文摘
Electrogenic events associated with the activity of the melibiose permease (MelB), a transporterfrom Escherichia coli, were investigated. Proteoliposomes containing purified MelB were adsorbed to asolid supported lipid membrane, activated by a substrate concentration jump, and transient currents weremeasured. When the transporter was preincubated with Na+ at saturating concentrations, a chargetranslocation in the protein upon melibiose binding could still be observed. This result demonstrates thatbinding of the uncharged substrate melibiose triggers a charge displacement in the protein. Further analysisshowed that the charge displacement is neither related to extra Na+ binding to the transporter, nor to thedisplacement of already bound Na+ within the transporter. The electrogenic melibiose binding process isexplained by a conformational change with concomitant displacement of charged amino acid side chainsand/or a reorientation of helix dipoles. A kinetic model is suggested, in which Na+ and melibiose bindingare distinct electrogenic processes associated with approximately the same charge displacement. Thesebinding reactions are fast in the presence of the respective cosubstrate (k > 50 s-1).