Collagen I

Collagen I

1. 25% total protein mass in mammals

   • Major proteins in ECM
     • Give tissues stability and found in skin, bones, tendons and teeth (histoarchitechture)
       • Collagen fibrils are essential in defining and maintaining shape and form of tissues
         • Major volume of extracellular space is filled with macromolecules, known as ECM
         • integral membrane proteins attach tissues to ECM
     • Other proteins include fibronectin, elastin and laminin
     • ECM roles in embryogenesis, angiogenesis, wound healing and tumour metastasis

2. 46 genes coding for collagen polypeptide in vertebrates

   • 28 different known subtypes of collagen
   • collagen polypeptide consists of around 1100 amino acids
     • 33% are Glycine
     • 10% is proline
     • 10% is 4-hydroxyproline
     • 0.1% is 3-hydroxyproline
       • Will be in position X only if 4-Hydroxyproline is at Y after.
     • Collagen sequences usually are highly repetitive- GLY-X-Y
       • X= standard proline
       • Y= 4-Hydroxyproline
       • so around 340 repeating triplets in collagen polypeptide
       • Can also have 3-hydroxyproline and 5-hydroxylysine, but this is in smaller amounts
         • 5-Hydroxylysine can be further modified by glycosylation at the 5 OH group in RER
       • So amino acids undergo post translational modifications after inclusion into polypeptide by RER
         • Proved my 1940 experiments and radioactive labelling
         • Enzymes= Prolyl and lysol hydroxylases
           • Prolyl hydroxylase
             • Require cofactor Vitamin C
               • Lack of this is reason you get unstable collagen and lead to diseases like scurvy
             • catalyse hydroxylation of prolines
             • 2 types for 4 and 3 hydroxyproline
               • 4-prolyl hydroxylase (P4H)
                 • Consists of 2 alpha and 2 beta subunits
                   • Beta subunit= protein disulphide isomerase (PDI)
               • 3-Prolyl hydroxylase
                 • catalyses proline at position X only if 4HP at Y
         • The addition of OH groups is vital for H bonding and structural stability

3. Single polypeptide chain of collagen forms a special type of left handed /polyproline II helix

   • NOT AN ALPHA HELIX, IT HAS LOTS OF PROLINE
     • Proline= helix breaker due to Ca-N torsion angles (not much more in phi/psi)
       • Proline also can't be donor in back bone , so no intra molecular H bonding, so proline makes structure rigid
   • 3.3 Residues per turn (alpha helix is 3.6)
     • 3A rise per turn (alpha helix is 1.5)
       • No intrastand H bonding like alpha helix

4. Collagen Triple helix

   • Right handed collagen triple helix
     • 3 collagen polypeptides come together
       • AXIAL REPEAT IS 29A LONG
       • not one polypeptide like alpha helix
     • IN STAGGERED FORMATION
       • GLY OF ONE, ADJACENT TO X OF ANOTHER (ALLOWS H BONDING)
         • Glycine amide (NH) donates a H bond to the C=O of residue X
       • This allows Glycine to fit and stack along centres of triple helix
       • also allows side chains to point out, allowing inter molecular H bonding
         • Stablises triple helix
           • AKA Tropocollagen
             • 285 kDa
               • 300nm long, 1.4 nm wide
         • Side chains pointing out make collagen hydrophobic protein and insoluble
           • But OH group exposed on proline increases interaction with solvent
             • It increases stability of helix
             • increases H bond network to Water
             • Hydroxyproline is vital for stability
               • Normal collagen melts at 39 degrees to gelatine
               • Mutants with mutant P4H melt at 24 degrees, so less stable
               • PROVIDES STABILITY IN 3 WAYS
                 • 1. INTERCHAIN H BONDING FROM AMIDE GLYCINE TO CO OF HYDROXYPROLINE
                 • 2. WATER MEDIATED H BONDS LINKING 2 CO GROUPS OF HYDOXYPROLINE AND GLYCINE
                 • 3. WATER MEDIATED H BONDING LINKING HYDROXYPROLINE OH GROUP AND CO OF ANOTHER HYP OR GLYCINE
                 • so 2x water interactions
           • collagen is backwards in sense that it exposes phobic residues
             • but it does back some proline side chains in core, and it can H bond to water and glycine
             • Phobic residues also stabilise tropocollagen
       • So staggering, along with modifications and inter stand H bonding make structure stable
     • Has grooves that allow unique electrostatic interactions of ligands for various functions