Comparison of energy-transducing capabilities of the two- and three-subunit cytochromes aa3 from Paracoccus denitrificans and the 13-subunit beef heart enzyme

In the accompanying paper, we have shown that the two-subunit cytochrome aa₃ isolated from Paracoccus denitrificans displays the same kind of complex and interactive redox behavior as the 13-subunit cytochrome aa₃ from beef heart. Therefore, the redox characteristics are not dependent on the additional 11 subunits. In the current work, we have examined the energy-transducing capabilities of both the two- and three-subunit enzymes obtained from Paracoccus denitrificans in relation to that of the 13-unit mammalian enzyme. We have found that in all of the tested functions, which included the development of ΔΨ and ΔpH, and the pumping of protons, that the two-subunit enzyme is at least as efficient as the structurally more complex mammalian enzyme. There is thus a correlation between the complex redox behavior and energy transducing capabilities of the two enzymes. There was also no difference in energy-transducing capabilities between the two- and three-subunit forms of the bacterial enzyme. It seems that only 2 subunits are required for an efficient energy-transducing cytochrome aa₃. The most likely role of the additional subunits in the mammalian enzyme, therefore, seems to be in regulation.