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The lactose permease (LacY) is arguably a paradigm for [http://en.wikipedia.org/wiki/Secondary_transporters secondary transporter proteins]. This membrane protein belongs to the Major Facilitator Superfamily and has been studied for decades to understand detailed mechanism of energy transduction and translocation reactions. This protein serves as the lactose and hydrogen ion (H+) [http://en.wikipedia.org/wiki/Symporter symporter], utilizing the free energy released from downhill movement of H+ to actively transport lactose. [http://en.wikipedia.org/wiki/Lactose Lactose] is a disaccharide that yields D-glucose and D-galactose when hydrolyzed. LacY is specific for disaccharides like lactose that contain a D-galactopyranosyl ring or D- galactose. Due to LacY, higher concentration of sugar molecules can be maintained inside a cell. Keeping higher level of sugar is essential because all bacteria must utilize the energy sources, such as [http://en.wikipedia.org/wiki/Carbohydrate carbohydrate], in their environment in order to produce [http://en.wikipedia.org/wiki/Adenosine_triphosphate ATP]. ATP provides energy for the biosynthetic processes that bacteria use for their maintenance and reproduction. LacY also uses the energy released from downhill movement of sugar to generate electrochemical H+ gradient <ref="gradient">PMID:16689628</ref>. These processes are found in many organisms and play a crucial role in many aspects of cell function. Therefore, many biochemical techniques to study LacY have been applied to the study of many other similar membrane proteins. Since LacY has been used as a model protein for secondary transporter proteins in this era, it is important to understand the structural basis for this transporter because it may give us detailed understanding of energy transduction mechanisms utilized in many living organisms <ref name="gradient"/>.