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| Content Provider | PubMed Central |
|---|---|
| Author | Kaback, H. Ronald |
| Copyright Year | 1997 |
| 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 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 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. |
| Starting Page | 5539 |
| File Format | |
| ISSN | 10916490 |
| e-ISSN | 10916490 |
| Journal | Proceedings of the National Academy of Sciences of the United States of America |
| Issue Number | 11 |
| Volume Number | 94 |
| Language | English |
| Publisher | The National Academy of Sciences of the USA |
| Publisher Date | 1997-05-27 |
| Access Restriction | Open |
| Rights Holder | The National Academy of Sciences of the USA |
| Subject Keyword | Research in Higher Education |
| Content Type | Text |
| Resource Type | Article |
| Subject | Multidisciplinary |
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