X-ray absorption spectroscopy of folded and unfolded copper(I) azurin

The structure of the copper site in folded and unfolded copper(I) azurin has been investigated by X-ray absorption spectroscopy (XAS). Analysis of the Cu K-edge spectra demonstrates that Cu(I) occupies a trigonal coordination site in the unfolded protein; and EXAFS data indicate a structure with 1.5–2 CuS(Cl) and 1.5–1 CuN(O) bonds. It is likely that the cysteine S and one of the two histidine N atoms remain coordinated in the unfolded structure, and that the third ligand is S(Met) or Cl−. The lengths of both the CuS(Cys) and CuN(His) bonds increase by ∼0.1 Å on unfolding, in accord with the view that copper coordination is constrained by the protein.