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Papers In Press, published online ahead of print September 26, 2006
J. Biol. Chem, 10.1074/jbc.M607232200
Submitted on July 31, 2006
Revised on September 11, 2006
Accepted on September 26, 2006

Energetics of ligand-induced conformational flexibility in the lactose permease of Escherichia coli

Yiling Nie, Irina Smirnova, Vladimir Kasho, and H. Ronald Kaback

Physiology, UCLA, Los Angeles, CA 90092-7327

Corresponding Author: rkaback{at}mednet.ucla.edu

Isothermal titration calorimetry has been applied to characterize the thermodynamics of ligand binding to wild-type lactose permease (LacY) and a mutant (Cys154Gly) that strongly favors an inward-facing conformation. The affinity of wild-type or mutant LacY for ligand and the change in free energy (G) upon binding are similar. However, with the wild type, the change in free energy upon binding is due primarily to an increase in entropy (TS), while in marked contrast, an increase in enthalpy (H) is responsible for G in the mutant. Thus, wild-type LacY behaves as if there are multiple ligand-bound conformational states, while the mutant is severely restricted. The findings also indicate that the structure of the mutant represents a conformational intermediate in the overall transport cycle.


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