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Alpha Helices Compose the Integral Protein Bacteriorhodopsin Bacteria light-harvesting protein that generates proton gradient α-Helix most common 2° membrane structure Helical (yellow) and charged (red)residues
Hydropathy Plots Identify Potential Trans-Membrane Peptide Regions Hydrophobicity Index: the free energy needed to transfer successive segments of a polypeptide from a non-polar solvent to water Hydropathy Plot
Porins – beta Barrel Trimers Located in outer membranes of bacteria, mitochondria and chloroplasts Always open allowing solutes to travel through in either direction; referred to as passive transport
Bacterial Channel - Porin Beta barrel with a hydrophobic exterior and a hydrophilic core Hydrophobic (yellow) and hydrophobic (white) residues shown
Transport Protein Classification by Operation Does not specify active or passive transport Single substance versus coupled movement in the same or opposite direction
Highly-Selective Potassium Channel 10,000 times more selective to K+ than Na+
Size Selective Channel Only Allows Potassium Ion Passage • K+ movement: • Down the concentration gradient • From cytosol to cell exterior Cell exterior Cytosol
Selectivity Filter Determines the Preference of K+ Over Other Ions Two of four trans-membrane alpha helices shown Dehydrated K+ ions move across the membrane Ionic diameter K+ = 2.66 Å and Na+ = 1.9 Å Why does Na+ not move through this pore?
Energetic Basis of Ion Selectivity K+ energy balance: Dehydration versus Carbonyl oxygen resolvation in selectivity-filter lining Does tight K+ binding slow transport across the channel?
Electrostatic Repulsion Forces Pushes and Speeds K+ Through Ion Channel • Selectivity filter contains 4 binding sites • Ions move down the concentration gradient
Aquaporins: Water-Specific Pores Hydrophobic pore with 2 Asn residues serving as a bridge as well as a H-bond disruptor Side view Top view
Gated Channels via Conformational Changes Channels open or close in response to a specific signal such as: pH, voltage, Ca+2, or phosphorylation
Transport via Alternate Protein Conformations Lactose permease ribbons blue to red (N- to C-terminus) Black – lactose disaccharide Space filling w/ 2 helices missing
Sodium Gradient Formed via a Na+-K+ATPaseAntiport Pump • P-type ATPases form a phosphorylaspartate and include: • Ca+2ATPases for muscle contraction • Gastric H+-K+ATPases Link • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
Secondary Transport via a Common Sodium Concentration Gradient • Na+-K+ATPaseantiporter gradient drives: • Glucose import via a Na+- glucose symporter • Calcium ion export • via a Na+-Ca+2 • antiporter
Digoxin Na+-K+ Pump Inhibition via Dephosphorylation Blocking Digoxin – a cardiotonic steroid medication used for atrial fluttering and heart failure. How can digoxin be chemically described? Digitalis purpurea
Na+ Channel Blocker Specific in blocking Na+ while having no effect on K+ Reversible binding: hydrated Na+ (nsec), tetrodotoxin (0.3 min) Lethal dose in humans: 10 ng Pufferfish: a Japanese delicacy and highly toxic How does this cork-like toxin position itself? Host-toxicity prevention mechanism?