welcome to the step one review study guide and in this lesson we learn about collagen the types of collagen synthesis and structure collagen is the most abundant protein in the human body it functions to strengthen the extracellular matrix as well as organize it there are four types of collagen type 1 makes up bone skin and tendon type 2 composed the cartilage type 3 makes up the reticulum and type 4 makes up the basement membrane be so totally cool read books let's go over these types in a little more detail type 1 imposes 90 of bone and is the most common type of collagen the cells that make the bone are osteoblasts type one collagen makes skin and tendon but also dentin fascia and the cornea in osteogenesis in perfected type 1 there is a decreased production of type 1 collagen type 2 makes up the cartilage including the hyaline cartilage vitreous body which is the clear fluid in the eye and the nucleus pulposus the nucleus pulposus is a jelly-like substance in the vertebrae that withstands compression and torsion type 3 composes the reticulum which includes the skin blood vessels uterus and fetal tissue an uncommon vascular type ehlers-danlos syndrome type 3 collagen is deficient type 4 collagen composes the basement membrane more specifically the basal lamina which is the layer of the extracellular matrix secreted by the epithelial cells it sits on remember that type 4 is under the floor and alloport syndrome type 4 collagen is defective and in good pasture syndrome the host cells target type 4 collagen by auto antibodies destroying the basement membrane let's go over synthesis first the collagen peptide sequence is glycine x y it's produced in the rough er collagen is one-third glycine and x and y are proline or lysine proline and lysine content can vary next specific proline and lysine residues are hydroxylated this process requires vitamin c a lack of vitamin c will lead to scurvy a hydroxyproline assay is a good candidate for collagen quantification because it can be found almost exclusively in collagen it can be reacted to produce a chromophore that can be quantified via absorbance now an individual chain of collagen called the pro-alpha chain are glycosylated at hydroxyl lysine residues three alpha chains then come together via hydrogen bonds and disulfide bridges into a triple helix if they cannot come together into a triple helix this leads to a condition called osteogenesis imperfecta the triple helix is then exocytosed from the cell into the ecm as the procollagen helix floats around the ecm enzymes then come to cleave the ends that are rich in disulfide bonds this creates an individual insoluble collagen fiber unit called the triple collagen many tropo collagens in the ecm are then crosslinked by enzymes containing copper called lysol oxidases they covalently link lysine to hydroxyl lysines to create the final collagen fibril if the organism is unable to cross link this results in menke's disease that's all for this lesson of step one review we'll see in the next video