title: Characterization of the potentiometric behavior of soluble cytochrome oxidase by magnetic circular dichroism. Evidence in support of heme-heme interaction.
abstract: Magnetic circular dichroism spectroscopy has been used to characterize the oxidation-reduction behavior of cytochromes a and a3 during potentiometric experiments. The experiments were complicated by the presence of slow, time-dependent changes during reduction, and it appears that reliable results can only be obtained during reoxidation or with enzyme that has been subjected to a cycle of reduction and oxidation. Under all experimental conditions the reduction levels of cytochrome a and a3 are comparable. This result cannot be reconciled with a model in which the two heme centers have defined and well resolved potentials. The most straightforward explanation of the data requires an oxidation-reduction coupling of the potentials of the two hemes, i.e. an allosteric or heme-heme interaction, which is about 2 K cal/mol in magnitude. There is a good correlation between magnetic circular dichroism and EPR measurements obtained on parallel samples. The kinetics of the slow, time-dependent processes have been characterized by measurement of a variety of spectral properties and enzyme activity. All parameters measured change at comparable rates, implying a common rate-controlling event. A new copper EPR signal has been observed at high pH. This signal appears to rise from the "EPR-undetectable" copper center.
mesh_terms: D000818:Animals; D002417:Cattle; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D009206:Myocardium / Q000201:enzymology; D010084:Oxidation-Reduction; D011199:Potentiometry; D011487:Protein Conformation
chemical_list: D003574:Cytochrome c Group; D006418:Heme; D003576:Electron Transport Complex IV