A theoretical study of double-inhibitor-titration curves in free-energy-transducing networks.

The general relationship between the double-inhibitor-titration curves and the kinetic properties of pumps in a delocalized chemiosmotic free-energy-transducing network is studied. The kinetic conditions for a delocalized system to generate the observed double-inhibitor-titration results are derived and the effectiveness of double-inhibitor experiments in discriminating between the localized and the delocalized proton-coupling mechanisms is assessed. It is found that, using simple enzymatic cycles for the kinetics of the pumps in a delocalized network, one can reproduce the experimentally measured double-inhibitor-titration curves that were widely used to argue against the delocalized mechanism. This implies that double-inhibitor-titration curves alone are not sufficient to discriminate between localized and delocalized coupling systems. Additional information concerning the kinetic responses of isolated pumps on the proton gradient across the membrane and inhibitor concentrations are required.