The nature of the ions that are translocated by Escherichia coli and Paracoccus denitrificans complexes I was investigated. We observed that E. coli complex I was capable of proton translocation in the same direction to the established ΔΨ, showing that in the tested conditions, the coupling ion is the H+. Furthermore, Na+transport to the opposite direction was also observed, and, although Na+was not necessary for the catalytic or proton transport activities, its presence increased the latter. We also observed H+translocation by P. denitrificans complex I, but in this case, H+transport was not influenced by Na+and also Na+transport was not observed. We concluded that E. coli complex I has two energy coupling sites (one Na+independent and the other Na+dependent), as previously observed for Rhodothermus marinus complex I, whereas the coupling mechanism of P. denitrificans enzyme is completely Na+independent. This work thus shows that complex I energy transduction by proton pumping and Na+/H+antiporting is not exclusive of the R. marinus enzyme. Nevertheless, the Na+/H+antiport activity seems not to be a general property of complex I, which may be correlated with the metabolic characteristics of the organisms. © 2010 Elsevier B.V.
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