Download Keratin

Protein Presentation:
Keratin
Group:
Mike Dibley
Chris Geiger
Noel Barkhooy
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.
• 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.
• 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.
• There are many forms of alpha-keratin.
• Individual alpha-keratin molecules all follow the
same pattern: head, rod and tail (100:300:100).
• 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.
Cytokeratin 12, type I
Cytokeratin 3, type II
Helical Structure Continued
• In a coil group of 7 residues, 1st & 4th positions contain hydrophobic
aa’s
• These nonpolar aa’s 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 aa’s
Homology
• Type I & II keratins actually share a low homology; <30%
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
The End