The caa(3) terminal oxidase of Rhodothermus marinus lacking the key glutamate of the D-channel is a proton pump

The thermohalophilic bacterium Rhodothermus marinus expresses a caa(3)-type dioxygen reductase as one of its terminal oxidases. The subunit I amino ac id sequence shows the presence of all the essential residues of the D- and K-proton channels, defined in most heme-copper oxidases, with the exception of the key glutamate residue located in the middle of the membrane dielect ric (E278 in Paracoccus denitrificans). On the basis of homology modeling s tudies, a tyrosine residue (Y256, R. marinus numbering) has been proposed t o act as a functional substitute [Pereira, M. M., Santana, M., Soares, C. M ., Mendes, J., Carita, J. N., Fernandes, A. S., Saraste, M., Carrondo, M. A ., and Teixeira, M. (1999) Biochim. Biophys. Acta 1413, 1-13]. Here, R. mar inus caa(3) oxidase was reconstituted in liposomes and shown to operate as a proton pump, translocating protons from the cytoplasmic side of the bacte rial inner membrane to the periplasmatic space with a stoichiometry of 1H()/e(-), as in the case in heme-copper oxidases that contain the glutamate r esidue. Possible mechanisms of proton transfer in the D-channel with the pa rticipation of the tyrosine residue are discussed. The observation that the tyrosine residue is conserved in several other members of the heme-copper oxidase superfamily suggests a common alternative mode of action for the D- channel.