Copper coordination in blue proteins

Abstract

The spectroscopic and electrochemical properties of blue copper proteins are strikingly different from those of inorganic copper complexes in aqueous solution. Over three decades ago this unusual behavior was ascribed to constrained coordination in the folded protein; consistent with this view, crystal structure determinations of blue proteins have demonstrated that the ligand positions are essentially unchanged on reduction as well as in the apoprotein. Blue copper reduction potentials are tuned to match the particular function of a given protein by exclusion of water from the metal site and strict control of the positions of axial ligands in the folded structure. Extensive experimental work has established that the reorganization energy of a prototypal protein, Pseudomonas aeruginosa azurin, is approximately 0.7 eV, a value that is much lower than those of inorganic copper complexes in aqueous solution. The lowered reorganization energy in the protein, which is attributable to constrained coordination, is critically important for function, since the driving forces for electron transfer often are low (approximately 0.1 eV) between blue copper centers and distant (>10 A) donors and acceptors.

Structure and function of type I copper in multicopper oxidases

“…Redox potentials of the copper centers in MCOs have been determined by measuring the absorption spectrum and potential of the partly reduced states of MCO in situ [6,16,[76][77][78]. The redox potential of type I copper ranges over ca.…”

“…Most of the blue copper proteins plastocyanin, azurin, pseudoazurin, amicyanin, rusticyanin, auracyanin, umecyanin, mavicyanin, plantacyanin (cucumber basic protein (CBP)), and stellacyanin have been crystallized and their structural data are available [4,16]. On the other hand, crystal structure analyses of MCOs comprised of more than 500 amino acids have been limited until recently.…”

Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin

“…One of the most intensively studied group of ET proteins is the blue copper proteins (e.g., plastocyanin, azurin, and the family of multi‐copper oxidases) 1. The copper binding sites were shown to confer unique spectroscopic and thermodynamic properties on the copper ions 2. These properties attracted studies aiming at understanding their role in the ET via proteins.…”

Bioenergetics and Biological Electron Transport

“…The blue copper proteins are another example where the potentials of the redox centre span a range of values [88], in this case from 0.37 V vs NHE in P. nigra plastocyanin to 0.785 V vs. NHE in Polyporus versicolor laccase. Again, these differences can be explained by differences in the axial ligands around the Cu centre and by the degree of hydrophobicity of the polypeptide surrounding the redox centre.…”