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UT BIOL 3030 - Transmembrane Transport

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Lecture 10: Transmembrane Transport- Overview of Transmembrane Transporto Simple diffusion: rate of substance movement across phospholipid part of a membrane is proportional to its concentration gradient, hydrophobicity, size, and/or electric potentialo Most molecules are actively transported- Selective Permeability of Lipid Bilayers- Bond Saturation and Membrane Permeabilityo Double bonds cause kinks, preventing tight packing and reducing hydrophobic interactionso Unsaturated: at least one double bond  more permeable membraneo Saturated: no double bonds  more chemical energy  much less permeable- Membrane Transport Proteinso Channels: Transport water, specific ions, or hydrophilic small molecules down concentration/electric potential gradients Facilitated transport  requires transport proteins, but not energy Non-gated channels  open much of time Gated channels  Only open in response to specific chemical/electric signalso Transporters: Move variety of ions, but much slower rate than channels Three types:- Uniporters: transport a single type of molecule down concentration gradient- Antiporters and Symporters: couples’ movement of one type of ion against its concentration gradient (secondary active transport) with movement of one or more different ions down its concentration gradiento ATP-powered Pumps: ATPases that use energy of ATP hydrolysis to move ions against a chemical gradient (active transport)- Membrane Transport Proteins Function Togethero Na has higher concentration outside cell, while K has higher concentration insideo Na/K ATPase uses energy realized by ATP hydrolysis to pump Na out and K ino Movement of K ion out of cell through K channels creates electric potential across membrane, cytosolic being negative with respect to extracellularo Na/lysine transporter moves two Na ions with one lysine from extracellular to cytosolic side Uphill movement of lysine is powered by downhill movement of Na, which in turn is powered by both the Na concentration gradient and (-) charged inside of the plasma membrane which attracts positive Na+o Na/K is important b/c it is the ultimate source of energy to power amino acid uptake- Facilitated Transport of Glucose and Watero Distinguishing features of uniporters from simple diffusion Faster rate of movement by uniporters Hydrophobicity is irrelevant Transport is limited by Vmax (transporters) and Km (transporter affinity) Transport is reversibleo GLUTs Glucose transporter, one of the best understood uniporters Glucose uptake by cells exhibits kinetics similar to those of simple enzyme-catalyzed reactions GLUT1 has higher affinity than GLUT 2o Aquaporin water channels increase rate of water transporto Uniporters alternate between two conformational states, usually preferring the state in which the binding site faces the higher concentration of that molecule- Tonicityo Isotonic: Equal number of solutes outside and inside of cello Hypotonic: Fewer solutes on outside, many on inside  water wants to move inside  cell could burst if too much water enterso Hypertonic: Fewer solutes on inside, many on outside  water wants to move out  cell could shrivel and die if too much water leaves- Aquaporins increase membrane water permeability o Frog oocytes example: Frog eggs normally are impermeable to water and do not have aquaporins When injected with aquaporin MRNA, cell expresses aquaporins and becomes semipermeable to water Oocytes with aquaporins swell and burst from influx of water, while those without aquaporins stay aliveo Aquaporins are tetramers of identical subunits Each aquaporin has 3 pairs of alpha helices connected by two hydrophilic loops that bend to form water-selective gate Single aquaporin unit:- Water molecules enter in single file line through gate- Channel contains highly conserved hydrophilic a.a. residues  side chains form H-bonds with transported water- Hydrophilic and hydrophobic a.a. lining the pore- H-bonds prevent passage of protons and other ions- No conformational change during transport, making it faster than uniporterso Antidiuretic hormone (ADH) Regulates amount of water excreted by kidneys by- Signaling reabsorption of water from filtrate, returning it to blood- Decreasing amount of water excretion- Drinking lots of water decreases ADHo Alcohol and caffeine inhibit release of ADH  why we piss a lot  can cause dehydrationo Diabetes insipidus Inactivates mutations, causing excretion of large volumes of dilute urine- ATP-Powered Pumpso 4 classes of transmembrane proteins couple energy released by ATP hydrolysis with energy-requiring transport of substances P-Class- Generates usual ionic environment of animal cells- Plants and fungi (H+ pump)- Higher eukaryotes (Na/K pump)- Mammalian stomach (H+/K+ pump)- All eukaryotic cells (Ca2+ pump)- Muscle cells (Ca2+ pump) V-class- ATPase- Vacuolar membrane in plants and fungi- Endosomal/lysosomal membrane in animal cells F-class- ATP synthase- Bacterial plasma membrane- Inner mitochondrial membrane- Thylakoid membrane ABC (ATP-Binding Cassette) superfamily- 2 transmembrane domains, 2 ATP binding domains, highly specific- Transport wide array of substances (drugs, phospholipids, peptides, proteins)- Bacterial plasma membrane- Mammalian plasma membrane- Ion gradients are established by pumpso Cytosolic pH is kept at 7.2, regardless of extracellular pHo Ion pumps are largely responsible for establishing/maintaining usual ionic gradientso Na/K pumps are majority b/cNo phosphoprotein intermediate K is needed for protein synthesis Na is needed for import of essential nutrients- P-class example: Ca2+ in SR membrane of skeletal muscle cellso Ca2+ is stored in SR of muscle cells  released into cytosol for muscle contractiono Muscle relaxation depends on Ca2+ ATPaseso ATP hydrolysis results in conformational changeo 1 ATP powers pumping of 2 Ca2+ into SR lumen (irreversible)o E1 state: high affinity for Ca2+, two bound Ca2+o E2 state: low affinity for Ca2+, no bound Ca2+ Difference between two states is conformations of the N and A domains Movement of these two domains power conformational change that constitute the Ca2+ binding sites, converting them from being accessible from the cytosolicface (E1) to one in which the loosely bound Ca2+ ions gain access to the exoplasmic face (E2)- Plasma Membrane Na/K ATPase Pumpo 3 Na out, 2 K in per ATP hydrolyzed o


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