Transmembrane must sheets have several strands to

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Unformatted text preview: gral Membrane Proteins ! Transmembrane helices !  Need 20 amino acids to span the bilayer !  Polar aromatic residues often occur where helix approaches polar head groups !  Sequence easy to spot Nonpolar: green Polar: purple + 37 Integral Membrane Proteins ! Transmembrane ! Must sheets have several strands to satisfy H-bonding of backbone ! Remember: peptide bonds are polar ! Form a barrel ! Smallest has 8 strands ! Narrow cavity + 38 Integral Membrane Proteins ! Transmembrane ! Exterior sheets surface has hydrophobic residues ! Those residues that face interior not necessarily hydrophobic ! Aromatic residues interface with lipid head groups ! Not easy to spot in sequence Hydrophobic: green Aromatics: gold + Lipid-linked Proteins ! Otherwise soluble proteins, except for lipid anchor ! Some may also have transmembrane regions ! May be anchored inside or out 39 + Lipid-linked Proteins (a) Protein 40 Carbohydrate residues (b) Lipid-carbohydrate anchor Lipid anchor Lipid anchor + Fluid Mosaic Model 41 !  Described by Singer and Nicolson in 1972 !  Membrane proteins float randomly in sea of lipids !  Mostly true – but not entirely free to move !  Movement restricted by: Interactions with other proteins !  Interactions with cytoskeletal proteins ! 0072507470/234417/mc_ch03_fig01.jpg + 42 Fluid Mosaic Model ! Membrane protein may: A) Be immobile !  Attached to cytoskeleton B) Mobile within small area C) Freely mobile (not shown) ! Proteins as well as lipids may be asymmetrical in membrane...
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