Energy extraction from air: structural basis of atmospheric hydrogen oxidation

Diverse aerobic bacteria use atmospheric H 2 as an energy source for growth and survival. This recently discovered yet globally significant process regulates the composition of the atmosphere, enhances soil biodiversity, and drives primary production in certain extreme environments. Atmospheric H 2 oxidation has been attributed to still uncharacterised members of the [NiFe]-hydrogenase superfamily. However, it is unresolved how these enzymes overcome the extraordinary catalytic challenge of selectively oxidizing picomolar levels of H 2 amid ambient levels of the catalytic poison O 2 , and how the derived electrons are transferred to the respiratory chain. Here we determined the 1.52 Å resolution CryoEM structure of the mycobacterial hydrogenase Huc and investigated its mechanism by integrating kinetics, electrochemistry, spectroscopy, mass spectrometry, and molecular dynamics simulations. Purified Huc is an oxygen-insensitive enzyme that couples the oxidation of atmospheric H 2 at its large subunit to the hydrogenation of the respiratory electron carrier menaquinone at its small subunit. The enzyme uses a narrow hydrophobic gas channel to selectively bind atmospheric H 2 at the expense of O 2 , while three [3Fe-4S] clusters and their unusual ligation by a D-histidine modulate the electrochemical properties of the enzyme such that atmospheric H 2 oxidation is energetically feasible. Huc forms an 833 kDa complex composed of an octamer of catalytic subunits around a membrane-associated central stalk, which extracts and transports menaquinone a remarkable 94 Å from the membrane, enabling its reduction. These findings provide a mechanistic basis for the biogeochemically and ecologically critical process of atmospheric H 2 oxidation. Through the first characterisation of a group 2 [NiFe]-hydrogenase, we also uncover a novel mode of energy coupling dependent on long-range quinone transport and pave way for the development of biocatalysts that oxidize H 2 in ambient air.