Collagen II

Collagen II

1. Collagen fibre = tightly packed array of Tropocollagen molecules

   • Collagen fibrils are insoluble are form due to interchain covalent crosslinks
     • These cross links involve lysine, 5-hydroxylysine and Histidine
       • CROSS-LINKING REQUIRES OXIDATION OF LYSINE TO ALLYSIDE (aldehyde)
         • Catalysed by Lysyl oxidase
           • happens outside cell!
           • Up to 4 polypeptide chains involved in cross linking
           • Copper dependant amino-oxidase
             • catalyses oxidative deamination of lysine and hydroxylysine residues
               • 3 scenarios that lead to covalent cross linking
                 • 1. 2 lysine residues can be oxidised to allysine which spontaneously form an aldol crosslink between 2 chains. Histidine and hydroxylysine allows the cross linking of 4 chains to a tetramer
                 • 2. One allysine reacts with Lys or Hyl in a neighbouring chain to form a Schiff base to ultimately form a dimer of tropocollagens. When histidine is near, a trimer can form.
                 • 3. 1 Hydroxylysine aldehyde (hydroxy allysine) reacts with a lysine or Hyl in a neighbouring chain to form another schiff base, which undergoes amadori rearrangement, leading to cross linking.
           • Toxin from sweet peas, B-aminopropionitrile, is an inhibitor to lysol oxidase, leads to disease lathyrism; abnormal weakened blood vessels and joints and bones
         • Cross-links are concentrated in the N and C terminal regions of chains
           • C terminus of one tropocollagen cross links to the n terminus of another via a staggered arrangement.
             • Staggered appearance explains striations
               • Light bands that correspond to overlap zones (where collagen lines up)
               • Dark bands correspond to the hole zones.
                 • spaces in-between staggered fibres
                   • Hole regions are of importance
                     • Sugars attached to collagen are found here, e.g glycosylation of hydroxylysine
                     • also roles in bone formation; Nucleation of hydroxyapatitite where calcium and phosphate are sequestered into the hole regions
     • essential for tensile strength and mechanical stability of fibres
   • Multiple tropocollagen molecules assemble into collagen fibrils
     • Fibrils pack together to form fibres.
       • Striations show where tropocollagen have packed together

2. Collagen Biosynthesis

   • Connective tissue collagen is synthesised in fibroblasts
   • Formed as preprocollagen with N terminal signal sequence thats cleaved by signal peptidase in ReR membrane
     • yields pro collagen
       • Pro collagen has N and C terminal extensions that are vital for folding
         • extensions allow fibres to align in register via disulphides either end
           • Polymerisation happens at C terminal end first
             • Rapid formation into tropocollagen before extensions are cleaved by collagen peptidases
               • These cleave at the Spacer regions (between cleaved thing and collagen)
                 • once cleavage occurs, spontaneous assembly into fibrils occur
               • Happens in extracellular space, if happened in cell, apoptosis would occur. so pro peptides prevent intracellular fibril formation
               • 2 types of collagen peptidase= aminoprocollagenpeptidase and carboxyprocollagen peptidase
   • Once mods done by RER to make pro collagen, tropocollagen is made and let out via exocytosis

3. 3 major classes of collagen

   • These have different amino acid sequences
     • Chains and structure differ
   • Fibrillar/ Fibril Collagen (focus )
     • TYPES 1,2,3,5,11
       • Type 1 is made up of three chains encoded by 2 genes
         • makes bone, skin and tendons etc
         • 90% of collagen in body
         • 2 chains same, 1 different
       • Type 2 is 3 identical chains
         • Makes cartilage etc
       • Type 1,2 and 3 are most abundant
     • Make staggered tropocollen arrays
   • Network forming collagen
     • 4 AND 7
     • Type 4 collagen forms basic fibrous network of all basil lamina (connective tissue underlying epithelium
       • forms long 400nm triple helices with the conserved GLY-X-Y repeat interrupted 24 times
         • These breaks makes structure less rigid and more dynamic
           • its N terminal domain forms 12-mers and C terminus forms hexameters (2xtropocolllagen)
             • These termini ARE NOT CLEAVED IN TYPE 4 NETWORK FORMING COLLAGENS
       • integrin connects collagen to cell surface
   • Fibril associated collagens
     • 9 AND 12
       • Half way between fibrillar and network forming as can form network but have GLY-X-Y sequence.
         • Have three different chains
         • Thought to function as linkers, connecting collagen chains in the ECM
   • Minor classes include transmembrane

4. Diseases and Collagen (genetic)

   • Brittle bone disease= mutation in type 1 collagen, where a point mutation changed glycine to stabilise structure.
   • Ehlers-Dalos syndrome (EDS) is an umbrella for lots of different mutations in collagen or enzymes. At least 10 disorders
     • Type 4 EDS effects type 3 collagen, mutation in glycine
     • Type 5 and 6 EDS is lysyl oxidase defective