Abstract
The phosphate carrier from the inner mitochondrial membrane is shown to catalyze different functions when analyzed in a reconstituted system. The well-known physiological functions of phosphate/phosphate antiport and unidirectional, proton-compensated phosphate uptake both follow a simultaneous mechanism of bisubstrate kinetics, including a ternary complex in the functionally active state. As a consequence, we interpret the unidirectional phosphate transport as phosphate/OH − -antiport in kinetic terms. Besides these two transport modes, after modification of particular cysteine residues, the mitochondrial phosphate carrier mediates unidirectional phosphate efflux, similar as has previously been shown for the aspartate/glutamate carrier and other mitochondrial transporters. This third mode is characterized by a surprisingly low substrate affinity and specificity at the internal binding site. The fully retained activation energy and the original substrate interaction at the external binding site, however, indicate a preserved carrier-type function also in this kind of action. Finally we could show, that mitochondrial carrier proteins, when integrated into planar lipid bilayer membranes, behave as true channels under particular experimental conditions.
Published Version
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