Characterization of the iron-sulfur clusters in the nitrogenase-like reductase CfbC/D required for coenzyme F430 biosynthesis.

Coenzyme F430 is a nickel-containing tetrapyrrole, serving as the prosthetic group of methyl-coenzyme M reductase in methanogenic and methanotrophic archaea. During coenzyme F430 biosynthesis, the tetrapyrrole macrocycle is reduced by the nitrogenase-like CfbC/D system consisting of the reductase component CfbC and the catalytic component CfbD. Both components are homodimeric proteins, each carrying a [4Fe-4S] cluster. Here, the ligands of the [4Fe-4S] clusters of CfbC2 and CfbD2 were identified revealing an all cysteine ligation of both clusters. Moreover, the midpoint potentials of the [4Fe-4S] clusters were determined to be -256 mV for CfbC2 and -407 mV for CfbD2. These midpoint potentials indicate that the consecutive thermodynamically unfavorable 6 individual "up-hill" electron transfers to the organic moiety of the Ni2+-sirohydrochlorin a,c-diamide substrate require an intricate interplay of ATP-binding, hydrolysis, protein complex formation and release to drive product formation, which is a common theme in nitrogenase-like systems.

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 H2-based energy conversion and the regeneration of nucleotide cofactors. While most hydrogenases are sensitive toward O2 and elevated temperatures, the soluble NAD+-reducing [NiFe] hydrogenase from Hydrogenophilus thermoluteolus (HtSH) is O2-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 HtSH 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 HtSH 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 O2 levels. We conclude that the kinetically controlled formation of this six-coordinate high-valent state represents a specific and precisely orchestrated stereoelectronic response toward O2 that could protect the enzyme from oxidative damage.