1. Membrane structure & function

2. Integral proteins Can have any number of transmembrane segments –Multiple transmembrane segments: often small molecule transport

3. Transport example: Bacteriorhodopsin Proton pump: establish H + gradient Subsequent ATP generation 7 transmembrane segments of ~20 AA –  -helical –Hydrophobic interaction anchor –Pore for H + movement –Interior of helices has some polar/charged character

4. 1° structure: predict transmembrane segments “Hydropathy plots” Predict whether sequence is Hydrophobic enough to cross membrane –Measure the  G when AA transferred from Hydrophobic into H 2 O –Calculate a ‘hydropathy index’ for a particular segment –If index of region > 0 → transmembrane segment

5. Tyr and Trp Higher presence at membrane interface in integral proteins –Can interact both with lipids and H 2 O –Tyr (orange), Trp (red), charged (purple)

6. How do molecules cross the membrane? Membrane fusion –uptake and release without “crossing a membrane” –Endocytosis: internalization of a vesicle –Exocytosis Requires –Two bilayers recognize each other –Bilayers become closely ‘apposed’ In position to fuse –Local disruption of bilayers –Fusion of bilayers to form a continuous surface Mediated by fusion proteins –Recognition and local distortion

7. Regulated exocytosis

8. Simple diffusion: permeable divider (ie. solute able to diffuse through the membrane) Uncharged species (polar or nonpolar) –Based on concentration gradient Solute: net diffusion toward dilute side At equilibrium: no net diffusion

9. Simple diffusion Charged species –Concentration gradient and electrical gradient (membrane potential V m ) Drives ions to reduce V m –Ion movement depends on the electrochemical potential Tend to equalize concentration AND equalize charge

10. Facilitated diffusion Transporters or permeases that decrease E a –Span lipid bilayer at least once –Movement only in thermodynamically favored direction –Affinity/specificity through weak forces Classes –Carriers Bind with high specificity Saturable Not very efficient Monomers –Channels Rapid transport Less stereospecificity Oligomers

11. Glucose transporter of erythrocytes Required for metabolism Transport of glucose from plasma into cells Uniport system (one solute) 50000 x faster transport ~12 transmembrane regions (hydropathy plots) –  helices that have an polar/electrostatic channel for transport

12. Glucose transporter of erythrocytes Behaves like a MM enzyme –Saturation effects –Model  one glucose binds at a time No covalent bonds Fully reversible process –Concentration gradient dependent –Passive transport

13. Active transport Solute accumulation against equilibrium –movement from low to high [solute] Thermodynamically unfavorable: requires energy 1° active transport –Coupled to exergonic chemical reaction –Commonly ATP hydrolysis P-type, F-type, V-type, multidrug type 2° active transport –Coupling of endergonic and exergonic transport of 2 different solutes Exergonic process drives endergonic transport

14. Transport ATPases P-type –Cation transporters –Reversible phosphorylation by ATP  conf. change V-type & F-type –H + transport –Acidification of intracellular compartments (lysosomes) –Drives ATP synthesis Multidrug transporters –Clinical significance –Transports drugs out of tumor cells or microbial cells –‘multi-drug resistance’

15. P-type ATPases Na + K + ATPase [Na + ] intra  low [K + ] intra  high Cells accumulate K + and release Na + Control of cell volume, action potentials, sugar and AA transport Each ATP hydrolyzed  3Na + out and 2 K + in –Membrane potential (V m )  -50-70 mV Maintenance  20-40% metabolic energy of most cells

16. Na + K + ATPase –Model EnzI has high Na + affinity Enz II has high K + affinity

17. Na + K + ATPase Inhibitors –Ouabain –Digitoxigenin –Used as cardiac glycosides to treat congestive heart failure –Stabilize the E 2 -P complex Na + accumulation in cells Antiporter (Ca 2+ in and Na + out) is activated elevated cytosolic Ca 2+  stimulates and strengthens contractions of heart muscles Digitoxigenin-foxglove Strophanthus gratus