O₂-Tolerant H₂ Activation by an Isolated Large Subunit of a [NiFe] Hydrogenase

The catalytic properties of hydrogenases are nature's answer to the seemingly simple reaction H₂ ⇌ 2H⁺ + 2e^-. Members of the phylogenetically diverse subgroup of [NiFe] hydrogenases generally consist of at least two subunits, where the large subunit harbors the H₂-activating [NiFe] site and the small subunit contains iron-sulfur clusters mediating e^- transfer. Typically, [NiFe] hydrogenases are susceptible to inhibition by O₂. Here, we conducted system minimization by isolating and analyzing the large subunit of one of the rare members of the group of O₂-tolerant [NiFe] hydrogenases, namely the preHoxG protein of the membrane-bound hydrogenase from Ralstonia eutropha. Unlike previous assumptions, preHoxG was able to activate H₂ as it clearly performed catalytic hydrogen/deuterium exchange. However, it did not execute the entire catalytic cycle described for [NiFe] hydrogenases. Remarkably, H₂ activation was performed by preHoxG even in the presence of O₂, although the unique [4Fe-3S] cluster located in the small subunit and described to be crucial for tolerance toward O₂ was absent. These findings challenge the current understanding of O₂ tolerance of [NiFe] hydrogenases. The applicability of this minimal hydrogenase in basic and applied research is discussed.