Monoclonal antibody 4B1 alters the pKa of a carboxylic acid at position 325 (helix X) of the lactose permease of Escherichia coli.

A carboxylic acid at position 325 in helix X is obligatory for lactose/H+ symport at a step corresponding to deprotonation of lactose permease [Carrasco, N. et al. (1989) Biochemistry 28, 2533-2539]. In this paper, pH profiles for active transport, efflux, and equilibrium exchange are analyzed for wild-type permease and mutant Glu325-->Asp. With respect to active transport and efflux down a concentration gradient, both of which involve net H+ translocation and are defective in the mutant, the wild-type and the mutant exhibit similar profiles, and at no pH is the mutant stimulated relative to the wild-type. Strikingly, exchange which does not involve H+ translocation is comparable in the wild-type and the Glu325-->Asp mutant below pH 7.5. Above pH 7.5, however, the exchange activity of the mutant is progressively and reversibly inhibited with a midpoint at about pH 8.5; while the exchange activity of wild-type permease is only mildly decreased above pH 9.5, and exchange by Glu325-->Ala or Glu325-->Gln permease is comparable to wild-type and unaffected by pH. The findings are consistent with the idea that translocation of the ternary complex between the permease, lactose, and H+ does not tolerate a negative charge at position 325. In wild-type permease, the electrostatic interaction between Glu325 (helix X) and Arg302 (helix IX) is sufficiently strong that the carboxylate is unaffected by pH. In contrast, with Asp at position 325, the electrostatic interaction is broken, the carboxylate becomes protonated, and the acid exhibits a pKa of about 8.5. Monoclonal antibody 4B1 binds to the periplasmic loop between helices VII and VIII of the permease [Sun, J. et al. (1996) Biochemistry 35, 990-998] and mimics the Glu325 mutants. Dramatically, 4B1 shifts the apparent pKa for exchange from about pH 8.5 to 7.5 in the Glu325-->Asp mutant with little or no effect on the wild-type or the Glu325-->Ala mutant. The findings are consistent with the conclusion that the uncoupling effect of 4B1 involves a conformational change in helix VII and/or VIII that secondarily alters the pKa of the essential carboxylic acid at position 325.