The F0 complex of the ATP synthase of Escherichia coli contains a proton pathway with large proton polarizability caused by collective proton fluctuation.

The F0 complex of the Escherichia coli ATP synthase embedded into cardiolipin liposomes was studied by FT-IR spectroscopy. For comparison, respective studies were performed with dried F0 liposomes and with F0 liposomes treated with N,N'-dicyclohexyl-carbodiimide (DCCD), which binds to Asp-61 of subunit c. Furthermore, the effect of H2O-->D2O exchange on the infrared spectrum was investigated. With F0 liposomes an infrared continuum is observed beginning at about 3000 cm-1 and extending toward smaller wavenumbers. In the DCCD-treated sample, this continuum is no longer observed. It vanishes also with drying of the liposomes. After H2O-->D2O exchange, this infrared continuum begins at about 2350 cm-1 and is less intense. All of these results demonstrate that a proton pathway in native F0 is present, in which the protons are shifted in a hydrogen-bonded chain with large proton polarizability due to collective proton tunneling. With the D2O-hydrated system, deuteron polarizability due to collective deuteron motion is observed, but the polarizability due to collective deuteron motion is smaller. Such pathways are very efficient, because they conduct protons or deuterons within picoseconds. These pathways lose their polarizability if the F0 complex is blocked by DCCD or if the liposomes are dried. On the basis of our results on the proton polarizability of hydrogen bonds and hydrogen-bonded systems and on the basis of structural data from the literature, the nature of the proton pathway of the F0 complex of E. coli is discussed.