Backbone dynamics of rusticyanin: the high hydrophobicity and rigidity of this blue copper protein is responsible for its thermodynamic properties

Local dynamics and solute-solvent exchange properties of rusticyanin (Rc) from Thiobacillus ferrooxidans have been studied by applying heteronuclear ((1)H, (15)N) NMR spectroscopy. (15)N relaxation parameters have been determined for the reduced protein, and a model-free analysis has been applied. The high average value of the generalized order parameter, S(2) (0.93), indicates that Rc is very rigid. The analysis of cross correlation rates recorded in both the reduced and the oxidized forms conclusively proves that Rc possesses the same dynamic features in both oxidation states. The accessibility of backbone amide protons to the solvent at different time scales has also been studied by applying specific heteronuclear pulse sequences and by H(2)O/D(2)O exchange experiments. These experiments reveal that rusticyanin is extremely hydrophobic. The first N-35 amino acids, not present in the other BCPs, protect the beta-barrel core from its interaction with the solvent, and thus, this is one of the main factors contributing to the hydrophobicity. Both characteristics (high rigidity and hydrophobicity) are maintained in the metal ion surroundings.