Passive vs. active transport

1. Passive vs. active transport • Passive transport is simply transport down an electrochemical gradient until equilibrium is reached • Active transport results in accumulation of solute beyond equilibrium, the unfavorable thermodynamics is driven by ATP hydrolysis

2. Passive vs. facilitated diffusion

3. Four types of transport ATPases • F-type – you are familiar with • P-type • V-type • Multi-drug transporter (ABC Transporter)

4. P-type ATPase • Cation transporter that is reversibly phosphorylated as part of the transport cycle • Vanadate sensitive • Na+, K+, Ca++ • Bacteria use to detoxify heavy metals • Widely distributed

5. A P-type ATPase maintains potassium and sodium gradient

7. This transport mechanism maintains a membrane potential

8. Ion gradients provide energy for secondary active transport • Ion gradients formed by transport of cations can be driving force for cotransport of other solutes

9. For example,

10. A composite look at transport

11. V-type ATPase and Multi-drug transporter • V-type • Works as a proton pump • Has key role in acidification of cellular compartments (including endosome) • Multi-drug transporter • Export numerous compounds in ATP-dependent manner

12. Ion channels are distinct from ion transporters • Ion channels provide a faster rate of transport • Ion channels cannot be saturated • Channels are gated, meaning they are open or closed in response to allosteric effectors

13. Channel structure provides insight into specificity and rate

14. Features of K+ channel • Negatively charged amino acids act as sink for cations • Pathway narrows (filter) to accommodate interactions specific for potassium • Appears to be a paradigm for ion channels (calcium, etc.)

15. Voltage gated sodium channels Change in membrane potential results in conformational change

16. Lastly, Ligand-gated channels • Acetylcholine receptor binds acetylcholine causing a conformational change opening the ion channel. • There are also intracellular ligand-gated channels

17. Ligand gated ion channels • Nicotinic acetylcholine receptor transports sodium, calcium and potassium ions through conformational changes

18. Membrane proteins such as channels, receptors have significant metabolic roles • Hormones and metabolites offer signaling or communication mechanisms within the cell

19. Qualities of Signal Transduction

20. Molecular cascades

21. Feedback inhibition

22. Integrated networks

23. Six types of signal transducers

24. Other two receptor types • Receptors with no intrinsic enzyme activity (can interact with enzymes though such as tyrosine kinases) to affect gene expression • Adhesion receptor, binds molecules in the extracellular matrix

25. Looking at receptor-ligand interactions • Experimentally must account for non-specific binding (ie. to the membrane, tube, etc.)

26. Scatchard analysis (like Lineweaver Burke)

27. Case study: Receptor Enzymes • Most commonly – • tyrosine kinases

28. Insulin receptor and a regulatory cascade

29. Important facets of this process • Phosphorylation alters protein structure/function • SH2 domain observed in many proteins, a conserved domain that mediates protein-protein interactions • G-protein activates kinases • Kinases act in a cascade to modulate transcriptional regulators (kinome)

30. IRS-1 can interact with other cellular components for network integration

31. G-proteins and signal transduction • b-adrenergic Signal pathway

32. How? • Epinephrine binds the serpentine receptor, causing GTP to replace GDP on the G-protein (this particular G-protein is distinct from Ras family) • G becomes active with GTP bound, and activates adenyl cyclase, which converts ATP to cAMP • Timing mechanism turns off G protein

34. cAMP activated protein kinases • Protein kinase A

36. Result of epinephrine cascade

37. Densensitization by phosphorylation

38. Second messenger cAMP

39. Other second messengers • Diacylglycerol • Inositol triphosphate • Calcium