The role played by the a-helix in the unfolding pathway and stability of azurin: switching between hierarchic and non hierarchic folding

The role played by the a-helix in determining the structure, the stability and the unfolding mechanism of azurin was addressed by studying a helix-depleted azurin variant produced by sitedirected mutagenesis. The protein structure was investigated by CD, 1D 1H NMR, fluorescence spectroscopy measurements and MD simulations, whilst EPR, UV-visible and cyclic voltammetry experiments were carried out to investigate the geometry and the properties of the CuII site. The effects of the a-helix depletion on the thermal stability and the unfolding pathway of the protein were determined by DSC, UV/visible and fluorescence measurements at increasing temperature. The results show that, in the absence of the a-helix segment, the overall protein structure is maintained, and that only the Cu site is slightly modified. In contrast, the protein stability is diminished by about 60% with respect to the wild-type azurin. Moreover, the unfolding pathway of the mutant azurin involves the presence of detectable intermediates. In comparison with previous studies concerning other small b-sheet cupredoxins, the results as a whole support the hypothesis that the presence of the a-helix can switch the folding of azurin from a hierarchic to a nonhierarchic mechanism in which the highly conserved b-sheet core provides a scaffold for cooperative folding of the wild-type protein.