Saccharide Chemistry & Function PDF

Saccharides Chemistry & Function Dr. Norjihada Izzah Ismail/Dr Norhana/Dr Nurizzati JKBSK, FKE, UTM Saccharides/carbohydrates are molecular compounds mainly consist of three elements: carbon (C), hydrogen (H) and oxygen (O), ratio (CH2O)n. Formed du...

Saccharides Chemistry & Function Dr. Norjihada Izzah Ismail/Dr Norhana/Dr Nurizzati JKBSK, FKE, UTM Saccharides/carbohydrates are molecular compounds mainly consist of three elements: carbon (C), hydrogen (H) and oxygen (O), ratio (CH2O)n. Formed during photosynthesis. Suffix -ose is used for saccharides. Functions: a source of energy for the body (e.g. glucose) and a store of energy (e.g. starch in plants). structural element (e.g. cellulose in plants and chitin in insects). components of other molecules (e.g. DNA, RNA, glycolipids, glycoproteins, ATP). precursors in the production of other biomolecules (e.g. amino acids, lipids). Saccharides/carbohydrates Saccharides/carbohydrates Can be divided into 4 types: monosaccharides disaccharides oligosaccharides polysaccharides  Monosaccharides are the simplest carbohydrates, often called simple sugars. General molecular formula (CH2O)n, where n = 3, 4, 5 or 6 D versus L configuration D configuration if the hydroxyl group is to the right of the last stereocenter carbon in a Fischer projection. L configuration is given if the hydroxyl group is to the left of the last stereocenter carbon. D or L is usually put in the beginning of the carbohydrate when naming the molecule.  Pentoses and hexoses can exist in two forms: cyclic (closed chain) non-cyclic (acyclic/open chain) A Haworth projection can be used to represent the cyclic form of monosaccharide. The open chain form of monosaccharides is illustrated with Fischer projections. Cyclic form The five-member cyclic form of a monosaccharide is known as a furanose. The six-member cyclic form of a monosaccharide is known as a pyranose. Glucose and other hexoses can exist in three forms: ▪ an open-chain linear structure ▪ a six-member (pyranose) ring → predominant ▪ a five-member (furanose) ring Open chain form In the open chain form, their structural formula show they contain either an aldehyde / ketone group. Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. ALDOSES KETOSES reducing sugars non-reducing sugars e.g. glucose e.g. dihydroxyacetone, fructose Ketoses must first tautomerize to aldoses before they can act as reducing sugars. Keto-enol tautomerism Monosaccharides exist as an equilibrium between the cyclic hemiacetal and open chain forms. In the open chain form, the aldehyde function is oxidized by Cu2+. Tautomer: any molecule that readily interconvert with each other through movement of a H atom. Ketoses are in equilibrium with aldoses, which can reduce Cu2+. Tautomer = constitutional isomer Isomers Compounds that have the same molecular formula but different chemical structures. Stereoisomers Two compounds with identical molecular formulas whose atoms are linked in the same order but in different spatial arrangements. Geometric isomers include the cis and trans forms of molecules containing a double bond. Anomers Isomers differing at C1 atom of an aldose or the C2 atom of a ketose. E.g. D-glucose can exist in two forms alpha-D-glucose and beta-D-glucose. α & β Anomers differ at the position of OH group at C1 of an aldose. Same side as C6 (beta anomer); Opposite site as C6 (alpha anomer) Optical isomer (enantiomer) The enantiomer that rotates plane polarized light clockwise is called dextrorotary [(+), or d-]. The other enantiomer that rotates plane polarized light counterclockwise is called levorotary [(-), or l-]. Are mirror images of one another, non-superimposable.  Most molecules in cells contain at least one chiral (asymmetric) carbon atom, which is bonded to four dissimilar atoms. Enantiomer molecules have identical chemical properties but completely different biological activities. In biological systems, nearly all amino acids are L isomers and nearly all sugars are D isomers.  Disaccharides 2 monosaccharides joined together by an O-glycosidic bond. Examples: α-glucose + β-glucose = maltose β-glucose + β-glucose = cellobiose Can be alpha- or beta- glucose  The "first" glucose in maltose is the alpha anomer, and the "first" glucose in cellobiose is the beta anomer. Lactose (eg. in milk) and sucrose (eg. in sugar cane). Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase (@ invertase). Lactose cannot be absorbed through the intestinal walls into the bloodstream. It must first be hydrolyzed back into glucose and galactose. Infants have an enzyme, lactase, which catalyzes this reaction. The levels of this enzyme may fall as we age and many adults cannot "digest" (hydrolyze) lactose in milk. This is referred to as lactose intolerance. Glycosidic linkage (bond) The covalent bond that can join one monosaccharide to another by a condensation reaction with the loss of a water molecule. 1 4 In the formation of any glycosidic bond, the C1 atom of one sugar molecule reacts with a hydroxyl group of another sugar molecule. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The linkage: α-1,4-glycosidic bond. Examples + + Oligosaccharides Molecules containing of 3 to 10 monosaccharides. The most important: disaccharides Connected by O-glycosidic bonds. Examples: raffinose, stachyose α-1,2 Sugar derivatives a) Sugar alcohols Organic compounds comprises of a class of polyols. Sweetening power, but sweetness is usually lower than the one of monosaccharide. b) Amino sugars c) Sugar phosphates Sugars that have added or substituted phosphate groups. Often used in biological systems to store or transfer energy. Also form the backbone for DNA and RNA. d) Glycosides Hemiacetal Acetal ▪ Glycosides are acetal, do not test positive with Benedict’s reagent. - Reason: acetal formation “locks” a ring so it cannot undergo oxidation or mutarotation. - Only hemiacetals act as reducing agents. Sugar + aglycone = glycoside Eg. Nucleoside = pentose sugar + nitrogenous base Non-sugar molecule (aglycone) adenine ribose (Adenine ribonucleoside) blue to blue-green or yellow-green is negative, yellowish to bright yellow is a moderate positive, and bright orange is a very strong positive. Polysaccharides Linear or branched polymer of monosaccharides, linked by glycosidic bonds, usually containing more than 15 residues. Examples: glycogen, cellulose, glycosaminoglycans. Based on composition, polysaccharides are of 2 types: Homopolysaccharides Heteropolysaccharides (aka heteroglycans) polysaccharides composed of one type of polysaccharides with more than one type sugar monomer (monosaccharides). of sugar monomer. Eg. Cellulose (polymer of glucose). Eg. hyaluronic acid , chondroitin sulphate. Polymer: Any large molecule composed of multiple identical or similar units (monomers) linked by covalent bonds. Glycogen: Storage function A very long, branched polysaccharide, composed of glucose units linked together by α(1,4) and α(1,6) glycosidic bonds. Similar in structure to amylopectin except that it has more branch points. Function: ▪ the primary carbohydrate storage molecule in animal cells ▪ found primarily in liver and muscle cells. HOMOPOLYSACCHARIDES Starch: Storage function A very long polysaccharide, composed of glucose units linked together by α(1,4) and α(1,6) glycosidic bonds. Functions: ▪ the energy reservoir of plant cells. ▪ a significant source of carbohydrate in human diet. Occurs in two forms: ▪ Amylose → long, unbranched chains of D-glucose residues linked through α(1,4) glycosidic bonds. - form helix structure. - give intense blue colour of Iodine test ▪ Amylopectin → branched polymer of D-glucose residues, contain both α(1,4) & α(1,6) glycosidic bonds. - α(1,6) branch points occur at every 20-25 glucose residues & prevent helix formation. - reddish colour of Iodine test , , Cellulose: non-storage function An unbranched polymer of glucose linked together by β(1,4) glycosidic bonds. Function: ▪ the major constituent of plant cell walls, structural framework (protect, support). Microfibrils: sheetlike strips consist of pairs of unbranched cellulose molecules held together by hydrogen bonding.  Human digestive enzymes can hydrolyze α(1,4) glycosidic bonds, but not β(1,4) bonds between glucose units; for this reason, humans can digest starch but not cellulose. A H atom in one molecule is electrostatically attracted to the N, O, or F atom in another molecule. SUMMARY: Homopolysaccharides Heteropolysaccharides/ heteroglycans Function: ▪ provide extracellular support for organisms - the bacteria cell envelope, the matrix that holds individual cells together in animal tissues. ▪ provides protection, shape and support to cells, tissues and organs. Examples: ▪ Glycosaminoglycans (GAGs) ▪ Murein@peptidoglycan – a major component of bacterial cell wall Glycosaminoglycans A linear polymer with disaccharide repeating units, many sugar residues are amino derivatives. GAGs Function Heparin Anticoagulant activity Hyaluronic acid Hydration (eg. skin) , lubrication of joints, a space filling capacity (eg. vitreous humor of eye, synovial fluid of joints). Summary Carbohydrates are a major product of photosynthesis and their oxidation provides a major energy source for both plants and animals. Monosaccharides Disaccharides and polysaccharides contain either aldehyde or ketone contain a glycosidic bond between group. two simple sugar molecules