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Updated: Aug 13 2021


  • Overview
  • Synthesis and Structure
    • Inside fibroblasts
      • pre-pro-collagen α chain formation
        • RER-bound ribosomes synthesize
        • contains hydrophobic translocation sequence
        • chain formed mainly of repeating tripeptide
          • Gly-X-Y
            • X and Y are proline, lysine
      • pro-collagen α chain formation
        • hydrophobic sequence cleaved
      • hydroxylated pro-collagen α chain formation
        • X and Y position prolines and lysines are hydroxylated to form hydroxylysine and hydroxyproline.
          • these amino acids are unique to collagen
          • hydroxylated as peptide chain passes into ER
          • performed by prolyl and lysyl hydroxylase
          • lack of lysyl hydroxylase function results in weak collagen chains
            • requires ascorbic acid (Vitamin C)
              • lack of vitamin C causes scurvy
                • presentation
                  • swollen gums, bruising, anemia, poor wound healing
            • can also be caused by defective lysyl hydroxylase gene
              • Ehlers-Danlos syndrome
                • nine different types
                • lysyl hydroxylase gene deficiency is one of many causes
                • presentation
                  • hyperextensible skin, hyperflexible joints, weak vessel walls (↑ risk for aneurysm)
      • glycosylated pro-collagen α chain formation
        • hydroxylysines are glycosylated
      • pro-collagen α chain trimer formation
        • three α chains associate
        • moved from RER to Golgi
        • secreted out of the fibroblast
    • Outside fibroblasts
      • collagen molecule (tropocollagen) formation
        • propeptides cleaved from ends and becomes insoluble
        • presence of propeptide does not allow assembly intracellularly
      • collagen fibril formation
        • catalyzed by lysyl oxidase
          • covalently links α chains by crosslinking hydroxylysines
          • copper required as cofactor
            • lack of copper results from Menkes disease
              • at low serum concentrations of copper this enzyme cannot function and weak collagen is formed
              • cause
                • X-linked gene mutation in ATP7A
                  • ATP-dependent copper efflux protein
                • aka Ehlers-Danlos syndrome type IX
                • inability of enterocytes to release absorbed copper
                • copper at toxic levels in small intestine and kidneys
                • copper in circulation and in brain at low levels
              • presentation
                • presents like a copper deficiency
                • seizures, failure to thrive, neurodegeneration
                • steel-colored and brittle hair
      • collagen fiber formation
        • fibrils aggregate to form final bundles of triple helix quaternary protein structure
  • Collagen Types
    • Type I
      • thick, rope-like bundles of collagen
        • strongest tensile form of collagen
      • majority of collagen in the body (approx. 90%)
      • found in locations where high tensile strength is needed
        • bone, fascia, tendons, teeth (dentin), cornea, skin
        • type III of early wound repair converted to type I in late wound repair
      • defective in osteogenesis imperfecta (OI) type I
        • aka brittle bone disease
        • AD, in most cases
        • presentation
          • multiple fractures with minimal force
            • first fractures may occur during delivery in severe form caused by collagen mutation
            • fractures will be milder and occur later in childhood with collagen deletion
          • blue sclerae
            • due to the translucency of the connective tissue over the choroid due to lack of collagen
          • deafness (50%)
            • abnormal middle ear bones
          • dental abnormalities
        • may be confused with child abuse
      • defective in various forms of Ehlers-Danlos syndrome
        • faulty collagen synthesis
        • see above
    • Type II
      • spongy collagen to absorb shock
      • found in tissues where there are compression forces
        • cartilage (including hyaline), vitreous body of the eye, nucleus pulposus of vertebral disc
    • Type III
      • web-like fibers where forces pull from many directions
        • aka reticulin
      • found in tissues where strength is needed (but not compression or tensile)
        • skin, blood vessels, uterus, fetal tissue
        • granulation tissue
          • type III of early wound repair converted to type I in late wound repair
      • defective in Ehlers-Danlos type IV
        • faulty collagen synthesis
        • associated with
          • joint dislocation
          • berry aneurysms
          • organ rupture
        • see above
    • Type IV
      • basement membrane
        • especially kidney, ears, eyes, skin
      • organizes/solidifies cellular structure
      • defective in Alport's syndrome
        • effects the tissues where type IV is most prominent
          • kidney → progressive hereditary nephritis
          • ears → deafness
          • eyes → ocular disturbances
        • majority of cases are X-linked dominant
      • one of the causes of epidermolysis bullosa
        • weak union of dermis and epidermis of the skin
        • easily formed blisters
      • Goodpasture's syndrome involves an auto-antibody against collagen type IV in pulmonary and glomerular capillaries
        • presents with hemoptysis and glomerular disease
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