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Lactose Permease (LacY)

 

As one of the most extensively studies members of the Major Facilitator Superfamily of membrane transport proteins, LacY catalyzes the coupled translocation o an H+ and a galactophyranosides (Galactoside/H+ symport). Since translocation across the membrane is obligatorily coupled, sugar accumulation against a concentration gradient (i.e., active transport) is achieved by usuing the free energy released from the downhill movement of H+ with electrochemical H+ gradient (Δμ̃H+; interior negative and /or alkaline). Conversely, downhill sugar translocation by LacY drives uphill translocation of the H+ with gernration of Δμ̃H+, the polarity of which depends on the direction of the sugar concentration gradient.

 

X-ray structures of a conformationally restricted mutant (C154G) and wild type LacY reveal 12 mostly irregular transmembrane helices arranged in 2 pseudo-symmetrical 6-helix bundles surrounding a large central water-filled cavity open to the cytoplasm only (an inward-facing conformation). However, multiple independent biochemical and biophysical studies, which include thiol cross-linking, site-directed alkylation, single molecule fluorescence resonance energy transfer, double electron-electron resonance and tryptophan fluorescence quenching or de-quenching, not only demonstrate that an outward-facing conformation exists, but that a periplasmic cavity must open and close for transport to occur. By this means, the cytoplasmic cavity closes with opening of a wide hydrophilic cavity on the periplasmic side of LacY, which allows exposure of the sugar-binding site to either side of the membrane (the alternating access mechanism).

 

 

 

 

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