Design principles of proton-pumping haem-copper oxidases

Transmembrane electrochemical proton gradients are used to store free energy in biological systems, and to drive the synthesis of biomolecules and transmembrane transport. These gradients are maintained by membrane-bound proton transporters that employ free energy provided by, for example, electron transfer or light. In recent years, the structures of several membrane proteins involved in proton translocation have been determined, and indicate that both protein-bound water molecules and protonatable amino acid residues play central roles in transmembrane proton conduction. From these structures, in combination with functional studies, have emerged general principles of proton transfer across membranes and control mechanisms for such reactions, in particular with regard to the electron-transfer-driven proton pump cytochrome c oxidase.