The active form of the cytochrome d terminal oxidase complex of Escherichia coli is a heterodimer containing one copy of each of the two subunits.

The cytochrome d complex is a component of the aerobic respiratory system of Escherichia coli. The enzyme functions as a terminal oxidase, oxidizing ubiquinol-8 within the cytoplasmic membrane and reducing oxygen to water. The enzyme is of particular interest because it is a coupling site in the electron transfer chain. The electron transfer reaction catalyzed by this enzyme is coupled to the translocations of protons across the membrane (H+/e-approximately equal to 1). The oxidase contains two subunits by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, with molecular weights of 58,000 and 43,000. In this paper, the question of the quaternary structure is addressed. Quantitative N-terminal analysis of the isolated enzyme and relative mass quantitation following sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicate the subunits are present in equimolar amounts. Sedimentation velocity and sedimentation equilibrium studies were used to characterize the hydrodynamic properties of the purified enzyme solubilized in Triton X-100, under conditions where the enzyme is active. It is concluded that the active enzyme in Triton X-100 is a heterodimer, containing one copy of each subunit. This is likely the structure of the enzyme in the E. coli membrane.