THE ROLE OF THE ELECTROCHEMICAL GRADIENT OF PROTONS IN SOLUTE TRANSPORT IN BACTERIA

Abstract

The chemiosmotic energy-generating processes comprise membrane-bound energy transducing systems such as electron-transfer chains and the Ca2+, Mg2+-stimulated ATPase complex. These systems act as electrogenic proton pumps that translocate protons across the cytoplasmic membrane from the cytoplasm to the external medium. As a result, an electrochemical gradient of protons is formed that exerts an inwardly directed force on the protons, the proton motive force. This proton motive force is the driving force for several energy-requiring processes in the cytoplasmic membrane such as solute and ion transport (secondary transport), and flagellar movement, reversed electron flow and the transhydrogenase reaction. The main proton motive force generating systems (primary transport systems) are the electron transfer chains and the Ca2+, Mg2+-stimulated ATPase complex. In aerobic bacteria, the proton motive force can be generated by a respiratory chain in which oxygen functions as the terminal electron acceptor. In phototrophic bacteria, the light-induced cyclic electron transfer systems, and in anaerobic bacteria, electron transfer to electron acceptors other than oxygen can generate a proton motive force. In fermentative bacteria, the proton-motive-force-driven solute transport systems can operate in the reversed direction so that efflux of end products can generate a proton-motive force.