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