Lecture 8

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Published on January 12, 2008

Author: Marietta1

Source: authorstream.com

Coupling of Active Transport to passive transport:  Coupling of Active Transport to passive transport Tight coupling between transport of 2 solutes allows these carriers to harvest the energy stored in the electrochemical gradient of one solute to transport another Free energy released during movement down an electrochemical gradient is used as the driving force to pump the other uphill against its electrochemical gradient Can work in either direction Symporter or antiporter Coupling of Active Transport to Ion Gradients:  Coupling of Active Transport to Ion Gradients In mammalian cells, Na+ electrochemical gradient is maintained across the plasma membrane by active transport of Na+ out of the cell, using ATP as an energy source This electrochemical gradient provides the driving force for the active transport of a 2nd solute E.g. in intestinal and kidney cells, symport systems driven by the Na+ gradient are used to transport sugars and amino acids into the cells Bigger the Na+ gradient, the greater the rate of solute entry (Fig 11-10) Slide3:  Carrier oscillates between state A and state B Binding of Na+ and glucose is cooperative (binding of either ligand induces a conformational change that enhances binding of the 2nd ligand) Since Na+ higher in the extracellular space (& very low inside), glucose more likely to bind in A state Accordingly, Na+ and glucose enter the cell (by an A to B transition) more often than they leave the cell Result is net transport of Na+ and glucose into the cell The Na+ gradient Is used to drive active transport of glucose Na+ pumped out by an ATP- driven pump Asymmetric Distribution of Carrier Proteins:  Asymmetric Distribution of Carrier Proteins In epithelial cells, e.g those absorbing nutrients from the gut, carrier proteins are distributed asymmetrically in the plasma membrane This allows for ‘transcellular transport’ of absorbed solutes Asymmetric Distribution of Carrier Proteins:  Asymmetric Distribution of Carrier Proteins Specifically, Na+ linked transporters in the apical absorptive domain transport nutrients into the cell Build up concentration gradients of these nutrients in the cell Na+ independent transport proteins in the basolateral domains allow nutrients to leave the cell passively down these concentration gradients (Fig. 11-12) Slide6:  Increases PM area Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane K+ is 10 to 20 X higher inside animal cells than outside Na+ is 10 to 20 X higher outside animal cells than inside These concentration gradients are maintained by the Na+ - K+ pump on the plasma membrane Pump operates as an antiporter, pumping K+ in and Na+ out (Fig. 11-13) Slide8:  Na+- K+ Pump These gradients are maintained by the Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane Transport cycle depends on autophosphorylation of the protein Terminal phosphate of ATP is transferred to an aspartic acid of the pump Ion pumps that autophosphorylate are called P-type transport ATPases (Fig. 11-14) Slide10:  See notes page Binding of Na+ & phosphorylation induce conformational change that transfers Na+ to outside and releases it Binding of K+ & dephosphorylation induce conformational change that transfers K+ to inside and releases it Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane The Na+- K+ pump is electrogenic It generates an electrical potential (known as membrane potential) across the membrane Reason: Pumps 3 Na+ ions out for every 2 K+ ions it pumps in Thus the inside of the cell is negative relative to the outside Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane Electrogenic effect of the pump contributes only ~10% of the membrane potential remaining 90% is only indirectly attributable to the Na+- K+ pump (discussed later) Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane The electrogenic nature of the pump plays a crucial role in regulating the tonicity of the cell Water moves slowly across the membrane, down its concentration gradient (called osmosis) (Fig. 11-16) Slide14:  Hypertonic external solution: concentration of water is low relative to its concentration inside the cell Water moves out down its concentration gradient and the cell shrinks Hypotonic external solution: concentration of water is high relative to its concentration inside the cell Water moves in down its concentration gradient and the cell swells Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane If the Na+- K+ pump is inhibited (e.g by treatment with ouabain), cells burst. Why?? Na+- K+ Pump on the Plasma Membrane:  Na+- K+ Pump on the Plasma Membrane Why?? Normally, the water pull of the solutes inside the cell is counteracted by the water pull of the high concentration of Na+ (and Cl- ) outside the cell The Na+- K+ pump maintains osmotic balance by pumping out the Na+ that leaks in down its concentration gradient The Cl- is kept out by the membrane potential (discussed later) Slide17:  When the Na+ – K+ pump stops, Na+ goes into the cell along its concentration gradient This adds to the solute concentration in the cytosol Water moves into the cell along its concentration gradient and the cell bursts Water draw is equal inside and outside Ca2+ & H+ pumps are also P-type Transport ATPases:  Ca2+ & H+ pumps are also P-type Transport ATPases Ca2+ pumps remove Ca2+ from the cytosol after signaling events Cytosolic Ca2+ concentration is very low (10-7 M) in comparison with extracellular Ca2+ (10-3M) Maintenance of the Ca2+ gradient is an essential feature of cell signaling (thus the importance of the Ca2+ pump) H+ pumps secrete acid from specialized epithelial cells in the lining of the stomach Ca2+ pump of the sarcoplasmic reticulum:  Ca2+ pump of the sarcoplasmic reticulum Sarcoplasmic reticulum is the endoplasmic reticulum of muscle cells network of membranous tubular sacs Serves as an intracellular store of Ca2+ Ca2+ is released from the sarcoplasmic reticulum through Ca2+ release channels when the muscle contracts Ca2+ pump of the sarcoplasmic reticulum:  Ca2+ pump of the sarcoplasmic reticulum Ca2+ pump comprises 90% of the sarcoplasmic reticulum membrane protein Responsible for restoring the Ca2+ gradient (pumps it back into the sarcoplasmic reticulum) Structure of the pump resolved by X-ray crystallography (Fig 11-15) Slide21:  See notes page ABC Transporters:  ABC Transporters Largest family of membrane transport proteins 78 genes (5% of genome) encode ABC transporters in E coli Many more in animal cells Known as the ABC transporter superfamily ABC Transporters:  ABC Transporters They use the energy derived from ATP hydrolysis to transport a variety of small molecules including: Amino acids, sugars, inorganic ions, peptides. ABC transporters also catalyze the flipping of lipids between monolayers in membranes ABC Transporters:  ABC Transporters All ABC transporters each contain 2 highly conserved ATP-binding domains (Fig. 11-19) Slide25:  ATP binding leads to dimerization of the ATP binding domains ATP hydrolysis leads to their dissociation A, Cross-sectional diagram B, diagramatic arrangement of polypeptide chains Structural changes in the ATP binding domains mediated by ATP hydrolysis cause conformational changes in the transmembrane segments that drive transport across the membrane ABC Transporters:  ABC Transporters The well-characterized multidrug resistance (MDR) protein is an ABC transporter The overexpression of this transporter in human cancer cells renders them resistant to cytotoxic drugs that are used for chemotherapy The transporter pumps drugs out of cancer cells, reducing their toxicity and conferring resistance to chemotherapy

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