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Lactose permease and the alternating access mechanism
Irina Smirnova1, Vladimir Kasho, H Ronald Kaback
1Department of Physiology and Department of Microbiology, University of California, Los Angeles, California 90095, United States.
Biochemistry
|October 15, 2011
Summary
The lactose permease (LacY) in E. coli uses an alternating access mechanism for sugar transport. Galactopyranoside binding triggers conformational changes, enabling sugar and proton translocation across the membrane.
Area of Science:
- Biochemistry
- Structural Biology
- Membrane Transport
Background:
- Lactose permease (LacY) from Escherichia coli facilitates the transport of lactose across the cytoplasmic membrane.
- Crystal structures reveal an inward-facing conformation with a periplasmic-sealed cavity and a cytoplasm-open cavity.
Purpose of the Study:
- To review biochemical and biophysical evidence supporting the alternating access mechanism of LacY.
- To investigate the dynamic conformational changes involved in LacY-mediated transport.
Main Methods:
- Review of existing biochemical and biophysical studies on LacY.
- Analysis of crystal structures in different functional states.
- Investigation of ligand-induced conformational dynamics.
Main Results:
- LacY undergoes significant global conformational changes upon galactopyranoside binding.
- The binding of sugar closes the inward-facing cavity and opens an outward-facing cavity.
- This supports an alternating access model for coupled sugar and proton translocation.
Conclusions:
- The alternating access mechanism, involving global conformational changes, is strongly supported for LacY function.
- The opening of the periplasmic cavity appears to be the rate-limiting step for substrate binding and transport.
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