NOISE-ANALYSIS OF ION CURRENT THROUGH THE OPEN AND THE SUGAR-INDUCED CLOSED STATE OF THE LAMB CHANNEL OF ESCHERICHIA-COLI OUTER-MEMBRANE - EVALUATION OF THE SUGAR BINDING-KINETICS TO THE CHANNEL INTERIOR

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was i nvestigated using fast Fourier transformation. The current noise throu gh the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral den sity of the noise of the open LamB channels was a quadratic function o f the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Si multaneously, the spectral density of the current noise increased dras tically, which indicated interaction of the sugars with the binding si te inside the channel. The frequency dependence of the spectral densit y was of Lorentzian type, although the power of its frequency dependen ce was not identical to -2. Analysis of the power density spectra usin g a previously proposed simple model (Bent, R., A. Schmid, and G. H. V os-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29.) allowed the evalu ation of the on- and the off-rate constants for the maltopentaose bind ing to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.