Biochemistry of Connective Tissue Handout PDF

Clinical Relavance Articular Cartilage A type of connective tissue that allow bones to glide against each other smoothly Loss cartilage à friction à pain, stiffness, swelling (eg. Osteoarthritis) https://www.nonamedicalarts.com/pain-conditions...

Clinical Relavance Articular Cartilage A type of connective tissue that allow bones to glide against each other smoothly Loss cartilage à friction à pain, stiffness, swelling (eg. Osteoarthritis) https://www.nonamedicalarts.com/pain-conditions/knee-pain/  Articular Cartilage Extracellular Matrix Extracellular matrix (ECM) Non-cellular component A complex network of molecules (collagen, glycoproteins, proteoglycans) that surround and support cells in tissues Functions Provide structual and mechanical support Elasticity & high tensile strength Absorb shock and weight Cell signaling and adhesion Collagen: provide structural support (firmness and strength) Proteoglycans and GAG : provide gel-like consistency of the matrix (absorb shock and resist compression) Chondrocyte Chondrocyte Cells that produce components of extracellular matrix Glycoproteins What, Where and Why (why do we need it)? A protein molecule with carbohydrate (oligosaccharides) attached (by glycosylation). Found on cell membrane, ECM, secretions, plasma Roles: ECM component, cell-cell communication, immune response, cell adhesion, cell surface recognition (cell signaling), enzyme, stability in circulation, antibodies, hormones (eg. LH, FSH, TSH, hCG), mucins in respiratory, GI tract and urogenital tracts. Almost all of the globular proteins of plasma are glycoproteins. Oligosaccharide Protein Both Glycoprotein and proteoglycan consist of carbohydrate + protein Glycoprotein: contains less/short/branched carbohydrate (eg. IgG contains < 4% of carbohydrate ) Proteoglycan: contains more/long/unbranched carbohydrate (eg. aggrecan contains > 80% of carbohydrate ) Structure of glycoproteins Oligosaccharides covalently linked to the proteins by N- or O-Glycosidic bonds. N-glycosidic bonds are formed between the sugar and N of Asparagine (Asn). O-glycosidic bonds are formed between the sugar and O of Serine or Threonine (Ser, Thr). N linked glycoproteins N-Linked Complex oligosaccharides Both contain the same pentasaccharide core N-Linked The complex group contains a diverse group of additional sugars. O-Linked P High mannose oligosaccharides* Man: mannose GlcNAc: N-acytylglucosamine Lippincott Fig. 14.14 Glycoproteins Synthesis Proteins are synthesized on ribosomes that are bound to the endoplasmic reticulum (rough, RER.) N-link Glycoproteins: glycosylation begins in ER, completed in Golgi O-link Glycoproteins: glycosylation happens in Golgi * * Transported to destination Membrane (integrated into the membrane of the Golgi secretory vesicles). * Secretory vesicles (stay in the lumen of the Golgi secretory vesicles) To cell Secretory vesicles Lysosome membrane  Glycosaminoglycans (GAGS) & Proteoglycans Proteoglycans are lubricants and support elements of connective tissue. Proteoglycans: consist of a core protein molecule attached with glycosaminoglycan (GAGs) Glycosaminoglycans (GAGs) are long, unbranched heteropolysaccharide chains Located primarily on the outer surface of cells or in the extracellular matrix (ECM) Serve as a flexible support for the ECM, interacting with the structural and adhesive proteins. Provide viscous, lubricating properties of mucous secretions (mucopolysaccharides)  Repeating disaccharide units Some monosaccharide Glycosaminoglycans (GAGS) units in GAGs GAGs are large complexes of negatively charged heteropolysaccharide chains Composed of repeating disaccharide units [acidic sugar and N-acetylated amino sugar]. A carboxyl group and sulfate groups, making GAGs highly negatively charged* Amino sugar (Amino group is acetylated, eliminating its positive change) Eg. -> N-acetylglucosamine (GlcNAc) -> N-acetylgalactosamine (GalNAc) Six classes of GAGs Hyaluronic acid (no sulfate), do not form proteoglycan Chondroitin sulfate (most abundant) Heparin Keratan sulfate Heparan sulfate Dermatan sulfate n Repeat unit of heparan sulfate Proteoglycans (Core protein + GAGs) Bottle brush model of a cartilage proteoglycan monomer Monomer structure: consist of a core protein attached to linear chains of GAGs (eg. in cartilage proteoglycans : GAGs include chondroitin sulfate and keratan sulfate). GAGs-protein linkage: GAGs attached to core protein via covalent linkage, most commonly through a trihexoside (galactose–galactose–xylose) and a serine residue in the protein. An O-glycosidic bond is formed between xylose and the OH group of the Serine Aggregate formation: Many proteoglycan monomers can associate with one molecule of hyaluronic acid to form proteoglycan aggregates (main component of cartilage’s ECM). The association is stabilized by additional small proteins called link proteins (trihexoside) Lippincott Fig. 14.5, 14.7 Synthesis of the amino sugars The synthetic pathway of amino sugars is very active in connective tissues (20% of glucose flows through this pathway) Synthesis of the acidic sugars Glucuronic acid and iduronic acid are essential components of GAGs. Glucuronic acid is also required for the detoxification of lipophilic compounds, such as bilirubin, steroids, and many drugs Core protein synthesis The core protein is made by ribosomes on RER, enters the RER lumen, and then moves to the Golgi, where it is glycosylated by membrane-bound glycosyltransferases.  GAGs Localization and Function GAG Localization Note Hyaluronic Acid Synovial fluid, vitreous humor, ECM of loose The only non-sulfated GAG*. (Hyaluronan) connective tissue, cartilage Do not covalently link to protein (attach to (Tissue hydration, lubricant, cell movement, shock absorbing) proteoglycans to form proteoglycan aggregates) Chondroitin sulfate Cartilage, tendon, ligaments bone, skin, vessels, heart valves. Most abundant GAG in the body. In cartilage, bind collagen and hold fibers in a tight, strong Form proteoglycan aggregates through network noncovalent association with hyaluronic acid Heparan sulfate Extracellular GAG found in basement membranes, cell surfaces. (cell adhesion, growth factor binding) Heparin Component of intracellular granules of mast cells that line the Serve as an anticoagulant. arteries of the lungs, liver and skin. Dermatan sulfate Skin, blood vessels, heart valves. Keratan sulfate Cartilage, bone, Cornea of the eye, loose connective tissue Proteoglycan aggregates with chondroitin sulfates. Structure Function Relationship* Biochemical shock absorbers Compressed Cartilage’s GAGs : ………………………..…and……………………….. Synovial fluid’s GAGs :……………………………. Large number of negative charges The chains are extended in solution Repel each other Surrounded by a shell of water molecules. Reduces friction When they are close proximity to each other, they slip past each other This produces the slippery consistency of mucous and synovial (joint) fluid. Absorb shock (GAGs absorb water) When they are compressed, the water is squeezed out and they occupy a smaller volume. Relaxed Once the compression is released they spring back (and are immediately rehydrated) to their original volume due to the repulsive forces of their negative charges. Lippincott Fig. 14.3 Collagen The most abundant protein in the human body. Long, rigid, three polypeptides (α chains) are wound around one another in a rope-like triple helix Collagen is composed a triple-stranded helical rod rich in glycine and proline residues. OH OH OH X : often proline Y : often hydroxyproline (but can be hydroxylysine) 2a1+a2 * ***Hydroxyproline is an amino acid unique to 3a1 * collagen (useful for measure collagen content) 3a1 * Three polypeptide chains are held together by interchain hydrogen bonds (Procollagen -> Tropocollagen) Dispersed as a gel that gives support to the structure (eg. ECM or the vitreous humor of the eye). Bundled in tight, parallel fibers that provide great strength (eg. tendons) Collagen of bone occurs as fibers arranged at an angle to each other so as to resist mechanical shear from any direction. Collagen Synthesis In RER 1. Prepro–α chains synthesis *** 2. Removal of signal sequence creating Pro–α chains *** 3. Hydroxylation of Proline/Lysine (Require Vitamin C) ***Hydroxyproline maximizes formation of interchain H bonds that stabilize the triple helical structure 4. Hydroxylysine may be glycosylated 5. Triple Helix formation (Procollagen) (from a Golgi vacuole) 6. Secreted into the ECM Procollagen peptidase In ECM 7. Procollagen is cleaved producing Tropocollagen Scurvy (vitamin C deficiency) 8. Tropocollagen spntaneously associate to form collagen fibrils Fail to stabilize the triple helix *** 9. Cross-link formation (stabilized fibrils) by Lysyl Oxidase Collagen fibril Ecchymoses (bruise-like) on the limbs (Copper (Cu+) is required as a cofactor) to form mature collagen fiber Capillary fragility Mature collagen fiber Subcutaneous leakage of blood Collagen Disorders Mutations in the major bone and cartilage collagens (types I, II, IX, X, and XI) give rise to highly variable presentations ranging from lethal disease to premature osteoarthritis (OA). Collagen that contains mutant chains may have altered structure, secretion, or distribution, and it frequently is degraded. Mutations of genes encode α chains Abnormal posttranslational modification Ehlers-Danlos Syndromes (EDS) Deficiency of cofactors There are several variants of EDS, all characterized by defects in collagen synthesis or assembly. Clinical features: fragile, hyperextensible skin, hypermobile joints; and ruptures involving the colon, cornea, or large arteries. Classic form of EDS (defect in type V collagen) skin and joint hypermobility Type V collagen is a fibrillar collagen (essential for fibrillation of types I and III collagen Vascular form (defect in collagen type III) associated with potentially lethal arterial rupture most serious form of EDS Osteogenesis Imperfecta (OI) or brittle bone disease The most common inherited disorder of connective tissue caused by deficiencies in type I collagen synthesis. It is caused by mutations in genes encoding the α1 and α2 chains of type I collagen (most common @ glycine) OI principally affects bone, but it also impacts other tissues rich in type I collagen (joints, eyes, ears, skin, and teeth). Too little bone, resulting in extreme skeletal fragility. Subtypes of Osteogenesis Imperfecta (OI) https://en.wikipedia.org/wiki/Osteogenesis_imperfecta Blue sclerae caused by decreased collagen content, making the sclera translucent and allowing partial visualization of the underlying choroid Skeletal radiograph of a fetus with lethal type 2 OI. Note the numerous fractures of virtually all bones, resulting in accordion-like shortening of the limbs. Robbins & Cotran Pathologic Basis of Disease