Metal−Ligand Interplay in Blue Copper Proteins Studied by 1H NMR Spectroscopy: Cu(II)−Pseudoazurin and Cu(II)−Rusticyanin

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Abstract
The blue copper proteins (BCPs), pseudoazurin from Achromobacter cycloclastes and rusticyanin from Thiobacillus ferrooxidans, have been investigated by 1H NMR at a magnetic field of 18.8 T. Hyperfine shifts of the protons belonging to the coordinated ligands have been identified by exchange spectroscopy, including the indirect detection for those resonances that cannot be directly observed (the β-CH2 of the Cys ligand, and the NH amide hydrogen bonded to the Sγ(Cys) atom). These data reveal that the Cu(II)−Cys interaction in pseudoazurin and rusticyanin is weakened compared to that in classic blue sites (plastocyanin and azurin). This weakening is not induced by a stronger interaction with the axial ligand, as found in stellacyanin, but might be determined by the protein folding around the metal site. The average chemical shift of the β-CH2 Cys ligand in all BCPs can be correlated to geometric factors of the metal site (the Cu−Sγ(Cys) distance and the angle between the CuNHisNHis plane and the Cu−Sγ(Cys) vector). It is concluded that the degree of tetragonal distortion is not necessarily related to the strength of the Cu(II)−Sγ(Cys) bond. The copper−His interaction is similar in all BCPs, even for the solvent-exposed His ligand. It is proposed that the copper xy magnetic axes in blue sites are determined by subtle geometrical differences, particularly the orientation of the His ligands. Finally, the observed chemical shifts for β-CH2 Cys and Ser NH protons in rusticyanin suggest that a less negative charge at the sulfur atom could contribute to the high redox potential (680 mV) of this protein.

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