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Carbohydrates: Chemical Structure and Biological Function
Principles of Biochemistry (BCH 3000) Carbohydrates: Chemical Structure and Biological Function Dr. Syahida Ahmad Department of Biochemistry Faculty of Biotechnology & Biomolecular Sciences UPM
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Carbohydrates? Most abundant and important class of biomolecules
(CH2O)n Contains carbonyl (C=O) and hydroxyl (OH) functional groups Predominant elements: C, H and O, some contains S, N or P Classified base on Number of sugar units Location of C=O stereochemistry Functions: As energy stores, fuels, and metabolic intermediates As structural components of cell walls As structural molecules in RNA and DNA Informational markers for molecular recognition – glycoproteins and glycolipids
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Types of Carbohydrates
Monosaccharides Single sugar units Disaccharides two sugar units Oligosaccharides 3-10 sugar units Polysaccharides More than 10 units
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Monosaccharides Glucose, fructose and galactose
Simple sugars with a single aldehyde or ketone & multiple hydroxyl groups (CH2O)n n range 3-7 Aldose If the carbonyl group is aldehyde Ketose If the carbonyl group is ketone D and L sugar (stereoisomers) The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group (Fischer projections) Naturally occurring carbohydrates is in D configuration Enantiomers and epimers
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Carbohydrate Classifications
Carbons Category Name Relevant examples 3 Triose Glyceraldehyde, Dihydroxyacetone 4 Tetrose Erythrose 5 Pentose Ribose, Ribulose, Xylulose 6 Hexose Glucose, Galactose, Mannose, Fructose 7 Heptose Sedoheptulose 9 Nonose Neuraminic acid also called sialic acid The major monosaccharides contain four to six carbon atoms.
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Monosaccharides cont. Howart projections
aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal forming a heterocyclic ring with an oxygen bridge between two carbon atoms Furanose (5-C ring) and Pyranose (6-C ring) forms or forms of sugar (Anomers) Chair or boat configuration Uses Major source of fuel for metabolism As an energy source Biosynthesis
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Reactions of Monosaccharides
Oxidation Oxidation of aldehyde to carboxylic acid E.g. Glucose catalyzed by dehydrogenase enzyme, assisted by the coenzyme NADP+ to lactone (a cyclic ester) Reduction Reduction of aldehyde group in glucose yields sorbitol (sweetening agent) Deoxy sugars – e.g. 2-deoxyribose, a component in DNA Esterification Phosphate ester –OH groups of sugars react with acids (phosphoric acid) Transfer of phosphoryl group (from ATP) to sugar –OH group (catalyzed by kinases) first step in Glycolysis
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Reactions of Monosaccharides cont.
Amino Derivatives Replacement of a –OH group on a carbohydrate by an amino group Glucosamine and galactosamine Acetylated glucosamine (N-acetylglucosamine and N-acetylmuramic acid), structural components of bacterial cell walls Glycoside Formation linking together two monosaccharides E.g. Maltose is formed by linking two glucose through glycosidic bond
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Acrylamide How does it get into foods?
An industrial chemical, which widely used in foods such as french fries, patato chips and other high-carbohydrate food that are fried or baked at elevated temperature Also used to make plastics and SDS-PAGE gels Neurotoxin and carcinogenic Question to raised… How does it get into foods? Mystery revealed… glucose + amino acids (asparagine) heated to above 120°C (248°F), acrylamide is produced
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Disaccharides Two monosaccharide units bound together by a glycosidic bond Maltose Glucose ( configuration) is link via its –OH group (on C1) to the –OH group on C4 of the 2nd glucose unit Linkage is called (14) glycosidic bond Cellobiose Glucose ( configuration) is link via its –OH group (on C1) link to the –OH group on C4 of the 2nd glucose unit Linkage is called (14) glycosidic bond Others Lactose Sucrose
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Polysaccharides Homopolysaccharides and heteropolysaccharides
Oligosaccharide Storage Starch Glycogen Structural Cellulose Chitin Mucopolysaccharides Peptidoglycans Glycoproteins
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Starch glucose storage polymer in plant
Mixture of two types of polymeric glucose, amylose and amylopectin Amylose A linear, unbranched chain of D-glucose units linked by (14) glycosidic bonds Amylopectin Main backbone composed of glucose unit linked by (14) glycosidic bonds Branches connected to the backbone via (16) glycosidic bonds -Amylase Catalyzes the hydrolysis of glycosidic bonds of starch in mouth
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Glycogen glucose storage polymer in animals
Identical to amylopectin, but has more (16) branches Present in both liver and muscle cells A linear, unbranched chain of D-glucose units linked by (14) glycosidic bonds
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Cellulose Cellulose Major structural component of plant cell walls
Unbranched polymer connecting D-glucose units together by (14) glycosidic linkages Strong and rigid networks, held together by intra- and intermolecular H bonding Stabilized sheet like arrangement of atoms – useful for clothing, paper, cardboard and building materials Animals do not produce enzymes that can catalyze the hydrolysis of (14) glycosidic bonds in cellulose Cellulases Enzymes secreted by wood-rot fungi and some bacteria for the decaying of dead wood Ruminant animals (cattle, sheep, goats, camels, giraffes) able to use cellulose because their 2nd stomachs contain bacteria that secrete the enzymes
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Do human digest chitin? Chitin Chitin
Protective exoskeletons of arthropods (insects, crabs, lobsters) Unbranched homopolysaccharide Also found in smaller amounts in the cell walls of yeasts, fungi and algae A polymer of N-acetylglucosamine, linked by (14) glycosidic bonds Exists in extended chains associated into fibers by intra- and intermolecular H bonding Do human digest chitin?
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Mucopolysaccharides Peptidoglycans
A thin, viscous, jellylike coating to cells Hyaluronic acids Has alternating monomeric units of N-acetylglucosamine and D-glucoronic acid Serve as a lubricant in the synovial fluid of joints and found in the extracellular matrix of connective tissue Peptidoglycans Rigid cell walls of bacteria Unbranced heteropolymer of N-acetylglucosamine and N-acetylmuramic acid
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Glycoproteins Proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide Glycosylation O-glycosylation N-glycosylation Monosaccharides found in eukaryotic glycoproteins Glucose, mannose, galactose, fucose, N-acetylgalactosamine and N-acetylglucosamine Biological functions Immunological protection Cell-cell recognition events Blood clotting Signal transduction processes Host-pathogen interactions Plasma membrane
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