Evidence for intraprotein charge transfer during the transport activity of the melibiose permease from Escherichia coli.

TLDR: It is suggested that the fast transient response reflects charge transfer within MelB during cosubstrate binding while the slow component is associated with charge transfer across the proteoliposome membrane.

Abstract: Electrogenic activity associated with the activity of the melibiose permease (MelB) of Escherichia coli was investigated by using proteoliposomes containing purified MelB adsorbed onto a solid-supported membrane. Transient currents were selectively recorded by applying concentration jumps of Na+ ions (or Li+) and/or of different sugar substrates of MelB (melibiose, thio-methyl galactoside, raffinose) using a fast-flow solution exchange system. Characteristically, the transient current response was fast, including a single decay exponential component (τ ≈ 15 ms) on applying a Na+ (or Li+) concentration jump in the absence of sugar. On imposing a Na+ (or Li+) jump on proteoliposomes preincubated with the sugar, a sugar jump in a preparation preincubated with the cation, or a simultaneous jump of the cation and sugar substrates, the electrical transients were biphasic and comprised both the fast and an additional slow (τ ≈ 350 ms) decay components. Finally, selective inactivation of the cosubstrate transloca...