SL Chemistry - Option C : Human Biochemistry Carbohydrates
General characteristics • The term carbohydrate is derived from the french: hydrate de carbone • Compounds composed of C, H, and O • (CH2O)n when n = 5 then C5H10O5 • Not all carbohydrates have this empirical formula: deoxysugars, aminosugars • Carbohydrates are the most abundant compounds found in nature (cellulose: 100 billion tons annually)
General characteristics • Most carbohydrates are found naturally in bound form rather than as simple sugars • Polysaccharides (starch, cellulose, inulin, gums) • Glycoproteins and proteoglycans (hormones, blood group substances, antibodies) • Glycolipids (cerebrosides, gangliosides) • Glycosides • Mucopolysaccharides (hyaluronic acid) • Nucleic acids
Functions • Sources of energy • Intermediates in the biosynthesis of other basic biochemical entities (fats and proteins) • Associated with other entities such as glycosides, vitamins and antibiotics) • Form structural tissues in plants and in microorganisms (cellulose, lignin, murein) • Participate in biological transport, cell-cell recognition, activation of growth factors, modulation of the immune system
Classification of carbohydrates • Monosaccharides (monoses or glycoses) • Trioses, tetroses, pentoses, hexoses • Oligosaccharides • Di, tri, tetra, penta, up to 9 or 10 • Most important are the disaccharides • Polysaccharides or glycans • Homopolysaccharides • Heteropolysaccharides • Complex carbohydrates
Monosaccharides • Also known as simple sugars • Classified by 1. the number of carbons and 2. whether aldoses or ketoses • Most (99%) are straight chain compounds • D-glyceraldehyde is the simplest of the aldoses (aldotriose) • All other sugars have the ending ose (glucose, galactose, ribose, lactose, etc…)
Monosaccharides Aldoses (e.g., glucose) have an aldehyde group at one end. Ketoses (e.g., fructose) have a keto group, usually at C2.
D- vs L- Designation D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde. The lower representations are Fischer Projections.
Sugar Nomenclature For sugars with more than one chiral center, D or Lrefers to the asymmetric C farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.
D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose. Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2n, where n is the number of asymmetric centers. The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).
Properties • Differences in structures of sugars are responsible for variations in properties • Physical • Crystalline form; solubility; rotatory power • Chemical • Reactions (oxidations, reductions, condensations) • Physiological • Nutritive value (human, bacterial); sweetness; absorption
Structural representation of sugars • Fisher projection: straight chain representation • Haworth projection: simple ring in perspective • Conformational representation: chair and boat configurations
Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH. Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran. These representations of the cyclic sugars are called Haworth projections.
Fructose forms either • 6-member pyranose ring: reaction of the C2 keto group with the OH on C6, or • 5-member furanose ring: reaction of the C2 keto group with the OH on C5.
Cyclization of glucose produces a new asymmetric centerat C1. The 2 stereoisomers are called anomers, a & b. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1: • a(OH below the ring) • b (OH above the ring).
Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar. The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.
Optical isomerism • A property exhibited by any compound whose mirror images are non-superimposable • Asymmetric compounds rotate plane polarized light
Polarimetry Measurement of optical activity in chiral or asymmetric molecules using plane polarized light Molecules may be chiral because of certain atoms or because of chiral axes or chiral planes Measurement uses an instrument called a polarimeter (Lippich type) Rotation is either (+) dextrorotatory or (-) levorotatory
Polarimetry Magnitude of rotation depends upon: 1. The nature of the compound 2. The length of the tube (cell or sample container) usually expressed in decimeters (dm) 3. The wavelength of the light source employed; usually either sodium D line at 589.3 nm or mercury vapor lamp at 546.1 nm 4. Temperature of sample 5. Concentration of analyte in grams per 100 ml
D = Na D line T = temperature oC a obs : observed rotation in degree (specify solvent) l = length of tube in decimeter c = concentration in grams/100ml [a] = specific rotation
Specific rotation of various carbohydrates at 20oC • D-glucose +52.7 • D-fructose -92.4 • D-galactose +80.2 • L-arabinose +104.5 • D-mannose +14.2 • D-arabinose -105.0 • D-xylose +18.8 • Lactose +55.4 • Sucrose +66.5 • Maltose+ +130.4 • Invert sugar -19.8 • Dextrin +195
Oligosaccharides • Most common are the disaccharides • Sucrose, lactose, and maltose • Maltose hydrolyzes to 2 molecules of D-glucose • Lactose hydrolyzes to a molecule of glucose and a molecule of galactose • Sucrose hydrolyzes to a moledule of glucose and a molecule of fructose
Glycosidic Bonds The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: R-OH + HO-R'R-O-R' + H2O e.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose).
Disaccharides: Maltose, a cleavage product of starch (e.g., amylose), is a disaccharide with an a(1®4) glycosidiclink between C1 - C4 OH of 2 glucoses. It is the a anomer (C1 O points down). Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C1 points up). The b(1®4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other.
Other disaccharides include: • Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(12). The full name of sucrose is a-D-glucopyranosyl-(12)-b-D-fructopyranose.) • Lactose, milk sugar, is composed of galactose & glucose, with b(14) linkage from the anomeric OH of galactose. Its full name is b-D-galactopyranosyl-(14)-a-D-glucopyranose
Sucrose a-D-glucopyranosido-b-D-fructofuranoside b-D-fructofuranosido-a-D-glucopyranoside • Also known as tablet sugar • Commercially obtained from sugar cane or sugar beet • Hydrolysis yield glucose and fructose (invert sugar) ( sucrose: +66.5o ; glucose +52.5o; fructose –92o) • Used pharmaceutically to make syrups, troches
Lactose b-D-galactose joined to a-D-glucose via b (1,4) linkage • Milk contains the a and b-anomers in a 2:3 ratio b-lactose is sweeter and more soluble than ordinary a- lactose • Used in infant formulations, medium for penicillin production and as a diluent in pharmaceuticals
Starch • Most common storage polysaccharide in plants • Composed of 10 – 30% a-amylose and 70-90% amylopectin depending on the source • The chains are of varying length, having molecular weights from several thousands to half a million
Polysaccharides Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with a(14) linkages. It adopts a helical conformation. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end.
Amylopectin is a glucose polymer with mainly a(14) linkages, but it also has branches formed by a(16) linkages. Branches are generally longer than shown above. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.
Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues
Glycogen • Also known as animal starch • Stored in muscle and liver • Present in cells as granules (high MW) • Contains both a(1,4) links and a(1,6) branches at every 8 to 12 glucose unit • Complete hydrolysis yields glucose • Glycogen and iodine gives a red-violet color • Hydrolyzed by both a and b-amylases and by glycogen phosphorylase
Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more a(16) branches. The highly branched structure permits rapid release of glucose from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.
Cellulose • Polymer of b-D-glucose attached by b(1,4) linkages • Yields glucose upon complete hydrolysis • Partial hydrolysis yields cellobiose • Most abundant of all carbohydrates • Cotton flax: 97-99% cellulose • Wood: ~ 50% cellulose • Gives no color with iodine • Held together with lignin in woody plant tissues
Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(1®4) linkages. Every other glucose is flipped over, due to the b linkages. This promotes intra-chain and inter-chain H-bonds and van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles called microfibrils.
Linear structures of cellulose and chitin (2 most abundant polysaccharides)
Suspensions of amylose in water adopt a helical conformation Iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch
(in starch) (in cellulose)
Oligosaccharides that are covalently attached to proteins or to membrane lipids may be linear or branched chains. O-linked oligosaccharide chains of glycoproteins vary in complexity. They link to a protein via a glycosidic bond between a sugar residue & a serine or threonineOH. O-linked oligosaccharides have roles in recognition, interaction, and enzyme regulation.