CARBOHYDRATES Carbohydrates Most abundant biomolecules on Earth Carbohydrates • Polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis • Many have the empirical formula (CH2O)n; some also contain nitrogen, phosphorus, or sulfur. Major Size Classes • Monosaccharides • Oligosaccharides • Polysaccharides “Saccharide” is derived from the Greek sakcharon, meaning “sugar” Monosaccharides • Simple sugars • Colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents • Most have a sweet taste • Most abundant in nature: D-glucose (dextrose) Monosaccharides • Consist of a single polyhydroxy aldehyde or ketone unit • Backbones are unbranched carbon chains in which all the carbon atoms are linked by single bonds • In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group Monosaccharides • According to Functional Group – Aldose: carbonyl group is at an end of the carbon chain – Ketose: carbonyl group is at any other position • According to Number of C – Trioses, tetroses, pentoses, hexoses, heptoses, etc. ALDOSE KETOSE Monosaccharides • Simplest monosaccharides: glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose Monosaccharides • Most common monosaccharides in nature: Dglucose and Dfructose Monosaccharides • Components of nucleotides and nucleic acids: D-ribose and 2-deoxy-D-ribose Monosaccharides • All the monosaccharides contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms • To represent three-dimensional sugar structures on paper, we often use Fischer projection formulas. Monosaccharides Monosaccharides Monosaccharides Monosaccharides Monosaccharides Monosaccharides • In general, a molecule with n chiral centers can have 2n stereoisomers • Stereoisomers of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. – D isomers: hydroxyl group on the reference carbon is on the right in the projection formula – L isomers. hydroxyl group on the reference carbon is on the right in the projection formula Monosaccharides • Carbons of a sugar are numbered beginning at the end of the chain nearest the carbonyl group. • The four- and five-carbon ketoses are designated by inserting “ul” into the name of a corresponding aldose • Two sugars that differ only in the configuration around one carbon atom are called epimers Monosaccharides • In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. Monosaccharides • Formation of ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals, which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. Monosaccharides • Six-membered ring compounds are called pyranoses • Five-membered ring compound are called furanoses. • Six-membered aldopyranose ring is much more stable than the aldofuranose ring and predominates in aldohexose solutions. • Only aldoses having five or more carbon atoms can form pyranose rings. • Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom (or anomeric carbon) are called anomers. Monosaccharide Derivatives • • • • • • • Alditol Aldonic acid Aldonate Uronic acid Uronate Amino sugar Deoxy sugars Derivatives of Monosaccharides Basis of Fehling’s Reaction = Glucose is Reducing This is the more sensitive and specific test for glucose Formation of Glycosidic Bond..Polysaccharides Written from Non-Reducing to Reducing End Oligosaccharides • Consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds • Most abundant: disaccharides like sucrose or cane sugar • Incells, most oligosaccharides consisting of three or more units are joined to nonsugar molecules Disaccharides • Consist of two monosaccharides joined covalently by an O-glycosidic bond Glycosidic bond • Formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other • Represents the formation of an acetal from a hemiacetal and an alcohol (a hydroxyl group of the second sugar molecule) • Readily hydrolyzed by acid but resist cleavage by base. Disaccharides • Can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid • Oxidation of a sugar’s anomeric carbon by cupric or ferric ion occurs only with the linear form, which exists in equilibrium with the cyclic form(s) Disaccharides • When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar What Is Sweet? Tasting Panels: Taste Relative to Sucrose….dilutions of the test molecules. Compound Sweetness Lactose 0.16 Glycerol 0.60 Glucose 0.75 Sucrose 1.0 Fructose 1.75 Aspartame 250 Na-saccharine 510 Sucralose 600 Lugduname 225,000 Because of sucralose intense sweetness, what is sold in the supermarket is mixed with a filler: fluffed glucose or fluffed maltodextrin. (Fluffed by the same process that makes Puffed Wheat or Puffed Rice). While sucralose can not be metabolized, the filler does have calories .. but less than 5 cal/gram. So the FDA allows the claim that sucralose products have no calories. What is the nutritional calorie compared to the biochemical calorie? What are the safety concerns? Polysaccharides • Sugar polymers containing more than 20 or so monosaccharide units • Most common form of carbohydrates in nature • Also called glycans • Do not have definite molecular weights • Most abundant: cellulose Polysaccharides • Differ from each other in: – identity of recurring monosaccharide units – length of their chains – types of bonds linking the units – degree of branching Classification of Polysaccharides • Homopolysaccharides contain only a single type of monomer. They usually serve as storage forms of monosaccharides that are used as fuels or as structural elements in plant cell walls and animal exoskeletons. • Heteropolysaccharides contain two or more different kinds of monomers. They usually provide extracellular support for organisms of all kingdoms. Homopolysaccharides • Most important storage polysaccharides are starch in plant cells and glycogen in animal cells which are both occur intracellularly as large clusters or granules and heavily hydrated, because they have many exposed hydroxyl groups Starch • Most plant cells have the ability to form starch, but it is especially abundant in tubers, such as potatoes, and in seeds. • Contains two types of glucose polymer, amylose and amylopectin • Amylose is unbranched, with molecular weight from a few thousand to more than a million. • Amylopectin is highly branched (occurring every 24 to 30 residues ), with molecular weight up to 100 million. Starch Glycogen • Main storage polysaccharide of animal cells • Like amylopectin, but more extensively branched (on average, every 8 to 12 residues) and more compact than starch • Especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle • With an average molecular weight of several million. • Has as many nonreducing ends as it has branches, but only one reducing end Glycogen Dextran • Bacterial and yeast polysaccharides made up of (α 1→6)-linked poly-D-glucose; all have (α1 → 3) branches, and some also have (α 1 → 2) or (α 1 → 4) branches • Found in dental plaque • Used in several commercial products for fractionation of proteins Cellulose • Fibrous, tough, water-insoluble substance, found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body • Constitutes much of the mass of wood and cotton • Linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units, but unlike amylose, in β configuration Chitin • Linear homopolysaccharide composed of Nacetylglucosamine residues in β linkage • Forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates • Principal component of the hard exoskeletons of arthropods and is probably the second most abundant polysaccharide, next to cellulose, in nature Chitin Homopolysaccharide Folding • Stabilized by weak interactions within or between molecules – Hydrogenbond – Hydrophobic and van der Waals interactions – Electrostatic interactions Homopolysacchaide Folding • Starch and glycogen : tightly coiled helix, stabilized by interchain hydrogen bonds • Along the amylose chain forms a 60° angle with the preceding residue, so the helical structure has six residues per turn Homopolysaccharide Folding • Cellulose: Each chair is turned 180°relative to its neighbors, yielding a straight, extended chain • Several chains lie side by side produces straight, stable supramolecular fibers of great tensile strength Peptidoglycan • Can be found in bacterial and algal cell walls • Heteropolymer of alternating (β1→4)-linked Nacetylglucosamine and N-acetylmuramic acid residues • Lie side by side in the cell wall, crosslinked by short peptides which weld the polysaccharide chains into a strong sheath Heteropolysaccharides • Lysozyme kills bacteria by hydrolyzing glycosidic bonds • Present in tears and produced by certain bacterial viruses Agar • Produced by certain marine red algae, including some of the seaweeds • Mixture of sulfated heteropolysaccharides made up of D-galactose and an L-galactose derivative ether-linked between C-3 and C-6 • Contains agarose (unbranched) and agaropectin (branched) • Agarose has gel-forming property • Forms a double helix when heated then cooled, trapping water molecules in the central cavity Agar • Used as inert supports for the electrophoretic separation of nucleic acids, an essential part of the DNA sequencing process • Used to form a surface for the growth of bacterial colonies • Used for the capsules in which some vitamins and drugs are packaged Extracellular Matrix • Gel-like material found in the extracellular space in the tissues of multicellular animals • Holds the cells together and provides a porous pathway for the diffusion of nutrients and oxygen to individual cells • Composed of an interlocking meshwork of fibrous proteins and glycosaminoglycans, a family of linear polymers composed of repeating disaccharide units Glycosaminoglycan • One of the two monosaccharides is always either N-acetylglucosamine or N-acetylgalactosamine; the other is in most cases a uronic acid • Has very high density of negative charge • Assume an extended conformation in solution • Attached to extracellular proteins to form proteoglycans Hyaluronic Acid/Hyaluronate • Have molecular weights greater than 1 million • Form clear, highly viscous solutions that serve as lubricants in the synovial fluid of joints and give the vitreous humor of the vertebrate eye its jellylike consistency • Essential component of the extracellular matrix of cartilage and tendons, to which it contributes tensile strength and elasticity Other Glycosaminoglycans • Much shorter polymers than hyaluronate and they are covalently linked to specific proteins • Includes – Chondroitin sulfate (contributes to the tensile strength of cartilage, tendons, ligaments, and the walls of the aorta) – Dermatan sulfate (contributes to the pliability of skin and is also present in blood vessels and heart valves) – Keratan sulfates (present in cornea, cartilage, bone, and a variety of horny structures formed of dead cells) Heparin • Natural anticoagulant made in mast cells (a type of white blood cell) and released into the blood • Inhibits blood coagulation by binding to the protein antithrombin • Has the highest negative charge density of any known biological macromolecule • Routinely added to blood samples obtained for clinical analysis, and to blood donated for transfusion, to prevent clotting Glycoconjugates • Sugar covalently joined to a protein or a lipid • Act in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, and wound healing Proteoglycans • 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 • Major components of connective tissue such as cartilage Glycoproteins • Have one or several oligosaccharides of varying complexity joined covalently to a protein • Found on the outer face of the plasma membrane, in the extracellular matrix, blood, Golgi complexes, secretory granules, and lysosomes • Rich in information, forming highly specific sites for recognition and high-affinity binding by other proteins. Glycolipids • Membrane lipids in which the hydrophilic head groups are oligosaccharides glycoproteins, act as specific sites for recognition by carbohydrate-binding proteins Carbohydrates Carbohydrates • glucose provides energy for the brain and ½ of energy for muscles and tissues • glycogen is stored glucose • glucose is immediate energy • glycogen is reserve energy Carbohydrates • all plant food • milk • carbohydrates are not equal – simple carbohydrates – complex carbohydrates Simple Carbohydrates • sugars – monosaccharides – single sugars – disaccharides – 2 monosaccharides Complex Carbohydrates • starches and fibers • polysaccharides – chains of monosaccharides Simple Carbs • monosaccharides – all are 6 carbon hexes • • • • 6 carbons 12 hydrogens 6 oxygens arrangement differs – accounts for varying sweetness – glucose, fructose, galactose Glucose • • • • mild sweet flavor known as blood sugar essential energy source found in every disaccharide and polysaccharide Fructose • sweetest sugar • found in fruits and honey • added to soft drinks, cereals, deserts Galactose • hardly tastes sweet • rarely found naturally as a single sugar Disaccharides • pairs of the monosaccharides – glucose is always present – 2nd of the pair could be fructose, galactose or another glucose – taken apart by hydrolysis – put together by condensation – hydrolysis and condensation occur with all energy nutrients – maltose, sucrose, lactose Condensation • making a disaccharide – chemical reaction linking 2 monosaccharides Hydrolysis • breaking a disaccharide – water molecule splits – occurs during digestion Maltose • 2 glucose units • produced when starch breaks down • not abundant Sucrose • fructose and glucose • tastes sweet – fruit, vegetables, grains • table sugar is refined sugarcane and sugar beets • brown, white, powdered Lactose • glucose and galactose • main carbohydrate in milk – known as milk sugar Complex Carbohydrates • polysaccharides – glycogen and starch • built entirely of glucose – fiber • variety of monosaccharides and other carbohydrate derivatives Glycogen • limited in meat and not found in plants – not an important dietary source of carbohydrate • BUT – all glucose is stored as glycogen – long chains allow for hydrolysis and release of energy Starches • stored in plant cells • body hydrolyzes plant starch to glucose Fiber • structural parts of plants – found in all plant derived food • bonds of fibers cannot be broken down during the digestive process – minimal or no energy available Fiber types • cellulose • pectins • lignins • resistant starches – classified as fibers – escape digestion and absorption Fiber Characteristics • soluble fibers, viscous, fermentable – easily digested by bacteria in colon – associated with protection against heart disease and diabetes • lower cholesterol and glucose levels – found in legumes and fruits Fiber • insoluble and not easily fermented – promote bowel movements – alleviate constipation – found in grains and vegetables DRI and Fiber • distinguish fibers by source – dietary fibers: naturally in intact plants – functional fibers: extracted from plants or manufactured – total fiber: sum of the 2 Carbohydrate Digestion • break down into glucose – body is able to absorb and use • large starch molecules – extensive breakdown • disaccharides – broken once • monosaccharides – don’t need to be broken down Carbohydrate Digestion • begins in mouth – chewing releases saliva – enzyme amylase hydrolyzes starch to polysaccharides and maltose • stomach – no enzymes available to break down starch – acid does some breakdown – fibers in starch provide feeling of fullness • small intestine – majority of carbohydrate digestion takes place here – pancreatic amylase reduces carbs to glucose chains or disaccharides – specific enzymes finish the job • maltase – maltose into 2 glucose • sucrase – sucrose into glucose and fructose • lactase – lactose into glucose and galactose • large intestine – 1-4 hours for sugars and starches to be digested – only fibers remain • attract water, which softens stool – bacteria ferment some fibers • water, gas, short-chain fatty acids (used for energy) Carbohydrate Absorption • glucose can be absorbed in the mouth • majority absorbed in small intestine – active transport • glucose and galactic – facilitated diffusion • fructose • smaller rise in blood glucose Lactose Intolerance • more lactose is consumed than can be digested – lactose molecules attract water • cause floating, abdominal discomfort, diarrhea – intestinal bacteria feed on undigested lactose • produce acid and gas Lactose Intolerance • age, damage, medication, diarrhea, malnutrition • management requires dietary change – 6 grams (1/2 cup) usually tolerable – take in gradually – hard cheeses & cottage cheese – enzyme drops or tablets • lactose free diet is extremely difficult to accomplish Carbohydrate Metabolism • 1. 2. 3. 4. 1/3 of body’s glycogen is stored in liver – released as glucose to bloodstream eat – intake glucose liver condenses extra glucose to glycogen blood glucose falls liver hydrolyzes glycogen to glucose Glycogen is bulky, so we store only so much: short term energy supply Fat is the long term energy supply. Glucose for Energy • enzymes break apart glucose – yielding energy • inadequate supply of carbohydrates – ketone bodies (fat fragments) are an alternate energy source during starvation – excess ketones can lead to ketosis: imbalance of acids in body • minimum of 50 – 100 grams of carbs/day are needed to avoid ketosis Glucose Homeostasis • maintaining an even balance of glucose is controlled by insulin and glucagon – insulin • moves glucose into the blood – glucagon • brings glucose out of storage • maintaining balance – balanced meals at regular intervals • fiber and some fat slow the digestive process down • glucose gets into the blood slow and steady 1 Intestine Maintaining Blood Glucose Homeostasis When a person eats, blood glucose rises. 2 Pancreas High blood glucose stimulates the pancreas to release insulin. Insulin 3 Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscles. Insulin also stimulates the conversion of excess glucose into fat for storage. Liver Fat cell Muscle 4 As the body's cells use glucose, blood levels decline. 5 Pancreas Glucagon 6 Glucose Insulin Glucagon Glycogen Low blood glucose stimulates the pancreas to release glucagon into the bloodstream. Glucagon stimulates liver cells to break down glycogen and release glucose into the blood.a Liver a The stress hormone epinephrine and other hormones also bring glucose out of storage. 7 Blood glucose begins to rise. Imbalance • diabetes – after food intake, blood glucose rises and is not regulated because insulin is inadequate • hypoglycemia – blood glucose drops dramatically • too much insulin, activity, inadequate food intake, illness • diet adjustment includes fiber-rich carbs and protein Glycemic Index • way of classifying food according to their ability to raise blood glucose • much controversy Sugar • ½ comes from natural sources, ½ from refined and added – sucrose, corn syrup, honey • excess can lead to nutrient deficiencies and tooth decay – empty calories – sugar and starch break down in the mouth Sugar • recommended intake – added sugar = no more than 10% of energy intake Starch and Fiber • diet that includes starch, fiber and natural sugars – whole grains, vegetables, legumes, fruits • • • • may protect against heart disease and stroke reduces the risk of type 2 diabetes enhances the health of the large intestine can promote weight loss Starch and Fiber • starch intake – 45-65% – 225 – 325 grams (DV is 300 grams) – 900-1300 kcal/2000 kcal – RDA is 130 grams • fiber intake – Daily Value is 25 grams/2000 kcal Groceries • grains: 1 serving = 15 grams • vegetables • • • • – ½ cup starchy = 15 grams – ½ cup nonstarchy = 5 grams fruit: 1 serving = 15 grams milk: 1 cup = 12 grams meat: none or little legumes: ½ cup = 15 grams Artificial Sweeteners • help keep sugar and energy intake down • anything we eat has FDA approval – saccharin – aspartame – acesulfame potassium – sucralose – neotame Sugar Replacers • sugar alcohols – provide bulk and sweetness • cookies, gum, candy, jelly – do contain minimal kcal – low glycemic response • absorbed slowly – do not cause dental caries
