Ouabain: Difference between revisions
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<StructureSection load='3n23' size=' | <StructureSection load='3n23' size='450' side='right' caption='Na+/K+ ATPase-Ouabain Binding, ([[3n23]])' scene='Sandbox_60/Ouabain_3d/1'> | ||
[[Ouabain]] is a cardiac glycoside that inhibits ATP-dependent sodium-potassium exchange across cell membranes. The binding of ouabain to the sodium-potassium pump (also called Na+/K+ ATPase) prevents the conformational changes necessary for its proper function. This affects intracellular ion composition in various ways with various effects, depending on the cell and the dosage. The compound has therefore been utilized in medicine, both as a [[Pharmaceutical Drugs|therapeutic]] and in research related to active membrane transport. | [[Ouabain]] is a cardiac glycoside that inhibits ATP-dependent sodium-potassium exchange across cell membranes. The binding of ouabain to the sodium-potassium pump (also called Na+/K+ ATPase) prevents the conformational changes necessary for its proper function. This affects intracellular ion composition in various ways with various effects, depending on the cell and the dosage. The compound has therefore been utilized in medicine, both as a [[Pharmaceutical Drugs|therapeutic]] and in research related to active [[Membrane Channels|active membrane transport]]. | ||
==Target Protein: Na+/K+ ATPase== | ==Target Protein: Na+/K+ ATPase== | ||
[[Image:498px-Scheme sodium-potassium pump-en-2.svg.png| | [[Image:498px-Scheme sodium-potassium pump-en-2.svg.png|420px|right|upright=3.0]] | ||
[[Na+/K+ ATPase]] actively transports potassium into the cell and sodium to the extra-cellular space according to the general scheme depicted below. This transmembrane protein is composed of three subunits, alpha, beta, and gamma, that may dimerize with other pumps ''in vivo''. The<scene name='Sandbox_60/Drug_in_complex_situated/1'>''in vitro''</scene>, crystal structure is a radially symmetrical hexamer, when in living tissues, the proteins most likely associate bilaterally. The alpha chain is shown in pink/blue and the beta chain in yellow/green. Of the two, the alpha subunit plays the leading role in the functioning of the protein. The bundle of alpha helices in the center of the protein position hydrophobic residues at the exterior of the motif, opening a portal through the lipid bilayer. It is inside this transmembrane domain that residues of the alpha subunit coordinate sodium or potassium ions. The large domain on the cytoplasmic side of the bilayer (furthest from beta subunit) floats in solution and is responsible for binding and hydrolyzing ATP. As with many protein enzymes of phosphoester hydrolysis, the phosphate groups are coordinated with a <scene name='Sandbox_60/Drug_in_complex_mg/1'>magnesium cofactor</scene>. The energy released from high-energy phosphate cleavage is transfered via conformational changes to the transmembrane domain, where ions are forced against their concentration gradient. On the extracellular side, the <scene name='Sandbox_60/Drug_in_complex_situated/1'>beta subunit</scene> moves to prevent the dissociation of potassium ions during their entry into the cell. A third gamma subunit (purple/orange), along with the beta subunit, contributes to the anchoring of transmembrane domains to the phospholipid bilayer. | [[Na+/K+ ATPase]] actively transports potassium into the cell and sodium to the extra-cellular space according to the general scheme depicted below. This transmembrane protein is composed of three subunits, alpha, beta, and gamma, that may dimerize with other pumps ''in vivo''. The <scene name='Sandbox_60/Drug_in_complex_situated/1'>''in vitro''</scene>, crystal structure is a radially symmetrical hexamer, when in living tissues, the proteins most likely associate bilaterally. The alpha chain is shown in pink/blue and the beta chain in yellow/green. Of the two, the alpha subunit plays the leading role in the functioning of the protein. The bundle of alpha helices in the center of the protein position hydrophobic residues at the exterior of the motif, opening a portal through the lipid bilayer. It is inside this transmembrane domain that residues of the alpha subunit coordinate sodium or potassium ions. The large domain on the cytoplasmic side of the bilayer (furthest from beta subunit) floats in solution and is responsible for binding and hydrolyzing ATP. As with many protein enzymes of phosphoester hydrolysis, the phosphate groups are coordinated with a <scene name='Sandbox_60/Drug_in_complex_mg/1'>magnesium cofactor</scene>. The energy released from high-energy phosphate cleavage is transfered via conformational changes to the transmembrane domain, where ions are forced against their concentration gradient. On the extracellular side, the <scene name='Sandbox_60/Drug_in_complex_situated/1'>beta subunit</scene> moves to prevent the dissociation of potassium ions during their entry into the cell. A third gamma subunit (purple/orange), along with the beta subunit, contributes to the anchoring of transmembrane domains to the phospholipid bilayer. | ||
Normal functioning Na+/K+ ATPase changes unidirectionally between two phases, E1 and E2. In the E1 phase, it binds Na+ and ATP on the inside of the cell. After hydrolysis, the phosphorelated protein ejects ADP and changes conformation to E2, allowing for 3Na+ to dissociate on the extracellular side of the membrane. The protein then binds 2K+ and hydrolyzes the bound inorganic phosphate, causing the reversion to E1. Just before the binding of K+ to the E2 state, Ouabain binds to residues inside the pump, preventing the transition back to E1 (thus the magnesium cofactor is coordinating an enzymatically bound <scene name='Sandbox_60/Drug_in_complex_mgp/1'>inorganic phosphate</scene>) | Normal functioning Na+/K+ ATPase changes unidirectionally between two phases, E1 and E2. In the E1 phase, it binds Na+ and ATP on the inside of the cell. After hydrolysis, the phosphorelated protein ejects ADP and changes conformation to E2, allowing for 3Na+ to dissociate on the extracellular side of the membrane. The protein then binds 2K+ and hydrolyzes the bound inorganic phosphate, causing the reversion to E1. Just before the binding of K+ to the E2 state, Ouabain binds to residues inside the pump, preventing the transition back to E1 (thus the magnesium cofactor is coordinating an enzymatically bound <scene name='Sandbox_60/Drug_in_complex_mgp/1'>inorganic phosphate</scene>) | ||