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Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - Carbohydrates are the most abundant biomolecules on Earth Conversion of more than 100 billion tons of CO2 and H2O into cellulose and other plant products (by photosynthesis each year) - Dietary staples, structural and protective elements in the cell walls and in the connective tissues, lubricants for skeletal joints, glycoconjugates - General formula: (CH2O)n; some contain nitrogen, phosphorus, or sulfur - There are three major size classes of carbohydrates: Monosaccharides, Oligosaccharides, Polysaccharides (> 20 sugar units) “Saccharide” is derived from the Greek sakcharon, meaning “sugar” Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - Monosaccharides (MSs): Colorless, crystalline solids, freely soluble in water, insoluble in nonpolar solvents, sweet taste - In the open-chain form, one of the carbon atoms is double bonded to an oxygen atom to form a carbonyl group If the carbonyl group is at the end (aldehyde) : Aldose If the carbonyl group is at any other position (ketone) : Ketose - The simplest MSs are the two three-carbon trioses Glyceraldehyde, an aldotriose Dihydroxyacetone, a ketotriose Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - MSs with four, five, six, and seven carbon atoms in their backbones are called, respectively, tetroses, pentoses, hexoses, and heptoses Aldotetroses and ketotetroses, aldopentoses and ketopentoses, and so on - The hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose, are the most common MSs in nature - The aldopentoses D-ribose and 2-deoxy-D-ribose are components of nucleotides Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - All the MSs (except dihydroxyacetone) contain one or more chiral atoms and thus occur in optically active isomeric forms Based on the configuration of the simplest aldose, glyceraldehyde, optical isomers (enantiomers) are classified into the D isomer and the L isomer. A molecule with n chiral centers can have 2n stereoisomers - The stereoisomers of MSs can be dived into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon Those in which the configuration at this reference carbon is the same as that of Dglyceraldehyde are designated D isomers (i.e., hydroxyl group at the right side) Those with the same configuration as L-glyceraldehyde are L isomers - Most of the hexoses of living organisms are D isomers Dept. of Chemical Engineering Chapter 16 Carbohydrates Monosaccharides Have Asymmetric Centers Dept. of Chemical Engineering Chapter 16 Carbohydrates Chiral carbon Dept. of Chemical Engineering Chapter 16 Carbohydrates Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - Two sugars that differ only in the configuration around one carbon atom are called epimers; D-glucose and D-mannose, which differ only in the stereochemistry at C-2, are epimers, as are D-glucose and D-galactose (which differ at C-4) Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry - In aqueous solution, all MSs with five or more carbon atoms occur predominantly as cyclic structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain Producing two more stereoisomers, designated and - The six-membered ring compounds are called pyranoses because they resemble pyran D-glucose: -D-glucopyranose and -D-glucopyranose - The five-membered ring compounds are called furanoses because they resemble furan D-fructose: -D-fructofuranose and -D-fructofuranose Dept. of Chemical Engineering Chapter 16 Carbohydrates Sugars: Their Structures and Stereochemistry Haworth Representation Chair Conformation Dept. of Chemical Engineering Chapter 16 Carbohydrates Organisms Contain a Variety of Hexose Derivatives - There are a number of sugar derivatives in which a hydroxyl group is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group Glucosamine, galactosamine, mannosamine N-acetylglucosamine: the part of many structural polymers N-acetylmuramic acid: the part of bacterial cell wall L-fucose, L-rhamnose: Found in plant polysaccharide - Oxidation of the carbonyl (aldehyde) carbon aldonic acid (gluconic acid) - Oxidation of the carbon at the other end: e.g., C-6 of glucose, galactose, or mannose Uronic acid (glucuronic, galacturonic, or mannuronic acid) Dept. of Chemical Engineering Chapter 16 Carbohydrates Dept. of Chemical Engineering Chapter 16 Carbohydrates Disaccharides Contain a Glycosidic Bond - Disaccharides (e.g., maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric (hemiacetal) carbon of the other - Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base Dept. of Chemical Engineering Chapter 16 Carbohydrates Disaccharides Contain a Glycosidic Bond Glc(14)Glc Dept. of Chemical Engineering Chapter 16 Carbohydrates Disaccharides Contain a Glycosidic Bond Sugar in the Milk Table Sugar For sucrose and trehalose, the anomeric carbons of both MSs are involved in the glycosidic bond A major constitutent of the circulating fluid of insects, serving as an energy-storage compound Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides - Polysaccharides, also called glycans, differ from each other in the identity of their recurring monosaccharide units, in the length of their chain, in the types of bonds linking the units, and in the degree of branching - Homopolysaccharides Storage forms of monosaccharides (e.g., starch and glycogen) Structural elements (e.g., cellulose and chitin) - Heteropolysaccharides Extracellular support for organisms Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides - Some homopolysaccharides are stored forms of fuel The most important storage polysaccharides are starch in plant cells and glycogen in animal cells Starch granules in a chloroplast (~ 1.0 m) Glycogen granules in a hepatocyte (~ 0.1 m) Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Cellulose - Cellulose is the major structural component of plants and is a linear homopolysaccharide of β-D-glucose which is linked in a β(14) glycosidic bond. Animals lack the enzymes, called cellulases, that hydrolyze cellulose to glucose The presence of bacteria (producing cellulases) explains why cows and horses can live on grass. Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Starch - Starches are polymers of α-D-glucose that occur in plant cells, usually as starch granules in the cytosol. 1) Amylose: Long, unbranched chains of D-glucose residues connected by (14) linkages 2) Amylopectin: Highly branched polymer connected by (14) and (16) linkages Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Starch - Both plants and animals contain enzymes that hydrolyze starches such as α- and βamylase which can attack by (14) linkages - The most usual conformation of amylose is a helix with six residues per turn Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Glycogen - Glycogen is the main storage polysaccharide of animal cells Similar to amylopectin but glycogen is more extensively branched and more compact It has only one reducing end but has many nonreducing ends which can be removed from polymer for their use as an energy source - Why not store glucose in its monomeric form? Hepatocytes store glycogen (0.01 M) equivalent to a glucose concentration of 0.4 M If the cytosol contained 0.4 M glucose, the osmolarity would be elevated Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Chitin - A polysaccharide that is similar to cellulose in both structure and function is chitin, a linear homopolysaccharide with all the residues linked in β(14) glycosidic bond. The monomer of chitin is N-acetyl-β-D-glucosamine It is a major structural component o f the exoskeletons of invertebrates such as insects and crustaceans. Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Peptidoglycan - The rigid component of bacterial cell walls is a heteropolymer of alternating (14)linked N-acetylglucosamine and N-acetylmuramic acid residues - The material that results from the cross-linking of polysaccharides by peptides is a peptidoglycan because it has both peptide and carbohydrate components Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Glycosaminoglycan - The extracellular space in the tissues of multicellular animals is filled with a gel-like material, the extracellular matrix (ECM) Holds the cells together and provides a porous pathway for the diffusion of oxygen and nutrients - ECM consists of an interlocking meshwork of heteropolysaccharides (GAGs) and fibrous proteins such as collagen, elastin, fibronectin, and laminin - The glycosaminoglycans (GAGs) are a family of linear polymers composed of repeating disaccharide units One of the two polysaccharides is either N-acetylglucosamine or Nacetylgalactosamine; the other is in most cases a uronic acid. Dept. of Chemical Engineering Chapter 16 Carbohydrates Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Glycosaminoglycan - The glycosaminoglycan hyaluronic acid contains alternating residues of D-glucuronic acid and N-acetylglucosamine Forming clear, highly viscous solutions that serve as lubricants in the synovial fluid of joints and giving the vitreous humor of the eye its jelly-like consistency Essential component of the extracellular matrix of cartilage and tendons - Chondroitin sulfate (Greek chondros, “cartilage”) contributes to the tensile strength of cartilage, tendons, ligaments, and the walls of the aorta - Dermatan sulfate (Greek derma, “skin”) contributes to the pliability of skin and is also present in blood vessels and heart valves Dept. of Chemical Engineering Chapter 16 Carbohydrates Structures and Functions of Polysaccharides: Glycosaminoglycan - Keratan sulfate (Greek keras, “horn”) have no uronic acid and their sulfate content is variable Present in cornea, cartilage, bone, and a variety of horny structures (horn, hair, hoofs, nails, and claws) - Heparin is a natural anticoagulant made in mast cells (one of the leukocytes) and released into the blood, where it inhibits blood coagulation by binding to the protein antithrombin Added to blood samples obtained for clinical analysis, and to blood donated for transfusion Dept. of Chemical Engineering Chapter 16 Carbohydrates Dept. of Chemical Engineering Chapter 16 Carbohydrates Glycoproteins - In addition to their important role as stored fuels and as structural materials, polysaccharides are information carriers They serve as destination labels for some proteins and as mediators of specific cell-cell interactions and interactions between cells and the extracellular matrix The informational carbohydrate is covalently joined to a protein or a lipid to form a glycoconjugate, which is the biologically active molecule - Proteoglycans are macromolecules of the cell surface or extracellular matrix in which one or more glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein - Gycoproteins have one or several oligosaccharides of varying complexity joined covalently to a protein, and they are found on the outer face of the plasma membrane, in the ECM, and in the blood - Glycolipid are membrane lipid in which the hydrophilic head groups are oligosaccharides Dept. of Chemical Engineering Chapter 16 Carbohydrates Dept. of Chemical Engineering Chapter 16 Carbohydrates Proteoglycans - The basic proteoglycan unit consists of a “core protein” with covalently attached glycosaminoglycans The point of attachment is commonly a Ser residue, to which the GAG is joined through a trisaccharide bridge. The Ser residue is generally in the sequence –Ser-Gly-X-Gly- Dept. of Chemical Engineering Chapter 16 Carbohydrates Glycoproteins - Glycoproteins are carbohydrate-protein conjugates in which the carbohydrate moieties are smaller and more structurally diverse than the GAGs of proteoglycans Dept. of Chemical Engineering Chapter 16 Carbohydrates Glycoproteins - The external surface of the plasma membrane has many membrane glycoproteins with arrays of covalently attached oligosaccharides of varying complexity E.g., glycophorin A of the erythrocyte membrane 60% carbohydrate by mass, in the form of 16 oligosaccharide chains covalently attached to amino acid residues near the amino terminus of the polypeptide chain - Many of the proteins secreted by eukaryotic cells are glycoproteins, including most of the proteins of blood. E.g., immunoglobulins (antibodies) and certain hormones Dept. of Chemical Engineering Chapter 16 Carbohydrates Glycolipids - Gangliosides are membrane lipids of eukaryotic cells in which the polar head group is a complex oligosaccharide containing sialic acid and other monosaccharide residues - Some of the oligosaccharide moieties of gangliosides contribute to blood group type determination Dept. of Chemical Engineering Chapter 16 Carbohydrates Lipopolysaccharides - Lipopolysaccharides are the dominant surface feature of the outer membrane of gramnegative bacteria such as Escherichia coli and Salmonella typhimurium These molecules are prime targets of the antibodies produced by the vertebrate immune system in response to bacterial infection - The lipopolysaccharides of S. typhimurium contain six fatty acids bound to two glucosamine residues, one of which is the point of attachment for a complex oligosacchairdes From S. typhimuium From E. coli Dept. of Chemical Engineering
