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 delta psi and delta pH, 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. © 1991, The Biophysical Society. All rights reserved.
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