The organisation of proton motive and non-proton motive redox loops in prokaryotic respiratory systems

Respiration is fundamental to the aerobic and anaerobic energy metabolism of many prokaryotic and most eukaryotic organisms. In principle, the free energy of a redox reaction catalysed by a membrane-bound electron transport chain is transduced via the generation of an electrochemical ion (usually proton) gradient across a coupling membrane that drives ATP synthesis. The proton motive force (pmf) can be built up by different mechanisms like proton pumping, quinone/quinol cycling or by a redox loop. The latter couples electron transport to a net proton transfer across the membrane without proton pumping. Instead, charge separation is achieved by quinone-reactive enzymes or enzyme complexes whose active sites for substrates and quinones are situated on different sides of the coupling membrane. The necessary transmembrane electron transport is usually accomplished by the presence of two haem groups that face opposite sides of the membrane. There are many different enzyme complexes that are part of redox loops and their catalysed redox reactions can be either electrogenic, electroneutral (non-proton motive) or even pmf-consuming. This article gives conceptual classification of different operational organisations of redox loops and uses this as a platform from which to explore the biodiversity of quinone/quinol-cycling redox systems.