UNIT 4. CARBOHYDRATES
OUTLINE 4.1. Introduction. 4.2. Classification. 4.3. Monosaccharides. Classification. Stereoisomers. Cyclic structures. Reducing sugars. Sugar derivatives 4.4. Oligosaccharides. Disaccharides. 4.5. Polysaccharides Homopolysaccharides: starch, glycogen, cellulose, chitin. Heteropolysaccharides: peptidoglican, glycosaminoglycans, glycoconjugates (Proteoglycans, Glycoproteins, Glycolipids). 4.6. Lectins.
4.1 INTRODUCTION: Carbohydrates are the single most abundant class of organic molecules found in nature. Polyhydroxyaldehydes or polyhydroxyketones. Basic molecular formula: (CH 2 O) n. Some of them contain phosphate, sulphate or amino groups. Biological roles: - Nutrients and energy storage. - Structural and protection roles. - Bones lubricants. - Cellular adhesion. - Glycoconjugates: intracellular signalling, cell recognition.
4.2. CLASSIFICATON: WHAT DO YOU HAVE TO KNOW? -Three groups classification: monosaccharides, oligosaccharides and polysaccharides. - Structure and chemistry of monosacharides: - Aldoses or ketoses (aldehyde function or ketone group) - Trioses, tetroses, pentoses, hexoses.
4.3. MONOSACCHARIDES: Single bounds between C. They contain a carbonyl group. Most abundant and important aldohexose: GLUCOSE. Most abundant ketohexose: Glyceraldehyde: aldose (carbonyl at the end of the chain) Dihydroxyacetone: ketose (carbonyl in the middle of the chain) FRUCTOSE. 1 2 2 1 3 3 4 4 5 6 D-Glucose 5 6 D-Fructose
4.3. MONOSACCHARIDES: Aldoses
4.3. MONOSACCHARIDES: Ketoses
4.3. MONOSACCHARIDES: WHAT DO YOU HAVE TO KNOW? -What is a chiral centre? - Why is Stereochemistry a prominent feature of monosaccharides? - How many stereoisomers do the aldoses have? And the ketoses? - What is an enantiomer? Give examples - Which kind of enantiomers are the most frequent within the carbohydrates group? - What is a diastereomer? Give examples - What is an epimer? Give examples - What is an anomer? Give examples - How are the sugars represented by means of Fischer and Haworth projections?
4.3. MONOSACCHARIDES: WHAT DO YOU HAVE TO KNOW? - How does the linear form of a monosaccharide undergo an intramolecular reaction to form a cyclic hemiacetal? - Which are the differences between hemiacetal and hemiketal cycles? - How many conformations of a pyranose sugar are there (chair, boat)? - Which is the meaning of the 'axial bond' and equatorial bond' (pyranose/furanose conformations)? - Which kind of process is called mutarotation?
4.3. MONOSACCHARIDES: REDUCING SUGARS Sugars with free anomeric carbon atoms that are reasonably good reducing agents and will reduce hydrogen peroxide, ferricyanide or certain metals such as Cu 2+.
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES A variety of chemical and enzymatic reactions produce derivatives of the simple sugars: 1) Reductions: alditols and deoxy sugars: Alditols: carbonyl group is reduced to alcohol (-itol).
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES 1) Reductions: alditols and deoxy sugars: Deoxy sugars: one or more hydroxyl groups are replaced by hydrogens. (they are part of glycoproteins and glycolipids)
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES 2) Amino sugars: amino group at the C-2 position. The amino groups can accept an acetyl group. The final products are sugars derivatives with important biological roles: (They are part of the bacterial wall)
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES 3) Oxidations: Oxidation at the aldoses C-end. Uronic acids Aldonic acids Aldaric acids
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES 4) Sugar esters: Phosphate and sulphate ester of monosaccharides.
4.4. OLIGOSACCHARIDES.DISACCHARIDES. O-Glycosidic bond: two monosaccharide units are linked by a covalent bond. Nonreducing end Reducing end Maltose (Component of malt). Reducing disaccharide
4.4. OLIGOSACCHARIDES.DISACCHARIDES. Component of milk. Reducing disaccharide. Sacarosa α-d-glucopiranosil-(1 2)-β-D-fructofuranosa Glc(α1 2β)Fru Sucrose (table sugar). Component of many higher plants. No free anomeric C. Nonreducing disaccharide.
4.5. POLYSACCHARIDES: Classification: - Homopolysaccharides. - Heteropolysaccharides. Linear chain Branched chain Linear chain: two types of monomers Branched chain: several types of monomers glucans, fructans, mannosans, galactans
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. STARCH: Storage polysaccharide in plants. Contains two polymers of glucose: Amylose, linear chain; and Amylopectin, branched chain. Amylose Fragment Branched point in amylopeptin
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. STARCH: Amylose chains (blue), and amylopeptin chains (pink) Helical conformation of amylose The enzyme α-amilase catalyses the α(1-4) bonds digestion.
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. GLYCOGEN: Storage polysaccharide in animals. It is stored in the liver and skeletal muscle. Its structure is similar to amylopeptin structure, but with higher branched points (high compact structure). Glycogen granules
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. CELLULOSE: Structural polysaccharide in plants. Linear chains constituted by D-glucose units β(1-4) linked. No hydrolysable by human beings enzymes. Hydrolysable by cellulases. Insoluble in water, fibrous and resistant. Cellulose chains interact by means of hydrogen bonds forming cellulose microfibres. The hydrogen bonds increase the strength of the structure. Cellulose microfibres
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. GLYCOGEN: Structural differences due to the nature of the bond: α(1 4) or β(1 4). Amylose helical conformation (a), or sheets composed by several cellulose chains (b).
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. CHITIN: Structural polysaccharide present in the cell walls of fungi and in the exoskeletons of crustaceans, insects and spiders. Linear chain constituted by N-acetyl-D-glucosamines in β(1-4) linkage.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. PEPTIDOGLYCAN: Component of the bacterial cell wall. β(1-4)-linked polymer of N- acetylglucosamine and N- acetylmuramic acid units. The polysaccharide is joined to a tetrapeptide (covalent bond). Lisozyme hydrolyze the glycosidic bonds. Peptidoglycan of the bacterial cell wall Gram-positive Staphylococcus aureus.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. GLYCOSAMINOGLYCANS (GAG): They are present in animal cells. The GAG are linear polymers constituted by disaccharides. One of the monosaccharides is always N-acetylglucosamine or N- acetylgalactosamine. The other one is usually uronic acid. They are negatively charged (sulphate or carboxilate groups). They promote molecule association (charge-charge interactions). GAGs recognize specific ligands (cell interactions).
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. GLYCOSAMINOGLYCANS (GAG): Hyaluronic acid (it is not protein linked). It is highly hydrated by virtue of strong interactions between water molecules and the polyanionic complex. It is present in cartilage and tendon, vitreous humour (eyes), extracellular matrix, mucosal surface and synovial fluid. Chondroitin sulfate. It promotes tension resistance in cartilage, tendon and arteries. It is present in brain, kidneys and lung. Dermatan sulfate. It is present in skin, veins and cardiac valves. Its concentrations increases when cells grow old. Keratan sulfate. Minor component of the proteoglycans. It is present in bones, cartilage, horns and inter vertebral discs. Heparin. Natural anticoagulant substance. It is present in mastocytes (type of leukocyte). It contains D-glucuronic acid.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. GLYCOSAMINOGLYCANS (GAG):
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. GLYCOCONJUGATES: Complexes of proteins or lipids with oligosaccharides and polysaccharides (covalent linked). They are present in the cellular surface and in the extracellular matrix. They are positive side of the cytoplasmic membrane orientated. Proteoglycans: proteins linked to polysaccharides GAG. Glycoproteins: proteins covalently linked to oligosaccharides. Glycolipids: membrane lipids (sphingolipids) covalently linked to oligosaccharides.
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: PROTEOGLYCANS: A 'Nucleus protein' is covalently linked to one o more than one GAG. a) GAG link to a nucleus protein by means of a trisaccahride. b) Proteoglycan containing a transmembrane protein. C) Agrecan structure (it is present in the extracellular matrix of cartilage).
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS: Proteins and carbohydrates are linked by means of N- or O-glycosidic bond. Oligosaccharides O-linked Oligosaccharides N-linked
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS: Glycoproteins structure is more complex that proteoglycans structure. The structure of the oligosaccharide is very important in the identification of the protein to interact with. Localization: Positive side of the cytoplasmic membrane (i.e. glycophorin A in erythrocytes). Proteins secreted by eukaryotic cells (i.e. immunoglobulins, hormones, coagulation factors). Proteins from lysosomes.
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS: Biological advantages: Polarity and solubility of a protein can be modified. Identify the final location of a just synthesized protein. Protection against enzymes digestion. Specific biological roles.
4.6. LECTINS: Class of proteins that bind carbohydrates with high specificity and affinity (non covalent bonds) They participate in several processes such as cell signalling and adhesion. They also control the intracellular location of the proteins that are just synthesised. In animals, the contact between cells is mediated by lectin-sugar interactions. Examples: - Physiological roles such as leukocyte movement (a). - Microorganisms infection (b) and (c). - Toxins mechanisms (d). Specific interaction between lectins and oligosaccharides.