Binuclear CopperA

Abstract

The purple CuA is a Cys‐bridged, mixed‐valence electron transfer center [Cu(1.5+)…Cu(1.5+)], found in cytochrome c oxidase and in nitrous oxide reductase. More recently, the NO reductase from Bacillus azotoformans was identified as the third CuA‐containing enzyme. There also exist engineered CuA sites which have been introduced by loop‐directed mutagenesis into blue type 1 copper proteins, such as azurin or amicyanin. Electron transfer mediated by CuA is very efficient. The reasons for utilizing a binuclear electron transfer center in cytochrome c oxidase and nitrous oxide reductase, instead of mononuclear type 1 Cu, are suggested to be its unidirectional electron transfer through the site or the lower energy of reorganization. Interest in CuA is directly related to its unique spectroscopic properties. Studies by paramagnetic resonance, magnetic circular dichroism, resonance Raman, and X‐ray absorption spectroscopy have provided a clear picture of the electronic properties of the CuA site and the interaction of the metal atoms with the neighboring amino acids.

Multifrequency EPR and Redox Reactivity Investigations of a Bis(μ-thiolato)-dicopper(II,II) Complex

“…Two N(His) and two additional ligands (one S(Met) and one O carbonyl of a glutamate in C c O 1-3 or of a tryptophan in N 2 OR 4 ) in axial positions complete the coordination sphere. The purple color of the Cu A center is the result of predominant S(Cys) → Cu charge-transfer transitions at ∼480 and ∼530 nm and a broad class III mixed-valent charge-transfer band around ∼830 nm. , The delocalized mixed-valent configuration Cu 1.5 Cu 1.5 is consistent with the multifrequency EPR data . The well-resolved seven-line pattern evidences the unpaired electron interaction with two I = 3/2 copper centers as described for N 2 OR …”

Kinetics of Copper Incorporation into a Biosynthetic Purple CuAAzurin: Characterization of Red, Blue, and a New Intermediate Species

“…The Cu A center is found in numerous organisms as the electron entry point and transfer hub for aerobic (cytochrome c oxidase, C c O, and a terminal oxidase, SoxH, in Sulfolobus acidocaldarius ) and anaerobic (N 2 OR and nitric oxide reductase, NOR , ) respiration. Its unique coordination and electron transfer (ET) properties have generated great interest in the Cu A site. ,− While the Cu A center itself is natively found in small cupredoxin-like domains, these domains are associated in some cases with large, membrane-bound complexes − and always with enzymes containing many other metallochromophores in the as-isolated form, which complicates studies of the Cu A site. ,− Thus, several systems have been developed to yield a soluble protein with a Cu A site that is free from other chromophores, including soluble truncates of native Cu A proteins − and biosynthetic models. − One such biosynthetic model, Cu A azurin (Cu A Az), was engineered from the blue copper cupredoxin, azurin, through loop-directed mutagenesis . Extensive spectroscopic, X-ray crystallographic, and electron transfer studies have shown Cu A Az to be an excellent electronic, structural, and functional model of native Cu A centers. ,,,− …”

Biological and Bioinspired Inorganic N–N Bond-Forming Reactions

“…Cu A is a binuclear Cu center, similar to the one present in cytochrome c oxidases, 366,367 that is found within a loop of the cupredoxin fold at the C-terminus of NosZ. Two fully conserved Cys residues are the bridging ligands of the two Cu ions that lie 2.5 Å apart (Figure 35a).…”

“…389 In its oxidized form, it is found in a mixed-valent [Cu A1 1.5+ : Cu A2 1.5+ ] state with a net charge of +3 and an S = 1 2 spin state, 375 similar to some HCOs. 366,390 In UV-vis absorption spectra of holoproteins, oxidized Cu A contributes absorption maxima at 480, 525-540, and 800 nm (Figure 39). The single unpaired electron that is fully delocalized over both Cu nuclei gives rise to a narrow seven-line hyperfine splitting in the g || region of EPR spectra, characteristic of two nuclear spins…”