Conformational stability of CopC and roles of residues Tyr79 and Trp83.

CopC is a periplasmic copper Chaperone protein that has a β-barrel fold and two metal-binding sites distinct for Cu(II) and Cu(I). In the article, four mutants (Y79F, Y79W, Y79WW83L, Y79WW83F) were obtained by site-directed mutagenesis. The far-UV CD spectra of the proteins were similar, suggesting that mutations did not bring any significant changes in secondary structures. Meanwhile the effects of mutations on the protein's function were manifested by Cu(II) binding. Fluorescence lifetime measurement and quenching of tryptophan fluorescence by acrylamide and KI showed that the microenvironment around Trp83 was more hydrophobic than that around Tyr79 in apoCopC. Unfolding experiments induced by guanidinium chloride (GdnHCl), urea provided the conformational stability of each protein. The Δ<ΔG(0) element > obtained using the model of structural elements was used to show the role of Tyr79 and Trp83. On the one hand, the <ΔG(0) element > induced by urea for Y79F, Y79W have a loss of 6.51, 2.03 kJ/mol, respectively, compared with apoCopC, proving that replacement of Tyr79 by Phe or Trp all decreased the protein stability, meaning that the hydrogen bonds interactions between Tyr79 and Thr75 played an important role in stabilizing apoCopC. On the other hand, the <ΔG(0) element > induced by urea for Y79WW83L have a loss of 11.44 kJ/mol, but for Y79WW83F did a raise of 1.82 kJ/mol compared with Y79W. The replacement of Trp83 by Phe and Leu yields opposite effects on protein stability, which suggested that the aromatic ring of Trp83 was important in maintaining the hydrophobic core of apoCopC.