Lecture 17 Flashcards
COLLAGEN - different types
All collagen contain 3 polypeptide chains (but the composition of the alpha chains varies)
Fibril forming:
Type I - associated with skin and bone, tendon, blood vessels, cornea
Type II - cartilage, intravertebral disc, vitreous body
Type III - blood vessels, fetal skin
Network forming:
Type IV - Basement membrane
Type VII - Beneath stratified squamous epithelial
Fibril-associated:
Type IX - Cartilage
Type XII - Tendon, liganment, some other tissues
What type of protein is collagen?
Fibrous
What is the basic unit of collagen?
Tropocollagen
Tropocollagen (number of polypeptides, structure, the amino acids)
- Type 1 collagen: 3 polypeptides (two alpha-1 chains and one alpha-2 chains)
- Different collagen types can be formed from different combinations of alpha chains
- Glycine in every 3rd position, along each alpha chain i.e. (Gly-X-Y)n repeat
- Characteristics triple helix (right-handed) (not alpha helices)
Triple helix characteristics
- Non-extensible (not able to be extended)
- Non-compressible (not capable of being compressed)
- High tensile strength (Max. stress can be applied before it breaks)
Formation of the helix and the ultimate assembly into larger structure = very high tensile strenght + makes them virtually non-compressible
What residues are in the alpha helix?
- Glycine residues
- Proline residues
- Hydroxy group
Role of glycine
Glycine is the only amino acid with a side chain small enough to fit in the middle of the helix. It leads to the formation of a triple helix.
Pic below:
In this image you can see a cross-section of a tropocollagen molecule with the Glycine (G) indicated projecting into the centre of the helix. You can see that this residue is the only one small enough to fit in this rather small space. Mutation which change these glycines can have a big effect on the structure and stability of a collagen fibre.
Role of proline
Prevents the polypeptide adopting alternative secondary structures like alpha helix or beta sheet
Hydroxy-group in hydroxyproline
The presence of hydroxylated residues in each of the alpha chains allows the formation of inter-chain hydrogen bonds which stabilises the structure and helps give collagen some of its tensile strength. Hydrogen bonds between alpha chains, NOT within the alpha chain
NOTE – this structure is different from an alpha helix where the H-bonds are within a single chain
What amino acids are usually in X and Y positions? (Gly-X-Y)
Mostly proline or hydroproline in X and some Y positions
What is the result is a mutation leads to the lack of these hydroxylated amino acid residues?
Mutants that lack these hydroxylated amino acid residues are much less strong and this can have physiological effects
Hydrogen bonds between alpha chains stabilise the structure
Summary of tropocollagen
- 300nm rod-shaped protein
- 3 polypeptides (alpha chains), each roughly 1000aa long
- gylcine in every 3rd position along each alpha chain i.e. (Gly-X-Y)n repeat
- Characteristic triple helix (right-handed)
- Mostly proline or hydroproline in X and some Y positions
- H-bonds between alpha chains stabilise structure
Summary of collagen
- A triple helical arrangement of collagen alpha chains
- Contains Gly-X-Y repeating sequence
- Hydrogen bonds stabilise interactions between chains
Individual tropocollagen molecules are assembled outside the cell to form collagen fibrils and ultimately fibres. The alignment of the tropocollagen molecules gives the characteristic banding pattern that is seen when viewed by electron microscopy..
Seperation of proteins by SDS-PAGE
Type of electrophoresis that seperates proteins on the basis of size
- SDS (detergent): unfolds proteins, leads to a linear polypeptide chain
- 2-mercaptoethanol: reduces disulphide bonds (seperates the 3 polypeptide chains)
SDS-PAGE - what would you expect to see of someone with normal collagen
2 bands with one band more intense than the other (as two alpha-1 chains and one alpha-2 chains)
