A spectral shift in cytochrome a induced by calcium ions

Ca2+ induces a red shift in the absorption spectrum of ferrocytochrome a when added to uncoupled mitochondria, sub-mitochondrial particles or isolated cytochrome aa3. The shift is identical within experimental error to the previously reported energy-linked shift in intact mitochondria (Wikström, M. K. F. (1972), Biochim. Biophys. Acta 283, 385-390). One mol of calcium produces the shift in one mole of cytochrome a, the KD being approx. 20-30 muM. The calcium-induced shift is readily reversed by chelating agents such as EDTA, ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA) and ATP and is insensitive to uncoupling agents and inhibitors of calcium transport (La3+ and ruthenium red). It is shown that the binding site for calcium that is responsible for the spectral shift is located on the outside of the permeability barrier of the mitochondrial cristae membrane. It is proposed that calcium simulates the energy-linked shift in cytochrome a by binding to a site of cytochrome aa3 that is occupied by protons in energized mitochondria and that is located at the external surface of the mitochondrial membrane.