Chapter 9 (part 3)

1. Chapter 9 (part 3) Membranes

2. Membrane transport • Membranes are selectively permeable barriers • Hydrophobic uncharged small molecules can freely diffuse across membranes. • Membranes are impermeable to polar and charged molecules. • Polar and charged molecules require transport proteins to cross membranes (translocators, permeases, carriers)

3. Transport of non-polar molecules • Non-polar gases, lipids, drugs etc… • Enter and leave cells through diffusion. • Move from side with high concentration to side of lower concentration. • Diffusion depends on concentration gradient. • Diffusion down concentration gradient is spontaneous process (-DG).

5. Transport of polar or charged compounds Involves three different types of integral membrane proteins • Channels and Pores • Passive transporters • Active transporters Transporters differ in kinetic and energy requirements

6. Channels and Pores • Have central passage that allows molecules cross the membrane. • Can cross in either direction by diffusing down concentration gradient. • Solutes of appropriate size and charge can use same pore. • Rate of diffusion is not saturable. • No energy input required

7. Porins • Present in bacteria plasma membrane and outer membrane of mitochondria • Weakly selective, act as sieves • Permanently open • 30-50 kD in size • exclusion limits 600-6000 • Most arrange in membrane as trimers

8. Passive Transport (Facilitated Diffusion) • Solutes only move in the thermodynamically favored direction • But proteins may "facilitate" transport, increasing the rates of transport • Two important distinguishing features: • solute flows only in the favored direction • transport displays saturation kinetics

9. Three types of transporters • Uniporter – carries single molecule across membrane • Symport – cotransports two different molecules in same direction across membrane • Antiport – cotransports two different molecules in opposite directions across membrane.

10. Saturation Kinetics of transport • Rate of diffusion is saturable. • Ktr = [S] when rate of transport is ½ maximun rate. • Similar to M-M kinetics • The lower the Ktr the higher the affinity for substrate.

11. Transporters undergo conformational change upon substrate binding • Allows substrate to transverse membrane • Once substrate is released, transported returns to origninal conformation.

12. Active Transport Systems • Some transport occur such that solutes flow against thermodynamic potential • Energy input drives transport • Energy source and transport machinery are "coupled" • Like passive transport systems active transporters are saturable

13. Primary active transport • Powered by direct source of energy(ATP, Light, concentration gradient) Secondary active transport • Powered by ion concentration gradient. • Transport of solute “A” is couple with the downhill transport of solute “B”. • Solute “B” is concnetrated by primary active transport.

14. Na+-K+ ATPase • Maintains intracellular Na low and K high • Crucial for all organs, but especially for neural tissue and the brain • ATP hydrolysis drives Na out and K in

15. Na+-K+ ATPase • Na+ & K+ concentration gradients are maintained by Na+-K+ ATPase • ATP driven antiportsystem. • imports two K+ and exports three Na+ for every ATP hydrolyzed • Each Na+-K+ ATPase can hydrolyze 100 ATPs per minute (~1/3 of total energy consumption of cell) • Na+ & K+ concentration gradients used for 2o active transport of glucose in the intestines

16. 1o active transport of Na+ 2o active transport of glucose

17. Transduction of extracellular signals • Cell Membranes have specific receptors that allow cell to respond to external chemical stimuli. • Hormone – molecules that are active at a distance. Produced in one cell, active in another. • Neurotransmitters – substances involved in the transmission of nerve impulse at synapses. • Growth factors – proteins that regulate cell proliferation and differentiation.

18. External stimuli(first messenger) – (hormone, etc…) • Membrane receptor – binds external stimuli • Transducer – membrane protein that passes signal to effector enzyme • Effector enzyme – generates an intracellular second messenger • Second messenger – small diffusible molecule that carrier signal to ultimate destination

19. G-Proteins • Signal transducers. • Three subunits, (a,b, g) a and g anchored to membrane via fatty acid and prenyl group • Catalyze hydrolysis of GTP to GDP. • GDP bound form is inactive/GTP bound form active • When hormone bound receptor complex interacts with G-protein, GDP leaves and GTP binds. • Once GTP -> GDP G-protein inactive • GTP hydrolysis occurs slowly (kcat= 3min-1) good timing mechanism

20. Epinephrine signaling pathway • Epinephrine regulation of glycogen degradation • Fight or Flight response • Ephinephrine primary messenger • G-protein mediated response. • G-protein activates Adenyl-cyclase to produce cAMP • cAMP is the second messenger • Activates protein kinase • Activates glycogen phosphorylase

22. Effect of Caffeine • Caffeine inhibits cAMP phosphodiesterase , prevents breakdown of cAMP. • Prolongs and intensifies Epinephrine effect.

23. Phosphatidylinositol (PI) Signaling Pathway • G-protein mediated • G-protein activates phospholipase C (PLC) • PLC cleaves PI to form inositol-triphosphate (IP3) and diacylglycerol (DAG) both act as 2nd messengers • IP3 stimulates Ca2+ releases from ER • DAG stimulates Protein kinase C