-backs of genome editing (off-targets, mosaicisms, etc.), and to design risk matrices and scenarios for a responsible use of this promising technology. In addition, this European steering committee will contribute in promoting an open debate on societal aspects prior to a translation into national and international legislation.

The membrane-bound [NiFe] hydrogenase (MBH) of Ralstonia eutropha H16 undergoes a complex maturation process comprising cofactor assembly and incorporation, subunit oligomerization, and finally twin-arginine-dependent membrane translocation. Due to its outstanding O ₂ and CO tolerance, the MBH is of biotechnological interest and serves as a molecular model for a robust hydrogen catalyst. Adaptation of the enzyme to oxygen exposure has to take into account not only the catalytic reaction but also biosynthesis of the intricate redox cofactors. Here, we report on the role of the MBH-specific accessory proteins HoxR and HoxT, which are key components in MBH maturation at ambient O ₂ levels. MBH-driven growth on H ₂ is inhibited or retarded at high O ₂ partial pressure (pO ₂ ) in mutants inactivated in the hoxR and hoxT genes. The ratio of mature and nonmature forms of the MBH small subunit is shifted toward the precursor form in extracts derived from the mutant cells grown at high pO ₂ . Lack of hoxR and hoxT can phenotypically be restored by providing O ₂ -limited growth conditions. Analysis of copurified maturation intermediates leads to the conclusion that the HoxR protein is a constituent of a large transient protein complex, whereas the HoxT protein appears to function at a final stage of MBH maturation. UV-visible spectroscopy of heterodimeric MBH purified from hoxR mutant cells points to alterations of the Fe-S cluster composition. Thus, HoxR may play a role in establishing a specific Fe-S cluster profile, whereas the HoxT protein seems to be beneficial for cofactor stability under aerobic conditions.

Molecular hydrogen is widely used by microorganisms as a source of energy. One of the best studied aerobic hydrogen oxidizers, the beta-proteobacterium Ralstonia eutropha (formerly Alcaligenes eutrophus), harbors two distinct [NiFe]-hydrogenases which catalyze the heterolytic cleavage of H2 into 2H+ and 2e-. The genes encoding the hydrogenase subunits are arranged in two large operons together with accessory and regulatory genes involved in hydrogenase biosynthesis. Both operons are transcribed from strong sigma54-dependent promoters. Transcription requires the activation by the HoxA protein, a member of the NtrC family of response regulators. HoxA is only active when H2 is present in the environment. H2 recognition is mediated by a signal transduction complex consisting of the soluble histidine protein kinase HoxJ and a regulatory [NiFe]-hydrogenase which acts as an H2 receptor. Biochemical and genetic data suggest that signal transduction between the RH and HoxJ involves an electron transport process. According to our current model the histidine protein kinase HoxJ inactivates HoxA by phosphorylation in the absence H2. This property of the HoxJ-HoxA regulator pair is quite different from the behaviour of common two-component regulatory systems. Phosphorylation of HoxA is blocked in the presence of H2 provided the RH can contact HoxJ and transmit the signal to the kinase. Furthermore, hydrogenase gene expression is subject to a global regulatory network in response to the carbon and energy source. HoxA is a major component of this epistatic control the molecular mechanism of which is not yet understood.