The Tetranuclear Copper-Sulfide Center of Nitrous Oxide Reductase.

Abstract

Nitrous oxide reductase catalyzes the reduction of nitrous oxide (N2O) to dinitrogen (N2) and water at a catalytic tetranuclear copper sulfide center, named CuZ, overcoming the high activation energy of this reaction. In this center each Cu atom is coordinated by two imidazole rings of histidine side-chains, with the exception of one named CuIV. This enzyme has been isolated with CuZ in two forms CuZ(4Cu1S) and CuZ(4Cu2S), which differ in the CuI-CuIV bridging ligand, leading to considerable differences in their spectroscopic and catalytic properties. The Cu atoms in CuZ(4Cu1S) can be reduced to the [4Cu1+] oxidation state, and its catalytic properties are compatible with the nitrous oxide reduction rates of whole cells, while in CuZ(4Cu2S) they can only be reduced to the [1Cu2C-3Cu1C] oxidation state, which has a very low turnover number. The catalytic cycle of this enzyme has been explored and one of the intermediates, CuZ0, has recently been identified and shown to be in the [1Cu2+-3Cu1+] oxidation state. Contrary to CuZ(4Cu2S), CuZ0 is rapidly reduced intramolecularly by the electron transferring center of the enzyme, CuA, to [4Cu1+] by a physiologically relevant redox partner. The three-dimensional structure of nitrous oxide reductase with the CuZ center either as CuZ(4Cu1S) or as CuZ(4Cu2S) shows that it is a unique functional dimer, with the CuZ of one subunit receiving electrons from CuA of the other subunit. The complex nature of this center has posed some questions relative to its assembly, which are only partially answered, as well as to which is the active form of CuZ in vivo. The structural, spectroscopic, and catalytic features of the two forms of CuZ will be addressed here, as well as its assembly. The understanding of its catalytic features, activation, and assembly is essential to develop strategies to decrease the release of nitrous oxide to the atmosphere and to reduce its concentration in the stratosphere, as well as to serve as inspiration to synthetic inorganic chemists to develop new models of this peculiar and challenging copper sulfide center.