Sang Joon Won/sandbox 1
From Proteopedia
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==mechanism== | ==mechanism== | ||
First, H+ binds to LacY, followed by lactose binding. A conformational change in LacY results in translocation of substrates and as they are released, LacY returns to its original conformation. | First, H+ binds to LacY, followed by lactose binding. A conformational change in LacY results in translocation of substrates and as they are released, LacY returns to its original conformation. | ||
| - | Intensive mutagenic analysis of LacY has revealed that <scene name='Sang_Joon_Won/sandbox_1/Lacy/5'>Glu126 and Arg144</scene> are crucial for substrate binding and <scene name='Sang_Joon_Won/sandbox_1/Lacy/ | + | Intensive mutagenic analysis of LacY has revealed that <scene name='Sang_Joon_Won/sandbox_1/Lacy/5'>Glu126 and Arg144</scene> are crucial for substrate binding and <scene name='Sang_Joon_Won/sandbox_1/Lacy/7'>Glu269</scene> is known to be involved in both H+ translocation and substrate binding. It has been postulated that Glu126 and Arg144 may interact with oxygen atoms of sugar via water molecules (3). Arg144 forms H-bonds with O3 and O4 atoms of the galactopyranosyl ring. Glu126 may also interact with the O4, O5 or O6 of the galactopyranosyl ring via water molecules (1). Furthermore, <scene name='Sang_Joon_Won/sandbox_1/Lacy/8'>Trp151</scene> may have a hydrophobic interaction with the galactopyranosyl ring (1). The hydrophobic interaction may orient the galactopyranosyl ring so that hydrogen bonds can be optimized. Glu269 and Arg144 form a salt bridge that can maintain the H-bond between Glu269 and Trp151 to keep Trp151 in the target orientation. Since Glu269 is also involved in H+ translocation, interaction between Glu269 in the C-terminal domain and Arg144 and Trp151 in the N- terminal domain may play a crucial role in the overall energy transduction mechanism that can change the overall conformation of LacY. |
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| + | <scene name='Sang_Joon_Won/sandbox_1/Lacy/9'>His322, Glu325 and Arg302</scene> are directly involved H+ translocation and it involves a series of protonation and deprotonation events. When LacY is in the outward facing conformation that faces the periplasm, H+ is on Glu269 or shared between Glu269 and His322. The sugar is recognized by Trp151, Arg144 and Glu126, which triggers H+ transfer to His322 and then to Glu325 as Glu269 is recruited to complete the binding site. As a result, transition to the inward-facing conformation is induced and sugar is then released into the cytoplasm, followed by release of H+. The release if H+ is induced by the change in pKa of Glu325 due to exposure to solvent in the hydrophilic cavity (1). After releasing the H+, transition back to the outward-facing conformation may be favored to accept another proton from periplasm. | ||
==experiments== | ==experiments== | ||
Revision as of 06:16, 15 April 2011
Contents |
Lactose Permease
Background
The lactose permease (LacY) is arguably a paradigm for 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+) symporter, utilizing the free energy released from downhill movement of H+ to actively transport 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 carbohydrate, in their environment in order to produce 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 [1]. 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 [1].
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| 1pv7, resolution 3.60Å () | |||||||||
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| Related: | 1pv6 | ||||||||
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| Resources: | FirstGlance, OCA, RCSB, PDBsum | ||||||||
| Coordinates: | save as pdb, mmCIF, xml | ||||||||
Structure
has 12 transmembrane helices in which the N- and C-terminal 6 helices form two bundles connected by a loop between helices VI and VII. The 6 helices domains have a pseudo-two fold axis of symmetry and they are positioned to form a large interior open on the cytoplasmic side (2). The cavity is composed of helices I, II, IV, and V of the N-terminal domain and helices VII, VIII, X and XI of the C-terminal domain. All the available structures up to date exhibits an inward facing conformation and many studies have indicated that the periplasmic barrier is tightly closed in the absence of sugar binding. Therefore, the inward-facing conformation, which faces the cytoplasm, represents the lowest free-energy state in the membrane (2). A single was bound to the cavity in the X-ray structure to mimic the actual sugar binding and there is only 1 binding site (3).
mechanism
First, H+ binds to LacY, followed by lactose binding. A conformational change in LacY results in translocation of substrates and as they are released, LacY returns to its original conformation. Intensive mutagenic analysis of LacY has revealed that are crucial for substrate binding and is known to be involved in both H+ translocation and substrate binding. It has been postulated that Glu126 and Arg144 may interact with oxygen atoms of sugar via water molecules (3). Arg144 forms H-bonds with O3 and O4 atoms of the galactopyranosyl ring. Glu126 may also interact with the O4, O5 or O6 of the galactopyranosyl ring via water molecules (1). Furthermore, may have a hydrophobic interaction with the galactopyranosyl ring (1). The hydrophobic interaction may orient the galactopyranosyl ring so that hydrogen bonds can be optimized. Glu269 and Arg144 form a salt bridge that can maintain the H-bond between Glu269 and Trp151 to keep Trp151 in the target orientation. Since Glu269 is also involved in H+ translocation, interaction between Glu269 in the C-terminal domain and Arg144 and Trp151 in the N- terminal domain may play a crucial role in the overall energy transduction mechanism that can change the overall conformation of LacY.
are directly involved H+ translocation and it involves a series of protonation and deprotonation events. When LacY is in the outward facing conformation that faces the periplasm, H+ is on Glu269 or shared between Glu269 and His322. The sugar is recognized by Trp151, Arg144 and Glu126, which triggers H+ transfer to His322 and then to Glu325 as Glu269 is recruited to complete the binding site. As a result, transition to the inward-facing conformation is induced and sugar is then released into the cytoplasm, followed by release of H+. The release if H+ is induced by the change in pKa of Glu325 due to exposure to solvent in the hydrophilic cavity (1). After releasing the H+, transition back to the outward-facing conformation may be favored to accept another proton from periplasm.
experiments
- ↑ 1.0 1.1 Guan L, Kaback HR. Lessons from lactose permease. Annu Rev Biophys Biomol Struct. 2006;35:67-91. PMID:16689628 doi:10.1146/annurev.biophys.35.040405.102005
