2 Objectives1- Structure of collagen 2- Distribution of collagen 2- Functions of collagen 3- Biosynthesis of collagen 4- Degeneration of collagen 5- Diseases of collagen (Clinical Applications)
3 What is collagen ?Collagen is the most abundant protein in the human body.Collagen is a fibrous proteinCollagen serves as having structural functions in the body .Collagen is found in components of- Skin- Connective tissue- Blood vessel walls- Sclera & cornea of the eye
4 general structure of collagen Collagen protein has primary & secondary levelsof structure.A typical collagen moleculeis a long, rigid structure in which three polypeptides(a chains) are wound around one anotherin a rope-like triple helix.Types and organization of collagen moleculesare governed by the structural role collagen plays in a particular organ.in some tissues, collagen is dispersed as a gel as in the vitreous humor of the eye.in other tissues, collagen may be arranged in parallel fibres that provide great strength as in tendonsin cornea of eye, collagen is stacked so as to transmit light.
6 Types of collagenCollagen super family of proteins includes more than 20 collagen typesThe three polypeptide a chains are held together by hydrogen bonds between chainsa chains are ~1000 amino acids long but with slightly different propertiesThese a chains are combined to form the various types of collagen found in tissues.Examples:Type 1 collagen: Most common collagencontains two a1 chains and one a2 chaina12 a2Type II collagen: contains three a1a13
9 detailed structure of collagen 1- Amino acid sequenceCollagen is rich in proline & glycineProline :by its ring structure facilitates the formation of the helical conformation of each a chain.Glycine (smallest amino acid):present in every third position of the chain.It fits into the spaces where the three chains of the helix come together.Glycines are parts of a repeated sequence [-Gly- X – Y – ] where:X is frequently Proline Pro & Y is often Hydroxyproline Hyp (can be Hydroxylsine Hyl)So, most of the a chain can be POLYTIPEPTIDE with sequence of:(- Gly- Pro – Hyp -) 333
11 2-Triple-helical structure detailed structure of collagen (cont.)2-Triple-helical structureCollagen as a fibrous protein has an elongated , triple-shaped helicalstructure in which side chains (R- groups) of its amino acid are on thesurface of the triple-helical molecule.This allows bond formation between the exposed R-groups ofneighbouring collagen monomers resulting in their aggregation into longfibres.
12 detailed structure of collagen (cont.) 3- hydroxyproline & hydroxylysine Hydroxyproline & hydroxylysine are not present in most proteins, however, collagen contains them. These two amino acids result from the hydroxylation of some proline & lysine amino acids after their incorporation into poly peptide chains during protein biosynthesis. (i.e. post-translational modification) Hydroxyproline is important in stabilizing the triple-helical structure of collagen because it increases (maximises) interchain hydrogen bond formation.
13 prolyl hydroxylase enzyme ascorbic acid (vitamin C) detailed structure of collagen (cont.)Proline is hydroxylated byprolyl hydroxylase enzymewhich requiresascorbic acid (vitamin C)as a coenzyme
14 detailed structure of collagen (cont.) 4- glycosylationThe hydoxyl group of hydroxlysine amino acids of collagen may be glycosylated byenzymatic reaction.Glucose & galactose molecules are attached to the polypeptide chain priorto triple helix formation.
15 biosynthesis of collagen The polypeptide precursors of collagen molecules are formed intracellularly in fibroblasts (or in osteoblasts of bone & chondroblasts of cartilage) & then secreted into the extracellular matrix.In the extracellular matrix, mature collagen monomers are formed.Collagen monomers aggregate & become cross-linked to formcollagen fibrils.
16 biosynthesis of collagen (cont.) intracellular stage (in fibroblasts)1- Formation of pro-a chain (protein biosynthesis on ribosomes of RER)2- Hydroxylation of some proline & lysine amino acids on Y-positionof Gly – X – Y sequence of pro-a chain by enzymes prolyl & lysyl hydroxylases (requires vitamin C)In cases of vitamin C deficiency (Scurvy):Hydoxylation of proline & lysine will not occur, with no cross linking resulting in decrease in the tensilestrength of collagen fibers.Clinical manifestations: bruises in the limbs due to subcutaneous extravasation of blood (capillary fragility)3- Glycosylation: some hydroxylysine amino acids are glycosylated by glucose or glucose-galactose4- Assembly & Secretion;Pro-a collagen form procollagen which is a precursor of collagen that has a central region of triple helix, flankedby non-helical amino – and caboxy- terminal extensions of the pro-a chain.Procollagen molecules are translocated to the Golgi apparatus where they are packaged in the secretoryvesicles which fuse with cell membrane causing the release of procollagen into extracellular space.
19 biosynthesis of collagen (cont.) Extracellular Stage (in matrix)5- Extracellular cleavage of procollagen molecules:Procollagen molecules are cleaved by N- & C- procollagen peptidaseswhich remove the terminal nonhelical peptides resulting in the formation of triple helical tropocollagenmolecules.6- Formation of collagen fibrils:Individual tropocollagen molecules associate to form a collagen fibril.Collagen fibril form an ordered, overlapping, parallel array with adjacent collagen molecules in astaggered pattern.7- Cross-link formation of collagen fibrilsby the enzyme lysyl oxidase which oxidatively demainates some lysine and hydroxylysine amino acidsin collagen forming aldhydes.An aldehyde from one collagen polypeptide link with lysine or hydroxylysine amino acids ofa neighboring collagen polypeptide to form covalent cross-links resulting in mature collagenformation.
20 Extracellular Stage of Collagen Synthesis (in matrix) ProcollagenTropocollagen(Collagen molecule)Collagen FibrilExtracellular StageofCollagen Synthesis(in matrix)
22 on one collagen polypeptide Cross-links formationincollagenAldhyde group formedon one collagen polypeptideNH2 of lysine onneighbor collagenpolypeptideCross-link between adjacent collagen polypeptides
23 degradation of collagen Half-lives of normal collagen are several years.Collagen fibrils is degraded by collagenases (matrix metalloproteinases) that cleave a collagen molecule into two fragments 3/4 & ¼ length.Then by other matrix proteases, these fragments are degraded to individual amino acids.
24 collagen diseasesDefects in any one of the steps of collagen fibre synthesis can result in a genetic disease involving inability of collagen to form fibres properly (& thus collagen will not provide tissues with the needed tensile strength).More than 1000 mutations have be identified in 22 genes coding for 12 of the collagen types.
25 collagen diseases (cont.) Ethlers-Danlos syndrome (EDS)Results from inherited defects in the metabolism of fibrillar collagen molecules.EDS can result from:Deficiency in collagen-processing enzymesas lysyl hydroxylase deficiency or procollagen peptidase deficiencyMutations in the amino acid sequences of collagen type I, II, III, or type V.(most important mutations are in of genes for type III)Collagen containing mutant chains is not secreted. So, it is degraded or accumulated in cells.Clinical Manifestations:Lethal vascular problems of arteries (Collagen III is an important component of arteries)Fragile & stretchy skinLoose joints
26 collagen diseases (cont.) Osteogenesis imperfecta (OI)(Brittle bone syndrome)Bones easily bend & fractureRetarded wound healingRotated & twisted spine leading to humped-back appearance