A molecular mechanism for energy coupling in a membrane transport protein, the lactose permease of Escherichia coli

A mechanism for the coupled translocation of substrate and H(+) by the lactose permease of Escherichia coli is proposed, based on a variety of experimental observations. The permease is composed of 12 α-helical rods that traverse the membrane with the N and C termini on the cytoplasmic face. Four residues are irreplaceable with respect to coupling, and the residues are paired—Arg-302 (helix IX) with Glu-325 (helix X) and His-322 (helix X) with Glu-269 (helix VIII). In an adjacent region of the molecule at the interface between helices VIII and V is the substrate translocation pathway. Because of this arrangement, interfacial changes between helices VIII and V are transmitted to the interface between helices IX and X and vice versa. Upon ligand binding, a structural change at the interface between helices V and VIII disrupts the interaction between Glu-269 and His-322, Glu-269 displaces Glu-325 from Arg-302, and Glu-325 is protonated. Simultaneously, protonated Glu-325 becomes inaccessible to water, which drastically increases its pK(a). In this configuration, the permease undergoes a freely reversible conformational change that corresponds to translocation of the ternary complex. To return to ground state after release of substrate, the Arg-302–Glu-325 interaction must be reestablished, which necessitates loss of H(+) from Glu-325. The H(+) is released into a water-filled crevice between helices IX and X which becomes transiently accessible to both sides of the membrane due to a change in helix tilt, where it is acted upon equally by either the membrane potential or the pH gradient across the membrane.