Molecular architecture of cytochrome oxidase and its transition on treatment with alkali or sodium dodecyl sulfate.
In dimeric cytochrome oxidase [EC 1.9.3.1], one of the two heme a molecules of one monomeric unit has been proposed to be converted by the other unit, thus becoming latent in terms of catalytic functions (1). As the dimer was split into two monomers by treatment with alkali or sodium dodecyl sulfate (SDS), it was shown that the intensity of circular dichroism (CD) in the Soret region due to heme a decreased, probably reflecting release of the strain on the latent heme. On the other hand, the profile of magnetic circular dichroism (MCD) was nearly unchanged during this conversion, except for a weakening of the signal due to deprotonation of the heme during the alkali treatment. When the monomer was further dissociated into constituent subunits in strong alkali or at high concentrations of SDS, the CD spectrum disappeared almost completely, indicating loss of the asymmetric interactions of the chromophoric heme a with its immediate environments, consisting of the subunit assembly. The MCD pattern also suffered a small change as the dissociation proceeded, and a specific pattern appeared as the Schiff base was finally formed. The Schiff base formation of cytochrome oxidase in strong alkali proceeded in two steps whether the heme iron was in the oxidized or reduced state. As a consequence of the initial rapid reaction, the enzyme was suggested to have been disintegrated into constituent subunits with heme a being attached nonspecifically to either one, and structural characteristics dependent on the redox state were completely lost. The Arrehenius plot for this rapid change showed a break, indicating a transition in the structure of the cytochrome oxidase assembly, although no such phenomenon was observed during the slow reaction. Activation parameters in the rapid and slow reactions for the oxidized and reduced oxidase are given. Based on these findings, as well as other considerations, a molecular architecture of this enzyme is proposed; the role of heme a in anchoring four 14,000-dalton polypeptides into the minimal functional unit catalyzing the aerobic oxidation of ferrocytochrome c is emphasized.
Molecular architecture of cytochrome oxidase and its transition on treatment with alkali or sodium dodecyl sulfate. In dimeric cytochrome oxidase [EC 1.9.3.1], one of the two heme a molecules of one monomeric unit has been proposed to be converted by the other unit, thus becoming latent in terms of catalytic functions (1). As the dimer was split into two monomers by treatment with alkali or sodium dodecyl sulfate (SDS), it was shown that the intensity of circular dichroism (CD) in the Soret region due to heme a decreased, probably reflecting release of the strain on the latent heme. On the other hand, the profile of magnetic circular dichroism (MCD) was nearly unchanged during this conversion, except for a weakening of the signal due to deprotonation of the heme during the alkali treatment. When the monomer was further dissociated into constituent subunits in strong alkali or at high concentrations of SDS, the CD spectrum disappeared almost completely, indicating loss of the asymmetric interactions of the chromophoric heme a with its immediate environments, consisting of the subunit assembly. The MCD pattern also suffered a small change as the dissociation proceeded, and a specific pattern appeared as the Schiff base was finally formed. The Schiff base formation of cytochrome oxidase in strong alkali proceeded in two steps whether the heme iron was in the oxidized or reduced state. As a consequence of the initial rapid reaction, the enzyme was suggested to have been disintegrated into constituent subunits with heme a being attached nonspecifically to either one, and structural characteristics dependent on the redox state were completely lost. The Arrehenius plot for this rapid change showed a break, indicating a transition in the structure of the cytochrome oxidase assembly, although no such phenomenon was observed during the slow reaction. Activation parameters in the rapid and slow reactions for the oxidized and reduced oxidase are given. Based on these findings, as well as other considerations, a molecular architecture of this enzyme is proposed; the role of heme a in anchoring four 14,000-dalton polypeptides into the minimal functional unit catalyzing the aerobic oxidation of ferrocytochrome c is emphasized.