BCH210H1 Lecture Notes - Lecture 16: Cytosol, Beta Barrel, Hydronium
49
BCH210H1 Full Course Notes
Verified Note
49 documents
Document Summary
Recall that aa in membrane spanning segments are mostly hydrophobic. Problem #1: peptide backbone is highly polar (nh and c=o) Intra-molecular h bonds bw one residue to (residue +4) to neutralize polar backbone. Helix traverses membrane with minimal disruption of lipid packing. Hydrophobic side chains point out toward hydrophobic membrane. All hydrophilic, polar items are within the helix. 10-30% of integral proteins have one membrane spanning segments. Always cross the membrane as an alpha-helix --> no other structure. Lipid bilayer is 3-4 nm, so 20-25 residues are required to span whole layer. Problem #2: hydrophobic amino acids generally have low propensity to form alpha-helices. Val, ile, phe are top beta-sheet formers but they need to be in the membrane. For example, glycophorin a is rich in beta-sheet residues and has only 3 glycines. Backbone neutralization + hydrophobicity supersede amino acid structural propensities. Chou-fasman still applies, but it"s always a battle between propensity and hydrophobicity.
