Amino sugars Proteoglycans

Amino sugars Proteoglycans ASSOC. PROF. DR.CEMİLE KOCA ANKARA ATATÜRK TRAINING AND RESEARCH HOSPITAL

Chapter 8

Glycoconjugates • Homoglycans: composed of a single type of monomer • (Starch, cellulose and chitin) • Heteroglycans: multiple types of monosaccharide units are joined together • Glycoconjugates: Polysaccharides (usually heteroglycans) are covalently linked with other chemical species (prot, lipids) • - Protein linkedGlycoconjugates can be organized into three main types: • Proteoglycans(mostly carbohydrate, some protein) • Peptidoglycans(short peptides joined to polysaccharide chains) • Glycoproteins(proteins with short carbohydrate chains)

Monosaccharides Are Joined to Molecules Through Glycosidic Bonds • CHOs can be attachedbyglycosidicbondstonon-CHO structures likepurineandpyrimidinebases (nucleicacids), aromaticrings(such as thosefound in steroidsandbilirubin), proteins (found in glycoproteinsandglycosaminoglycans), andlipids (found in glycolipids) • O-glycosidic bond : bond formed between the anomeric carbon atom of a sugarand the hydroxyl oxygen –OH atom of othermolecule (CHO, alcohol, prot, lipid …) • N-glycosidic bond: betweenanomericcarbon atom of a sugar andthe nitrogen atomof an amine-NH2groupon thenon-carbohydratemolecule

Amino Sugars • Amino sugars are monosaccharides where one or more of the hydroxyl groups -OH have been replaced with amino -NH2groups • Only three amino sugars are common in nature • N-Acetyl-D-glucosamine is a derivative of D-glucosamine

Amino Sugars • One simple amino sugar is a-D-glucosamine (GlcN), which has an amino group on the C2 carbon • The amino group is often further modified by acylation, as seen in N-acetyl-a-D-galactosamine(GalNAc)

Amino Sugars (Hexosamines) areessential components of ; • Glycosaminoglycans • Glycoproteins • Glycolipids • also found in some antibiotics • The synthetic pathway of amino sugars is very active in connective tissues, where as much as 20% of glucose flows through this pathway • Several antibiotics (eg, erythromycin) contain amino sugars, which are important for their antibiotic activity

Proteoglycans(mostly carbohydrate, some protein) • Peptidoglycans(short peptides joined to polysaccharide chains) • Glycoproteins(proteins with short carbohydrate chains)

Extracellular Macromolecules A major component of connective tissue (polysaccarides + proteins) Glycosaminoglycans Proteoglycans GlycoproteinsMucins Ground substance Amino sugars are used as components in glycosaminoglycans These are composed of repeating disaccharide units, often further modified with anionic groups, to increase viscosity and serve as lubricants

Extracellular Macromoleculesground substance + fibers macromolecule% carb. glycosaminoglycans* (GAGs) 100 proteoglycans* 90-95 glycoproteins 2-30 fibrous proteins 1-2 Examples of functions: mechanical support lubrication cushioning adhesives cell spacers selective filters * mucopolysaccharides, mucoproteins, respectively 1

Synthesis of the amino sugars N-Acetylglucosamine (GlcNAc) & N-acetylgalactosamine (GalNAc): • fructose 6-phosphate is the precursor of GlcNAc, GalNAcandthesialicacids, including N-acetylneuraminicacid (NANA, a nine-carbon, acidicmonosaccharide)

N-Acetylneuraminic acid(NANA): • is a member of the family of sialic acids, each of which is acylated at a different site • Before NANA can be added to a growing oligosaccharide, it must be converted into its active form by reacting with cytidine triphosphate (CTP) • This is the only nucleotide sugar in human metabolism in which the carrier nucleotide is a monophosphate • The carbons and nitrogens in NANA come from N-acetylmannosamine and phosphoenolpyruvate

Proteoglycans (PGs) aregiantmoleculecomplexes consisting of CHO (95%) & proteins(5%) bottlebrush-shaped structure composed of as many as 200 GAG chains covalently bonded to a core protein = protein core + GAGs

Each proteoglycan is covalently linked to hyaluronic acid forming huge macromolecules

Proteoglycans (PGs) aggrecan, the major proteoglycan of cartilage • macromolecules of the cell surface or extracellular matrix, connective tissues • These molecules are often highly hydrated and occupy a large volume because their glycosaminoglycan components contain polar and ionic groups • This hydration produces elasticity and resistance to compression, allowing the cartilage and joints to absorb and recover from mechanical shock and vibration

Proteoglycans (PGs) • at least 30 members of the proteoglycan superfamily in mammaliancells Exp: Aggrecan (major component of cartilage) Syndecan (act as transmembrane cell surface receptors Perlecan (basementmembrane) Neurocan….

Functions of Proteoglycans- 1 • regulate the extracellularassembly of collagenfibrils • Resist compression and retard the rapid movement of microorgani and metastatic cells • Form molecular filter (in association with basal lamina) of varying pore sizes and charge distributions that selectively screen and retard macromolecules as they pass through them

Functions of Proteoglycans- 2 • Contain binding sites of certain signaling molecules eg. TGF-ßso mediate the activitiesof various growth factors • influence thedevelopment of specialized tissues • Epithelial branching and differentiation • Eye development • Limb development

 1m  Example PGs: Aggrecan • major GAG–PGin cartilage • link proteins bind noncovalently • with bound H2O,disperses shocks,compressive force • adhesion proteins link to collagen & cells • degraded by chondroitin sulfatase, etc core protein link proteins hyalur-onan keratansulfate chondroitinsulfate

Example PGs: Syndecan • cell-surface PG • core protein domains • intracellular • transmembrane • extracellular 5 GAGs attached • functions • interactions • cell-cell • cell-matrix • growth factor receptor GAG chains outside inside core protein

Peptidoglycans • heteroglycan chains linked to peptides • Major component of bacterial cell walls • Heteroglycan composed of alternating N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) • b-(1-4) linkages connect the units • The antibiotic penicillin works by interfering with the structure

Glycosaminoglycans(GAGs) • are large complexes of negatively charged heteropolysaccharide chains • unbranchedheteroglycans • repeating disaccharides (AB)nABABAB… • A is usually 1 uronic acid (hexose with C6 as COO– ) (glucuronic acidor its carbon-5 epimer, L-iduronic acid) • B is 1 amino sugar (GlcNac, orGalNac) • Many of the amino sugars arealso be esterified with sulfuric acid (sulfated) on carbon 4 or 6 or on a nonacetylated nitrogen also, furtherincreasing their polarity A sugar B sugar

Addition of sulfate groups to GAGs • Sulfation of the carbohydrate chain occurs after the monosaccharide to be sulfated has been incorporated into the growing carbohydrate chain • The source of the sulfate is 3'-phosphoadenosyl-5'-phosphosulfate (PAPS, a molecule of AMP with a sulfate group attached to the 5'-phosphate) • Sulfotransferases cause the sulfation of the carbohydrate chain at specific sites

Glycosaminoglycans(GAGs) • exist as: • independent moleculese.g., hyaluronate & heparin • parts of larger structures e.g., in proteoglycans • The proteoglycansbind large amounts of water, producing the gel-like matrix and fill thegaps between the fibrillar components (collagen…) ofthe ECM • forms the basis of the body's ground substance • Thisinhibits the spread of pathogens in the ECM, for example

Linkage region of glycosaminoglycans

GAGs 1. Hyalurinic acid (hyaluronate) 2. Chondroitin sulfate 3. Dermatan sulfate 4. Heparansulfate 5. Heparin 6. Keratan sulfate

GAG structure: repeating units GAG A sugar B sugarhyaluronate glucuronate N-acetyl glucosamine chondroitin sulfate glucuronate N-Ac galactosamine 4-SO4 dermatan sulfate iduronate " heparan sulfate glucuronate glucosamine N-SO3, 6-SO4 heparin iduronate 2-SO4 " keratan sulfate galactose N-Ac glucosamine 6-SO4 *opposite configuration in iduronate glucuronate/iduronate: epimers at C5 glucose/galactose: epimers at C4 * 2 5

Degradation of Glycosaminoglycans • degraded in lysosomes, which contain hydrolytic enzymes that are most active at a pH of approximately 5 • The low pH optimum is a protective mechanism that prevents the enzymes from destroying the cell should leakage occur into the cytosol where the pH is neutral • Relatively short half lifes: 3-10 days (hyaluronic acid, chondroitin and dermatan sulfate, respectively) • Exception: keratan sulfate, half life more than 120 days

Mucopolysaccharidoses • hereditary disorders (1:25,000 births), clinically progressive • characterized by accumulation of GAGs in various tissues, • causing varied symptoms, such as skeletal and extracellular matrix deformities, and mental retardation • Mucopolysaccharidoses are caused by a deficiency of any one of the lysosomal hydrolases normally involved in the degradation of heparan sulfate and/or dermatan sulfate • This results in the presence of oligosaccharides in the urine, because of incomplete lysosomal degradation of glycosaminoglycans • These fragments can be used to diagnose

Children who are homozygous for one of these diseases are apparently normal at birth, then gradually deteriorate • In severe cases, death occurs in childhood • All of the deficiencies are autosomal and recessively inherited except Hunter syndrome, which is X-linked • Bone marrow and cord blood transplants have been used to treat Hunter syndrome

Mucopoly-saccharidoses

Glucosamine and Osteoarthritis • With old age, cartilage begins to deteriorate, resulting in abnormal wear and stress on the joints and bones in the arms and legs • The resulting joint pain and inflammation are symptoms of the disease osteoarthritus, which afflicts most population worldwide • Although analgesics such as aspirin and acetaminophen are commonly used to reduce the pain of osteoarthritus, recent studies have indicated that nutritional supplements of glucosamineand chondroitin sulfate may help to reduce these symptoms as well, although mostly in moderate and severe cases

Amino sugars Proteoglycans