Membranes and Transport

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Membranes and Transport
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Membrane Proteins

• Integral
• Peripheral

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Fig 12-11
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Peripheral Anchoring Links

• Cytosolic
• Thioester link from protein cys residue to fa
• Fatty amide link to N terminal
• Thioether link to isoprenoid
• Outside
• Glycosylated derivatives of phosphatidyl inositol
  (GPI)

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Fig 12-13
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Integral Links

• Hydrophobic strands of proteins
• At least 6 types (there are barrels, too.)
• One transmembrane helix, amino outside
• One transmembrane helix, carboxyl outside
• Multiple helices, single chain
• Multiple helices, multiples chains
• Peripheral type
• Helices and GPI anchors

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Primary sequence of membrane spanning helices

• 20-25 residues ( 0.15 nm per aa)
• Hydropathy measure of hydrophobicity
• Energetics of removal from hydrophobic phase
• Hydropathy index v. residue number
• 7 residue rolling average (window)
• ONLY works for helices

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Fig 12-17
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Fig 12-17 and 12-15
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Membrane Fusion

• Exocytosis
• Endocytosis
• Cell Division
• Fusion of egg and sperm
• Viral entry
• Transport

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Fig 12-21b
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Transport

• Passive
• Simple Diffusion
• Limited to small, nearly all hydrophobic
  molecules
• O2, N2, CH4, H2O
• Not fast enough in some instances (H2O)
• Facilitated Diffusion
• Water, many others

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Aquaporin

• Six helices per chain
• Four chains form a pore
• Single file, 5? 108 molecules per second
• Cf. 4? 107 s-1 turnover number for catalase
• Water flows in the direction of the osmotic
  gradient

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Fig 12-24
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Glucose Transporter

• 12 transmembrane helices per chain
• 5 chains form a pore
• GluT1 (erythrocytes) Kt 1.5 mM
• GluT2 (liver and other) Kt 66 mM
• GluT4 (myocytes and adipocytes) insulin
  stimulates installation in membrane

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Fig 12-25a and c
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Classes of Transport

• Uniport
• Cotransport
• Symport
• Antiport

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Kinetics of Transport

• Similar to Michaelis-Menten kinetics
• Sout T ? Sout?T, etc
• V0 VmaxSout/(Kt Sout)

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Chloride-Bicarbonate Exchanger

• In respiring tissues
• CO2 in by simple diffusion, then HCO3- out, Cl-
  in
• In lungs
• HCO3- in, made into CO2, Cl- out
• Antiport

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Fig 12-28