Redox thermodynamics of blue copper proteins

The thermodynamic parameters of protein reduction (Delta H degrees'(rc) and Delta S degrees'(rc)) were measured for a number of blue copper proteins i ncluding spinach plastocyanin, cucumber plastocyanin, Pseudomonas aeruginos a azurin, Rhus vernicifera stellacyanin, cucumber stellacyanin, and horsera dish 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 entha lpic term (-Delta H degrees'(rc)/F) turns out to be the dominant contributi on to the reduction potential in this protein class. However, the entropic term (T Delta S degrees'(rc)/F) heavily affects E degrees', especially for the azurins. These data were analyzed in the light of the structural and dy namic information available on protein folding, geometric and electronic fe atures of copper ligation, and solvation properties of the two redox states . It is clearly seen that the reduction enthalpy of the subfamily of the "p hytocyanins" is less negative as compared to that of the other cupredoxins, most likely owing to a stronger axial ligation of the copper ion (which re sults in a nearly tetrahedral coordination geometry) and the greater exposi tion 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 inst ead apparently related to the salvation properties of the site. In addition , by analogy with class I cytochromes c, an increase in protein rigidity co uld also contribute to the entropy loss on reduction. Finally, it is appare nt that the strategy of protein control of the reduction thermodynamics in high-potential electron-transfer metalloproteins (blue copper proteins, cla ss I cytochromes c, HiPIPs) is the same: a dominant enthalpic term arising from ligand-binding interactions and electrostatic factors at the metal/pro tein interface, which strongly stabilizes the reduced state, is most often opposed by a weaker entropic term due to changes in protein dynamics and sa lvation properties, which disfavors protein reduction.