PMID:10984523
| Citation |
Sahin-Tóth, M, Karlin, A and Kaback, HR (2000) Unraveling the mechanism of the lactose permease of Escherichia coli. Proc. Natl. Acad. Sci. U.S.A. 97:10729-32 |
|---|---|
| Abstract |
We studied the effect of pH on ligand binding in wild-type lactose permease or mutants in the four residues-Glu-269, Arg-302, His-322, and Glu-325-that are the key participants in H(+) translocation and coupling between sugar and H(+) translocation. Although wild-type permease or mutants in Glu-325 and Arg-302 exhibit marked decreases in affinity at alkaline pH, mutants in either His-322 or Glu-269 do not titrate. The results offer a mechanistic model for lactose/H(+) symport. In the ground state, the permease is protonated, the H(+) is shared between His-322 and Glu-269, Glu-325 is charge-paired with Arg-302, and substrate is bound with high affinity at the outside surface. Substrate binding induces a conformational change that leads to transfer of the H(+) from His-322/Glu-269 to Glu-325 and reorientation of the binding site to the inner surface with a decrease in affinity. Glu-325 then is deprotonated on the inside because of rejuxtaposition with Arg-302. The His-322/Glu-269 complex then is reprotonated from the outside surface to reinitiate the cycle. |
| Links |
PubMed PMC27091 Online version:10.1073/pnas.200351797 |
| Keywords |
Bacterial Proteins/genetics; Bacterial Proteins/metabolism; Escherichia coli/enzymology; Escherichia coli Proteins; Hydrogen-Ion Concentration; Ligands; Membrane Transport Proteins/genetics; Membrane Transport Proteins/metabolism; Monosaccharide Transport Proteins; Point Mutation; Structure-Activity Relationship; Substrate Specificity; Symporters |
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