Spectroscopic and density functional studies of the red copper site in nitrosocyanin: role of the protein in determining active site geometric and electronic structure.
The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the sigma rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 A) compared to plastocyanin (2.08 A) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSC(beta) planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by approximately 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.
Basumallick
Lipika
L
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Sarangi
Ritimukta
R
DeBeer George
Serena
S
Elmore
Brad
B
Hooper
Alan B
AB
Hedman
Britt
B
Hodgson
Keith O
KO
Solomon
Edward I
EI
D001426:Bacterial Proteins / Q000737:chemistry* / Q000302:isolation & purification; D001665:Binding Sites; D002942:Circular Dichroism; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D008667:Metalloproteins / Q000737:chemistry* / Q000302:isolation & purification; D008958:Models, Molecular; D015394:Molecular Structure; D043370:Nitrosomonas europaea / Q000737:chemistry; D010084:Oxidation-Reduction; D013059:Spectrum Analysis, Raman
D001426:Bacterial Proteins; D008667:Metalloproteins; C431624:red copper protein, Nitrosomonas europaea; D003300:Copper