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Protein Presentation: Keratin

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Individual molecules combine to form insoluble structures. Keratins are the molecular basis for hair, nails, wool, feathers, beaks, claws and horns. ... – PowerPoint PPT presentation

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Title: Protein Presentation: Keratin


1
Protein PresentationKeratin
  • Group
  • Mike Dibley
  • Chris Geiger
  • Noel Barkhooy

2
Keratin
  • Fibrous (structural) proteins.
  • Individual molecules combine to form insoluble
    structures.
  • Keratins are the molecular basis for hair, nails,
    wool, feathers, beaks, claws and horns.
  • Keratin is also found in the cytoskeleton of
    cells.
  • Keratin clusters are found on chromosomes 12 and
    17.

3
  • alpha (cysteine rich) isomer found in
    cytoskeleton and hair.
  • beta (cysteine poor) isomer found mostly in birds
    and reptiles. It is the building block of
    scales, feathers and claws. It is rich in
    residues with small side chains glycine, alanine
    and serine.
  • alpha form can be stretched up to 120 in moist
    heat. beta form is rigid.
  • Cysteine can form disulfide bridges with other
    cysteine residues. These cross-linkages decrease
    the elasticity of alpha-keratin.

4
  • In the early 1950s Linus Pauling and R.B. Corey
    in proposed several structures for keratin.
  • Observed shorter than expected amide C-N bond.
    They deduced that the peptide bond was planar.
  • A planar peptide bond reduced the number of
    conformations of a poly-peptide chain and led to
    their proposal of the alpha helix and the beta
    sheet.
  • alpha-helix explained the x-ray data which showed
    a repeat unit of 0.50 0.55 nm. This distance
    corresponds to the height of the rise per
    revolution of helix.
  • alpha-helix also explained a repeat unit of 0.15
    nm. This distance corresponds to the height of
    the rise per residue. The ratio of these two
    numbers give the number of amino acids per
    revolution 3.6
  • Hydrogen bonding occurs between carbonyl oxygen
    and the amide hydrogen on next twist of helix.

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6
  • There are many forms of alpha-keratin.
  • Individual alpha-keratin molecules all follow the
    same pattern head, rod and tail (100300100).
  • The rod section contains 3 individual
    alpha-helices with short interleaving sections
    separating them.
  • Type I (40-55 kDa) are acidic. (k9-k20)
  • Type II (56-70 kDa) are neutral or basic. (k1-k8)
  • Many of the subclasses contain allelic versions
    (there are 6 forms of k6 (human)).
  • Two type I and two type II molecules form a
    heterotetramer. The arrangement of this molecule
    is a left-handed coil.

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8
Cytokeratin 12, type I
Cytokeratin 3, type II
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11
Helical Structure Continued
  • In a coil group of 7 residues, 1st 4th
    positions contain hydrophobic aas
  • These nonpolar aas on different helical chains
    attract each other and make up the inside
    positions of the double coils
  • These hydrophobic reactions stabilize the coil
    structure
  • The outside positions are mostly polar aas

12
Homology
  • Type I II keratins actually share a low
    homology lt30 at nuc. Acid level
  • High homology between same type keratins 60-90
  • Suggests a major sequence divergence
  • So although the aa sequences may differ
    significantly the structures the proteins take on
    is always the same coiled coil and interact
    with each other

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The End
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