Infrared difference spectroscopy analysis of the purified melibiose permease of Escherichia coli reconstituted into liposomes was carried out as a function of the presence of the two symporter substrates (Na+, melibiose) in either H2O or in D2O media. Essentially, the data first show that addition of Na+ induces appearance of peaks assigned to changes in the environment and/or orientation of α-helical domains of purified melibiose permease. Likewise, melibiose addition in the presence of Na+ produces peaks corresponding to additional changes of α-helix environment or tilt. In addition to these changes, a pair of peaks (1599 (+) cm-1/1576 (-) cm-1) appearing in the Na+-induced difference spectrum is assigned to the antisymmetric stretching of COO- groups, since they show practically no shift upon H/D exchange. It is proposed that these acidic groups participate in Na + coordination. A corresponding pair of peaks, again fairly insensitive to H/D substitution (1591 (-) cm-1/1567 (+) cm -1), appear in the melibiose-induced difference spectra, and may again be assigned to COO- groups. The latter carboxyl groups may correspond to part or all of the acidic residues interacting with Lys or Arg in the resting state that become free upon melibiose binding. © 2006 by the Biophysical Society.
|Publication status||Published - 1 Jan 2006|