Possible molecular mechanisms of the protonmotive function of cytochrome systems

The object of this paper is to help to evolve a conceptual framework suitable for exploring and explaining the mechanisms of protonmotive cytochrome systems.

A review of some of the knowledge of the kinetic and equilibrium behaviour of the classical cytochrome systems of mitochondria indicates that the simple redox loop concept is not adequate for building a realistic conceptual model. In particular, the remarkable behaviour of the components of the cytochrome b- c ₁ complex, which has long been regarded as puzzling, cannot be explained on the basis of simple redox loop formulations. However, the newly introduced concepts of the protonmotive ubiquinone cycle, or Q cycle, and of the cyclic loop 2-3 system, which represent developments of the redox loop concept, are shown to provide a promising basis for the evolution of a satisfactory theory.

The mechanism of the Q cycle is discussed and developed in the light of experimental knowledge of classical mitochondrial cytochrome systems. The possible operation of a Q cycle in chloroplasts and bacteria is briefly discussed, and attention is drawn to bacterial cytochrome systems that appear to be organized as simple redox loops rather than as cyclic systems.

Some aspects of notions of direct chemiosmotic coupling and indirect conformational coupling are compared in the context of research strategy.