Reversibility of Energy Transformations in the Respiratory Chain

In oxidative phosphorylation, the energy from the combustion of substrate hydrogen can be reversibly transformed into phosphate‐bond energy. This is the reason that oxidation‐reduction reactions in the respiratory chain can take place against the redox potential gradient if energy is supplied. The latter can come from ATP or directly from energy‐rich intermediates arising in the course of oxidative phosphorylation. – At the center of the present considerations is the postulate that there is an equilibrium prevailing in the respiratory chain that results from the reversibility of the reactions. At the same time this postulate provides a unifying approach to various phenomena related to the process of oxidative phosphorylation. Such an equilibrium may involve several components of the respiratory chain. Respiration thus corresponds to a dynamic equilibrium in the respiratory chain which deviates increasingly from the static equilibrium the greater the rate of respiration. Consequently, respiration can be considered as being regulated by the phosphorylation potential. – The stationary redox state of the components of the respiratory chain can be viewed as a simulataneous function of the phosphorylation potential and of the redox potential that effects both ends of the respiratory chain. The latter reacts to either extreme, i.e. to minimum or maximum differences in the redox potential, by forming characteristic patterns of the state of reduction of its components. Differences in redox potential amounting to as much as 280 mV can be over‐come by the phosphorylation potential for a single phosphorylative step. – The conditions for an equilibrium in the respiratory chain are also fulfilled from a kinetic viewpoint as the rates of the reverse reactions involving either electron or proton transfer are of the same order of magnitude as those of the forward reactions.