|LacY proton/sugar symporter|
Crystal Structure of Lactose Permease in Complex with an Affinity Inactivator. PDB 
Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the gradient of H+ towards the cell to transport lactose in the same direction into the cell.
The protein has twelve transmembrane helices and exhibits an internal two-fold symmetry, relating the N-terminal six helices onto the C-terminal helices.
The sugar lies in a pocket in the center of the protein which is accessible from the periplasm. On binding, a large conformational change takes place which makes the sugar binding site accessible from the cytoplasm.
Mechanism: hydrogen from the outside of the cell binds to a carboxyl group on the enzyme that allows it to undergo a conformational change. This form of lactose permease can bind lactose from outside the cell. The enzyme then everts and lactose is transported inward.
- Chaptal, V.; Kwon, S.; Sawaya, M. R.; Guan, L.; Kaback, H. R.; Abramson, J. (2011). "Crystal structure of lactose permease in complex with an affinity inactivator yields unique insight into sugar recognition". Proceedings of the National Academy of Sciences 108 (23): 9361–9366. doi:10.1073/pnas.1105687108. PMC 3111295. PMID 21593407.
- Abramson, J.; Smirnova, I.; Kasho, V.; Verner, G.; Kaback, H. R.; Iwata, S. (2003). "Structure and Mechanism of the Lactose Permease of Escherichia coli". Science 301 (5633): 610–615. doi:10.1126/science.1088196. PMID 12893935.
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