Protein III: collagen

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Collagen is an insoluble fibrous protein and is the most common protein in mammals. 33% of bone and 50% of cartilage is made of collagen it also surrounds cells. As it is insoluble a soluble precursor tropocollagen is used to work on.

Tropocollagen

Cells that produce tropocollagen:

Fibroblasts (skin)

Osteoblasts (bone)

Chondroblasts (cartilage)

Structure:

285,000 Dalton molecular mass. Rod shape 300nm long and 1.5 nm wide. Triple helix ( 3 polypeptide chains).

Amino Acid Composition:

- 33% Glysine -10% Proline -10% Hyroxyproline - 15 Hyrdoxylysine

No Cystine or Tryptophan

Triple Helix:

Each polypeptide chain is an open left handed helix. Pitch: 0.87nm as the proline sidechain get in the way of each other. The sequence glycine - proline - hydroxyproline reoccurs frequently and every 3rd amino acid is glycine.

3 chains are together in a right - handed helix which is resistant to being stretched. Amido nitogen of glycine hydrogen bonds to adjacent C=O in another chain. Only gylcine side chains can fit into centre of helix. IT IS NOT AN ALPHA-HELIX.

Carbohydrates can be attached to the hydroxyl of hydroxylysine.

Hydroxylation of amino acids in collagen

Enzyme act on...

Prolyl - 3 - hydroxylase on prol - hydro - gly

Prolyl - 4 - hydroxylase on Gly - X - Pro

Lyslyl hydroxylase Gly - x - Lys

Reaction:

Amino acid + O2 + alpha-oxoglutarate ---> Hydroxyamino acid + CO2 + succinate

Cofactors/requirements:

Fe2+, O2 and ascorbic acid (vitamin C keeps iron as Fe2+)

Hydroxylation: growing polypeptide chain. Free amino acids are not substrates.

Scurvy: vitamin C deficiency

Advanced scurvy:lots of small haemorrhages caused by fragile blood vessels, tooth loss, bone pain and degeneration. It is caused by inadequate hydroxylation of collagen so the protein doesn't fold up correctly and is degraded by the cells proof reading apparatus.

1519-1522: magellan lost 80% of his crew to scurvy in the first circumnavigation of the globe

1749: A controlled trial showed lemon juice protected sailors from scurvy

Proline causes steric hinderance as there is no N-H for hydrogen bonding in the peptide bond

Biosynthesis of Collagen

Translation on ribosome (mRNA), polypeptide chain is fed into the ER

Hydroxylation of Pro and Lys

Glycosylation

Assembly into procollagen which has a globular head and tail

Procollagen is secreted into the extracellular space

Procollagen petidase breaks off globular regions to form tropocollagen.

It is assembled into fibrillar collagen in a staggered array, shown by the banding pattern in the electron microscope.

Cross linking of tropocollagen

Lysine side chain ---(lysylamino oxidase) ---> Allysine + Allysine + Lysine

Allysine -- > aldo cross link

Lysine --> Lysinonorleucine

Collagen type IV

No Proteolytic processing

Collagen triple helix is interrupted by short non-helical regions (gives more flexibility)

Monomer had globular head and tail