PMID:9159108
| Citation |
Kaback, HR (1997) A molecular mechanism for energy coupling in a membrane transport protein, the lactose permease of Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 94:5539-43 |
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| Abstract |
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 alpha-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 pKa. 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. |
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| Keywords |
Amino Acid Sequence; Binding Sites; Escherichia coli/enzymology; Escherichia coli Proteins; Hydrogen-Ion Concentration; Kinetics; Membrane Transport Proteins/chemistry; Membrane Transport Proteins/metabolism; Models, Chemical; Models, Structural; Molecular Sequence Data; Monosaccharide Transport Proteins; Mutagenesis, Site-Directed; Protein Structure, Secondary; Protein Structure, Tertiary; Recombinant Proteins/chemistry; Recombinant Proteins/metabolism; Symporters |
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