Reversible Glutamate Coordination to High-Valent Nickel Protects the Active Site of a [NiFe] Hydrogenase from Oxygen.

NAD + -reducing [NiFe] hydrogenases are valuable biocatalysts for H 2 -based energy conversion and the regeneration of nucleotide cofactors. While most hydrogenases are sensitive toward O 2 and elevated temperatures, the soluble NAD + -reducing [NiFe] hydrogenase from Hydrogenophilus thermoluteolus ( Ht SH) is O 2 -tolerant and thermostable. Thus, it represents a promising candidate for biotechnological applications. Here, we have investigated the catalytic activity and active-site structure of native Ht SH and variants in which a glutamate residue in the active-site cavity was replaced by glutamine, alanine, and aspartate. Our biochemical, spectroscopic, and theoretical studies reveal that at least two active-site states of oxidized Ht SH feature an unusual architecture in which the glutamate acts as a terminal ligand of the active-site nickel. This observation demonstrates that crystallographically observed glutamate coordination represents a native feature of the enzyme. One of these states is diamagnetic and characterized by a very high stretching frequency of an iron-bound active-site CO ligand. Supported by density-functional-theory calculations, we identify this state as a high-valent species with a biologically unprecedented formal Ni(IV) ground state. Detailed insights into its structure and dynamics were obtained by ultrafast and two-dimensional infrared spectroscopy, demonstrating that it represents a conformationally strained state with unusual bond properties. Our data further show that this state is selectively and reversibly formed under oxic conditions, especially upon rapid exposure to high O 2 levels. We conclude that the kinetically controlled formation of this six-coordinate high-valent state represents a specific and precisely orchestrated stereoelectronic response toward O 2 that could protect the enzyme from oxidative damage.