4 - THE CHEMIOSMOTIC PROTON CIRCUIT

Abstract

This chapter discusses the functioning of the proton circuit in a wide range of chemiosmotic energy transductions. It discusses the close analogy between the proton circuit and the equivalent electrical circuit, not only as a simple model but also because the same laws govern the flow of energy around both circuits. The chapter also discusses the measurement of proton-motive force, the stoichiometry of proton extrusion by the respiratory chain, experimental determination of H+/O ratios, the stoichiometry of proton uptake by the ATP synthase, proton current, proton conductance and respiratory control, and overall parameters of energy transduction. In an electrical circuit, the two fundamental parameters are potential difference and current. From measurements of these functions, other factors may be derived, such as the rate of energy transmission or the resistance of components in the circuit. In an open circuit, electrical potential is maximal, but no current flows as the redox potential difference generated by the battery is precisely balanced by the back-pressure of the electrical potential. The tight coupling of the redox reactions within the battery to electron flow prevents any net chemical reaction. In the case of the mitochondrion, the proton circuit is open-circuited when there is no pathway for the protons extruded by the respiratory chain to re-enter the matrix. As with the electrical circuit, the potential across the membrane is maximal under these conditions, and there would be a thermodynamic equilibrium between the proton-translocating regions of the respiratory chain and proton-motive force. As the redox reactions are tightly coupled to proton extrusion there would be no respiration in this condition.