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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
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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
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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
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Bacterial Channel - Porin
Beta barrel with a hydrophobic exterior and a hydrophilic core Hydrophobic (yellow) and hydrophobic (white) residues shown
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Transport Protein Classification by Operation
Does not specify active or passive transport Single substance versus coupled movement in the same or opposite direction
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Highly-Selective Potassium Channel
10,000 times more selective to K+ than Na+
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Size Selective Channel Only Allows Potassium Ion Passage
K+ movement: Down the concentration gradient From cytosol to cell exterior Cell exterior Cytosol
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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?
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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?
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Electrostatic Repulsion Forces Pushes and Speeds K+ Through Ion Channel
Selectivity filter contains 4 binding sites Ions move down the concentration gradient
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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
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Gated Channels via Conformational Changes
Channels open or close in response to a specific signal such as: pH, voltage, Ca+2, or phosphorylation
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Transport via Alternate Protein Conformations
Lactose permease ribbons blue to red (N- to C-terminus) Black – lactose disaccharide Space filling w/ 2 helices missing
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Sodium Gradient Formed via a Na+-K+ ATPase Antiport Pump
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Na+-K+-ATPase: Active Transport
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Sodium Gradient Formed via a Na+-K+ ATPase Antiport Pump
P-type ATPases form a phosphorylaspartate and include: Ca+2 ATPases for muscle contraction Gastric H+-K+ ATPases Link
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Secondary Transport via a Common Sodium Concentration Gradient
Na+-K+ ATPase antiporter gradient drives: Glucose import via a Na glucose symporter Calcium ion export via a Na+-Ca+2 antiporter
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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
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Pufferfish: a Japanese delicacy and highly toxic
Na+ Channel Blocker Pufferfish: a Japanese delicacy and highly toxic 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 How does this cork-like toxin position itself? Host-toxicity prevention mechanism?
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Chapter 9 Problems: 11, 13, 17, 21, 25, 33 and 37
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