Interpreting the effects of specific protein modification on antiport coupling mechanisms: the case of the aspartate/glutamate exchanger.

Reaction of two cysteine residues in the aspartate/glutamate carrier of mitochondria is reported to abolish exchange but to actuate a passive one-way exit of extremely low substrate affinity and specificity, but with the same activation energy as antiport (Dierks, T., Salentin, A. and Krämer, R. (1990) Biochim. Biophys. Acta 1028, 281-288). This behaviour, reminiscent of a channel, becomes understandable when the required control over carrier mobility by the substrate is allowed for. Whether the transport mechanism involves a substrate site alternately exposed on opposite sides of the membrane or sites simultaneously exposed on both sides, and whether the substrate acts by converting an immobile carrier conformation to an inherently mobile intermediate or by stabilizing the transition state in carrier movement, the same fundamental relationship emerges: the ratio of coupled to uncoupled rates (antiport relative to net flux) is limited by the ratio of substrate dissociation constants in successive carrier conformations, one immobile, the other mobile; the increment in the binding energy in the two forms must therefore be large. Shifts in the equilibrium between these conformations and shifts in their relative affinities for the substrate can account for the properties of the modified transport system, which, it is concluded, functions as a carrier, not a channel.