Redox Thermodynamics of Blue Copper Proteins

Abstract

The thermodynamic parameters of protein reduction (ΔH°‘rc and ΔS°‘rc) were measured for a number of blue copper proteins including spinach plastocyanin, cucumber plastocyanin, Pseudomonas aeruginosa azurin, Rhus vernicifera stellacyanin, cucumber stellacyanin, and horseradish umecyanin through voltammetric techniques in nonisothermal experiments at neutral pH. Including previous estimates for other members of the same protein family, we discuss here the thermodynamics of the electron-exchange reaction for twelve blue copper proteins from different sources. The enthalpic term (-ΔH°‘rc/F) turns out to be the dominant contribution to the reduction potential in this protein class. However, the entropic term (TΔS°‘rc/F) heavily affects E°‘, especially for the azurins. These data were analyzed in the light of the structural and dynamic information available on protein folding, geometric and electronic features of copper ligation, and solvation properties of the two redox states. It is clearly seen that the reduction enthalpy of the subfamily of the “phytocyanins” is less negative as compared to that of the other cupredoxins, most likely owing to a stronger axial ligation of the copper ion (which results in a nearly tetrahedral coordination geometry) and the greater exposition of the site to the solvent, which are both factors that stabilize the Cu(II) ion. The reduction entropy, which in most cases is negative, is instead apparently related to the solvation properties of the site. In addition, by analogy with class I cytochromes c, an increase in protein rigidity could also contribute to the entropy loss on reduction. Finally, it is apparent that the strategy of protein control of the reduction thermodynamics in high-potential electron-transfer metalloproteins (blue copper proteins, class I cytochromes c, HiPIPs) is the same: a dominant enthalpic term arising from ligand-binding interactions and electrostatic factors at the metal/protein interface, which strongly stabilizes the reduced state, is most often opposed by a weaker entropic term due to changes in protein dynamics and solvation properties, which disfavors protein reduction.