Physical mechanism for regulation of proton solute symport in Escherichia coli.

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Abstract
The activity of E. coli transport proteins for lactose and proline can be altered by changing the redox state of the dithiols in these carriers. A series of lipophilic oxidizing agents inhibits full activity; subsequent addition of dithiothreitol restores full activity. Both systems are irreversibly inhibited by N-ethylmaleimide, but prior addition of oxidizing agents protects against this inhibition. Studies on the inhibitory effect of the dithiol-specific reagent phenylarsine oxide show that the redox-sensitive step is the conversion of a dithiol to a disulfide. Measurement of the initial rate as a function of the lactose and L-proline concentrations shows that the oxidation of a dithiol to a disulfide increases the Km of the carriers for lactose and L-proline. The reduced (dithiol) form of the carrier has a low Km and the oxidized (disulfide) form has a high Km for its substrate. The changes in Km brought about by reduction and oxidation are the same as those that accompany the generation and dissipation, respectively, of an electrochemical proton gradient (.DELTA..hivin..mu.H+). These results support a mechanism in which an .DELTA..hivin..mu.H+ or one of its components alters the ligand affinities of the carrier during a single transport cycle through conversion from the reduced to the oxidized form.

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