The ligand geometry of copper determines the stability of amicyanin

Solution differential scanning calorimetry (DSC) of oxidized amicyanin, a Type I copper protein, at pH 7.5 reveals two thermal transitions. The major transition at 67.7 °C corresponds to the disruption of the Cys92 thiolate to Cu(II) charge transfer as evidenced by a corresponding temperature-dependent loss of amicyanin visible absorbance. A minor transition at 75.5 °C describes the further irreversible protein unfolding. Reduced amicyanin exhibits a pH-dependent change of the copper ligand geometry. At pH 8.5 where the Type I tetrahedral geometry is maintained, DSC reveals two thermal transitions with Tm values similar to that of oxidized amicyanin. At pH 6.2 where the Cu(I) coordination is trigonal planar, reduced amicyanin exhibits a single thermal transition with a lower Tm of 64.0 °C. Apoamicyanin, from which copper has been removed, also exhibits a single thermal transition but with a much lower Tm of 51.8 °C. Thus, the thermal stability of amicyanin is dictated both by the presence or absence of copper and its ligand geometry, but not its redox state. The physiological relevance of these data is discussed.