Carbohydrates AG - the biochemistry of sugars (Part I).pdf

Carbohydrates (Part I) The Biochemistry of Sugars: An introduction to sugars Heparin segment Dr Annie Godwin ([email protected])  CHAPTER 7 Carbohydrates and Glycobiology  Checklist: what we need to know Carbohydrates: Par...

Carbohydrates (Part I) The Biochemistry of Sugars: An introduction to sugars Heparin segment Dr Annie Godwin ([email protected])  CHAPTER 7 Carbohydrates and Glycobiology  Checklist: what we need to know Carbohydrates: Part I Key terminology 1. What are carbohydrates & why are Carbohydrate: A sugar they important? (monosaccharide) or one of its dimers (disaccharide) or polymers 2. Understand the structures and (polysaccharide). nomenclature of monosaccharides. 3. Understand the chemistry of sugars, Monosaccharide: A carbohydrate consisting of a single sugar unit. carbonyl groups and chiral centres. 4. Understand the open-chain and ring Disaccharide: A carbohydrate consisting of two covalently joined forms of monosaccharides. monosaccharide units. 5. Understand the structures and properties of disaccharides. Polysaccharide: A linear or 6. Understand the biological function branched polymer of monosaccharide units linked by of polysaccharides. glycosidic bonds. The ABCs of biochemistry RECAP: The organic compounds from which most cellular materials are constructed. Proteins Nucleic acids Lipids Carbohydrates  The importance of Carbohydrates Most abundant Annually, 100 billion metric tons of Sugar & starch is a dietary staple biomolecule on Earth CO2 and H20 into plant derivatives in many areas of the world Cellulose Structural protective element in cell walls of bacteria/ Role in cell recognition/ adhesion plants & in connective tissues or joints of animals & metabolism/ localisation/ fate Carbohydrates Carbo (sugar) - hydrate (water). Named because many have the formula (CH2O)n – where ‘n’ is the number of carbon atoms ≥ 3 Size ranges from small (e.g. glyceraldehyde, Mw= 90 Da) to huge (e.g. amylopectin, Mw= 200,000,000 Da). Fulfill a wide range of functions, including: – Energy source and energy storage. – Structural component of cell walls and exoskeletons. – Informational molecules in cell-cell signaling. Can be covalently linked with proteins to form glycoproteins, glycolipids and proteoglycans. Carbohydrates Common functional groups of biomolecules A functional group is a group of atoms within a molecule with distinctive chemical properties. When two or more substituents are shown in a molecule, they are designated R1, R2, and so forth. Monosaccharides: Aldoses and Ketoses Two classes of sugars, based on which carbonyl group they possess. An aldose contains an aldehyde group. Monosaccharide: A A ketose contains a ketone group. carbohydrate consisting of a single sugar unit. Aldose: A simple sugar 1 in which the carbonyl 1 carbon atom is an 2 aldehyde; that is, the 2 carbonyl carbon is at 3 3 one end of the carbon chain. Ketose: A simple monosaccharide in which the carbonyl group is a ketone. C1 = carbonyl carbon C2 = carbonyl carbon Monosaccharides: Aldoses and Ketoses …also classified based on the number of carbon 1 1 atoms they have. 2 2 Hexoses have 6 carbon 3 3 atoms: C6H12O6. 4 4 The carbon atoms are numbered based on the 5 5 carbonyl carbon. This will be the end of the 6 6 monosaccharide which is numbered lowest. Glucose Fructose …have the suffix ‘-ose’. an aldose sugar a ketose sugar  Aldoses having 3-6 carbons Carbohydrates have many isomers because of the arrangement of the −OH groups in their structures. These differences affect the properties of the monosaccharides. Hexoses Trioses Tetroses Pentoses  Ketoses having 3-6 carbons Trioses Tetroses Pentoses Hexoses Fischer projection formulas: A 2D method for representing 3D molecules to show the configuration of groups around chiral centers. REMEMBER: The carbonyl group is situated at the top and last achiral CH2OH at the bottom.  Chiral centers and enantiomers L R Glyceraldehyde is the simplest carbohydrate to exist in two isomeric forms (stereoisomers) that are mirror images of each other. Triose Chiral carbon. Enantiomer = mirror image. 4 different substituents attached Type of stereoisomer. to the carbon atom. Interacts with polarized light. Achiral objects can be superimposed. Monosaccharides & polarized light Light can be polarised by passing it through a David Attenborough polarising filter. A polarising filter has all its molecules aligned in the same direction. & crustaceans As a result, only waves with vibration aligned in A fun way to learn the same direction can pass through. Relative configuration of optically active compounds more! Plane polarised light is passed through a filter and then a solution of optical isomers. The chiral compound causes the plane of vibration to rotate. Chiral centers and enantiomers In sugars that contain many chiral centers, Chiral compound: A compound that contains only the one that is most distant from the an asymmetric center carbonyl carbon is designated as D (right - (chiral atom or chiral clockwise) or L (left - anticlockwise). center) and thus can occur in two non- – Chirality at other carbons determines identity superposable mirror- D and L isomers of a sugar are image forms. enantiomers. Chiral center: An atom – L and D enantiomers often have the same with substituents physical properties e.g. water solubility. arranged so that the molecule is non – Differ in the direction of rotation of plane- superposable on its polarized light. mirror image. Most hexoses in living organisms are D- Enantiomers: stereoisomers. Stereoisomers that are Some simple sugars occur in the L-form, non-superposable mirror images of each other. such as L-arabinose. The importance of enantiomers Obtaining enantiomerically pure compounds is important in medicine. The 2 enantiomers of a chiral molecule can have very different biological properties: bioavailability, rate of metabolism, metabolites, excretion, potency and selectivity for receptors, transporters and/or enzymes, and toxicity. Epimers Epimers are two sugars that differ only in the configuration around one carbon atom. D-glucose and 2 Epimers Epimer: Two stereoisomers differing in configuration at one asymmetric center in a compound having two or more asymmetric centers. Where are we up to? 1. Sugars (saccharides) are compounds containing an aldehyde or ketone group and two or more hydroxyl groups. 2. They are classified based on the number of carbon atoms they have & the functional ‘carbonyl’ group they possess. 3. Monosaccharides contain chiral carbons and exist in a number of stereochemical forms represented on paper as Fischer projections. 4. Epimers are sugars that differ in configuration at only one carbon atom. Carbohydrates: Part II 1. Understand the open-chain and ring formations of monosaccharides. 2. Understand the basic structures and properties of disaccharides. Definitions Aldose: A simple sugar in which the carbonyl carbon atom is an aldehyde; that is, the carbonyl carbon is at one end of the carbon chain. Carbohydrate: A sugar (monosaccharide) or one of its dimers (disaccharide) or polymers (polysaccharide). Chiral center: An atom with substituents arranged so that the molecule is non superposable on its mirror image. Chiral compound: A compound that contains an asymmetric center (chiral atom or chiral center) and thus can occur in two non-superposable mirror-image forms. Disaccharide: A carbohydrate consisting of two covalently joined monosaccharide units. Enantiomers: Stereoisomers that are non-superposable mirror images of each other (L - laevorotatory and D - dextrorotatory). Fischer projection formulas: A 2D method for representing 3D molecules to show the configuration of groups around chiral centers. Glycosidic bond: Bonds between a sugar and another molecule (typically an alcohol, purine, pyrimidine or sugar) through an intervening oxygen. Isomers: Any two molecules with the same molecular formula but a different arrangement of molecular groups. Ketose: A simple monosaccharide in which the carbonyl group is a ketone. Monosaccharide: A carbohydrate consisting of a single sugar unit. Oligosaccharide: Several monosaccharide groups joined by glycosidic bonds. Polysaccharide: A linear or branched polymer of monosaccharide units linked by glycosidic bonds. Monosaccharides: Aldoses and Ketoses (Quiz) Identify these sugars: Aldose or Ketose, Enantiomer, Number of Carbons Glucose Ribulose Glyceraldehyde