Carbohydrates Structure and Function PDF

Carbohydrates – Structure and Function  Section 18.2 Occurrence and Functions of Carbohydrates Carbohydrates storehouse of chemical energy (glucose, starch, glycogen) The mo...

Carbohydrates – Structure and Function  Section 18.2 Occurrence and Functions of Carbohydrates Carbohydrates storehouse of chemical energy (glucose, starch, glycogen) The most abundant class of – a gram of digested bioorganic molecules on earth carbohydrate gives about 4 produced by the kcal of energy photosynthetic activity of the – complex carbohydrates are green plants best for diet also referred to as saccharides supportive structural components in plants and some animals because of the sweet taste of (cellulose, chitin) many carbohydrates form part of the structural (Latin, saccharum, meaning framework of DNA & RNA sugar) carbohydrate “markers” on cell surfaces play key roles in cell-cell recognition processes. Copyright © Cengage Learning. All rights reserved 2  Section 18.3 Classification of Carbohydrates Simpler Formula: Classification based on – CnH2nOn or Cn(H2O)n products of acid hydrolysis: (hydrates of C) Monosaccharides – n= number of atoms – the simple sugars polyhydroxy aldehydes or – contain a single polyhydroxy aldehyde or polyhydroxy ketone polyhydroxy ketones or unit compounds that produce such – cannot be degraded into simpler substances upon hydrolysis. products by hydrolysis reactions – pure monosaccharides are water- soluble, white, crystalline solids Disaccharides – contains 2 monosaccharide units covalently bonded to each other – crystalline and water soluble substances – upon hydrolysis they produce monosaccharides Copyright © Cengage Learning. All rights reserved 3  Carbonyl group A carbonyl group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom --> [C=O] The simplest carbonyl groups are Aldehydes and Ketones usually attached to another carbon compound.  Section 18.3 Classification of Carbohydrates Oligosaccharides – contains 3-10 monosaccharide – homopolysacchrides – polymers units - covalently bonded of a single monosaccharide – disaccharides are the most (glycogen, cellulose, starch) common type – heteropolysaccharides – contain – trisaccharides (raffinose) more than one kind of monosaccharide (hyaluronic acid, – tetrasaccharides (stachyose) heparin, chondroitin sulfate) – free oligosaccharides, other than disaccharides, are less common Derived carbohydrates in nature – those where carbohydrate – usually found associated with moieties have undergone some proteins and lipids in complex reactions converting them into molecules that serve structural other products and regulatory functions – sugar acids, sugar alcohols, deoxysugars, and sugar amines Polysaccharides – consist of tens of thousands of monosaccharide units covalently bonded Copyright © Cengage Learning. All rights reserved 5  Section 18.8 Classification of Monosaccharides carbohydrates that have the general formula CnH2nOn – n varies from 3 – 8. grouped together according to the number of carbons they contain – C3H6O3 triose – C4H8O4 tetrose – C5H10O5 pentose – C6H12O6 hexose – C7H14O7 heptose -presence of a ketone group is usually indicated by using the – C8H16O8 octose ending “ ulose” in naming the may either be: sugar – an aldose – contains aldehyde group - e.g., levulose – a ketose – contains ketone group Copyright © Cengage Learning. All rights reserved 6  Section 18.8 Classification of Monosaccharides Exercise Classify each of the following monosaccharides according to both the number of carbon atoms and the type of carbonyl group present. Copyright © Cengage Learning. All rights reserved 7  Section 18.8 Classification of Monosaccharides Exercise Classify each of the following monosaccharides according to both the number of carbon atoms and the type of carbonyl group present. Answers: a. aldopentose; b. ketohexose; c. aldohexose; d. ketopentose Copyright © Cengage Learning. All rights reserved 8  Section 18.8 Classification of Monosaccharides Trioses Pentoses the parent member of the family of monosaccharides aldopentoses from them emanates the other – D-(-)-xylose members of the monosaccharide a constituent of the heart family. muscle the final form of carbohydrate into – D-(-)-ribose which all carbohydrates, regardless of their complexity, are degraded in ribose and 2-deoxyribose – the body during carbohydrate present as intermediates in metabolism. metaolic pathways and are important building blocks D(+)- glyceraldehyde - (the of RNA and DNA enantiomer- OH group is on the right side of the asymmetric carbon atom) ketopentoses – an aldotriose – D-ribulose Dihydroxyacetone – D-xylulose – a ketotriose Copyright © Cengage Learning. All rights reserved 9  Section 18.8 Classification of Monosaccharides Hexoses D-mannose the most common of all the – found in certain bacteria, fungi, and plants monosaccharides – converted to usable glucose in the aldohexoses body, but has no real – D-(+)-mannose physiological significance – D-(+)-glucose A 5% (m/v) glucose solution is often used in hospitals as an intravenous source of nourishment for patients who cannot take food by mouth. – D-(+)-galactose ketohexose – D-(-)-fructose Copyright © Cengage Learning. All rights reserved 10  Section 18.8 Classification of Monosaccharides Monosaccharides can be classified based on their spatial orientation (stereochemistry). A monosaccharide can be classified as a D or L isomer, depending on the spatial orientation of the –H and –OH groups attached to the carbon atom adjacent to the terminal primary alcohol group. The D isomer is represented when the –OH is written to the right of this carbon in the Fischer projection formula. The L isomer is represented when this –OH is written to the left. Copyright © Cengage Learning. All rights reserved 11  Section 18.9 Biochemically Important Monosaccharides Glucose and Fructose 1. Most abundant in nature 2. Nutritionally most important 3. Grape fruit good source of glucose (20 - 30% by mass) -- also named grape sugar, dextrose and blood sugar (70 - 100 mg/100 mL of blood) 4. Six membered cyclic form 1. Ketohexose 2. Sweetest of all sugars; the fruit sugar 3. Found in many fruits and in honey 4. Good dietary sugar-- due to higher sweetness 5. Five membered cyclic form Copyright © Cengage Learning. All rights reserved 12  Section 18.9 Biochemically Important Monosaccharides Galactose and Ribose 1. A component of milk sugar 2. Synthesized in human 3. Also called brain sugar-- part of brain and nerve tissue 4. Used to differentiate between blood types 5. Six membered cyclic form 6. Galactosemia - a result of genetic deficiency in the infant – the gene responsible for the enzyme that converts D-galactose to D-glucose. Such infants cannot metabolize galactose and it builds up in the blood and tissue. 1. Part of RNA 2. Part of ATP 3. Part of DNA 4. Five membered cyclic form Copyright © Cengage Learning. All rights reserved 14  Section 18.10 Cyclic Forms of Monosaccharides Hemiacetals and Hemiketals The dominant form of Two types of ring structures are monosaccharides with 5 or more possible: C atoms is cyclic – five-membered ring, or furanose ring, derived from parent compound furan Hemiacetals and hemiketals are – six-membered ring, or pyranose ring, formed from the reaction between derived from parent compound pyran two functional groups: aldehyde or ketone and alcohol – may take place either intermolecularly or intramolecularly as in the case of sugars, provided there are sufficient number of carbons between the aldehyde or ketone and the alcohol group to permit a stable ring formation – five- or six-membered hemiacetal rings are stable Copyright © Cengage Learning. All rights reserved 15  Section 18.10 Cyclic Forms of Monosaccharides Cyclic Hemiacetal Forms of D-Glucose In the cyclic hemiacetals of glucose, C1*, is now a chiral center (an anomeric carbon) – two anomers of D-glucose: -D- glucose & -D-glucose The cyclic hemiacetals are readily interconvertible in aqueous solution – this intercoversion of - and - anomers in solution is accompanied by a change in specific rotation called MUTAROTATION. – only sugars that form hemiacetal or hemiketal structure mutarotate. Copyright © Cengage Learning. All rights reserved 16 The Cis and Trans The key difference between alpha and beta anomers is that in alpha anomer, the hydroxyl group at the anomeric carbon is cis to the exocyclic oxygen at the anomeric center, whereas in beta anomer, the hydroxyl group is trans to the exocyclic oxygen. Cis-trans isomers exhibit a type of stereoisomerism where the atoms have different spatial arrangements in three-dimensional space. In the field of organic chemistry, cis isomers contain functional groups on the same side of the carbon chain whereas the functional groups are on opposite sides in trans isomers. This type of isomerism can arise in both organic and inorganic molecules. The prefix “cis” and “trans” have Latin roots and can be translated as “this side of” and “other side of” respectively. Some coordination complexes have cis-trans isomers as well.  Section 18.10 Cyclic Forms of Monosaccharides 2 anomeric forms of D- Any —OH group at a chiral center that is to the right in a Fischer glucose: projection formula points down in – Alpha-form: -OH of C1 and the Haworth projection formula CH2OH of C5 are on and any —OH group to the left in opposite sides a Fischer projection formula points – Beta-form: -OH of C1 and up in the Haworth projection CH2OH of C5 are on same formula. sides 6 6 CH2OH Anomeric CH2OH Anomers: Cyclic O Carbon O OH 5 5 monosaccharides that differ 4 1 4 OH OH 1 only in the position of the OH 2 OH 2 OH 3 substituents on the anomeric OH 3 OH Anomeric Carbon carbon atom. -D-Glucose -D-Glucose Copyright © Cengage Learning. All rights reserved 19  Section 18.11 Haworth Projection Formulas Practice Exercise Which of the monosaccharides glucose, fructose, galactose, and ribose has each of the following structural characteristics? (There may be more than one correct answer for a given characteristic) a. It is a pentose. b. It is a ketose. c. Its cyclic form has a 6-membered ring. Copyright © Cengage Learning. All rights reserved 20  Section 18.11 Haworth Projection Formulas Practice Exercise Which of the monosaccharides glucose, fructose, galactose, and ribose has each of the following structural characteristics? (There may be more than one correct answer for a given characteristic) a. It is a pentose. b. It is a ketose. c. Its cyclic form has a 6-membered ring. d. Its cyclic form has two carbon atoms outside the ring. Answers: a. Ribose b. Fructose c. Glucose, galactose Copyright © Cengage Learning. All rights reserved 21  Section 18.13 Disaccharides The two monosaccharides are Condensation of the hydroxyl linked together by acetal function of the hemiacetal formation to form disaccharide group of one monosaccharide One monosaccharide act as a with the hydroxyl group of hemiacetal and other as another monosaccharide forms alcohol and the resulting ether the bond, called a glycosidic bond is a glycosidic linkage. bond, joining the 2 saccharide units. Copyright © Cengage Learning. All rights reserved 22  Section 18.13 Disaccharides Maltose – -1,4-glycosidic bond means that Malt sugar, found in corn syrup, the first sugar is in -configuration malt, and germinating seeds and its C#1 is linked to C#4 of the consists of two molecules of second sugar component glucose joined by -1,4-glycosidic – the second sugar may be either bond an α- or a β-anomer. Copyright © Cengage Learning. All rights reserved 23 Alpha and Beta  Section 18.13 Disaccharides Cellobiose one of the major fragments the 2 glucose units are joined isolated after extensive by a -1,4-glycosidic linkage hydrolysis of cellulose Maltose is digested easily by humans because we have enzymes that can break α- (1→4) linkages but not β- (1→4) linkages of cellobiose Copyright © Cengage Learning. All rights reserved 25  Section 18.13 Disaccharides Lactose Lactose intolerance: a condition in Milk sugar which people lack the enzyme – human - 7%–8% lactose lactase needed to hydrolyze – cow’s milk - 4%–5% lactose lactose to galactose and glucose. consists of -galactose with a Lactose intolerance is unpleasant, -1,4-glycosidic linkage to - but its effects can be avoided by a glucose (or -glucose) diet that rigorously excluded milk and milk products. Copyright © Cengage Learning. All rights reserved 26  Section 18.13 Disaccharides Galactosemia: the genetic disease Lactose intolerance caused by the absence of the vs Galactosemia enzymes needed for conversion of galactose to glucose. When undigested, lactose attracts A reduced form of galactose, water causing fullness, discomfort, called dulcitol (galactitol), a toxic cramping, nausea, and diarrhea. metabolite, is produced and Bacterial fermentation of lactose accumulates. along the intestinal tract produces It can lead to severe mental acid (lactic acid) and gas, adding retardation, cataracts, and early to the discomfort. deaths Copyright © Cengage Learning. All rights reserved 27  Section 18.13 Disaccharides Sucrose the common table sugar & the most abundant of all disaccharides found in plants. produced commercially from the juice of sugar cane and sugar beets. the -anomeric carbon 1 of glucose joins the -anomeric carbon 2 of fructose (-1,2- glycosidic bond) Copyright © Cengage Learning. All rights reserved 28  Section 18.13 Disaccharides Invert sugar Optical activities: – Sucrose : +66.5 – Glucose : +53 – Fructose : -92 – Invert sugar : -39 invert sugar has a much greater Honeybees and many other tendency to remain in solution. insects possess an enzyme called invertase that hydrolyzes sucrose In the manufacture of jelly and to invert sugar. candy and in the canning of fruit, crystallization of the sugar is Thus honey is predominantly a undesirable, therefore conditions mixture of D-glucose and D- leading to the hydrolysis of fructose with some unhydrolyzed sucrose are employed in these sucrose. processes; in addition, fructose is sweeter than sucrose Copyright © Cengage Learning. All rights reserved 29  Section 18.13 Disaccharides Artificial sweeteners Copyright © Cengage Learning. All rights reserved 30  Section 18.14 Oligosaccharides Commonly found in onions, cabbage, broccoli and wheat In humans, intestinal bacteria action on the undigestable raffinose and stachyose present in beans produces gaseous products that can cause discomfort and flatulence. Copyright © Cengage Learning. All rights reserved 31  Section 18.14 Oligosaccharides Solanin - a potato toxin, is a oligosaccharide found in association with an alkaloid bitter taste of potatoes is due to relatively higher levels of solanin. Copyright © Cengage Learning. All rights reserved 32  Section 18.15 General Characteristics of Polysaccharides The Polymer Chain many monosaccharide units bonded with glycosidic linkages branched or unbranched homopolysaccharide or heteropolysaccharides Copyright © Cengage Learning. All rights reserved 33  Section 18.15 General Characteristics of Polysaccharides alternate name is glycan not sweet and don’t show Homopolysaccharides positive tests with Tollen’s and – starch Benedict’s solutions – glycogen limited water solubility – cellulose Storage polysaccharides – chitin – starch – carageenan – glycogen Heteropolysaccharides Structural polysaccharides – hyaluronic acid – cellulose – heparin – chitin – chondroitin sulfate Acidic polysaccharides – alginic acid – heparin – hyaluronic acid Copyright © Cengage Learning. All rights reserved 34  Section 18.16 Storage Polysaccharides Starch the chief caloric distributor in the diet; the reserve carbohydrates for plants Amylose - straight chain polymer; 15 - 20% of the starch; water-soluble fraction; 60 – 300 glucose units joined by -1,4-glycosidic bonds experimental evidence indicates that the molecule is actually coiled like a spring and is not a straight chain of glucose units. When coiled in this fashion the molecule has just enough room in its core to accommodate an iodine molecule. The characteristic blue color that starch gives when treated with iodine is due to the formation of the amylose-I2 complex. Copyright © Cengage Learning. All rights reserved 35  Section 18.16 Storage Polysaccharides Starch Amylopectin – branched chain polymer – 80 - 85 % of the starch – the water-insoluble fraction – composed of 300 – 6000 glucose units joined primarily by -1,4- glycosidic bonds and occasionally by -1,6- glycosidic bonds – -1,6 bonds are responsible for branching which occurs about once every 25-30 units. Copyright © Cengage Learning. All rights reserved 36  Section 18.16 Storage Polysaccharides Glycogen the animal starch glucose storage molecule of animals stored in granules in liver and muscle cells like amylopectin, is a nonlinear polymer of glucose units joined by -1,4- and -1,6-glycosidic bonds but has lower molecular weight more highly branched structure its branches are shorter gives red-brown color with I2 Copyright © Cengage Learning. All rights reserved 37  Section 18.17 Structural Polysaccharides Cellulose It serves as dietary fiber in food-- readily absorbs water and results a fibrous carbohydrate found in all in softer stools plants where it serves as the 20 - 35 g of dietary fiber is desired structural component of the plant’s everyday cell wall a linear polymer of glucose units joined by -1,4-glycosidic bonds linear nature of chains allows close packing into fibers, making it difficult for solvent molecules to pull the chains apart, thus cellulose is inert towards most solvents Cotton ~95% cellulose and wood ~50% cellulose Copyright © Cengage Learning. All rights reserved 38  Section 18.17 Structural Polysaccharides Cellulose yields D-glucose upon hydrolysis yet man & the carnivorous animals can’t utilize cellulose as a source of glucose. human‘s digestive juices lack the enzyme cellulase that hydrolyze - 1,4-glycosidic linkages. ruminants (cows, goats) and termites have, within their digestive tracts, microorganisms that produce cellulase Copyright © Cengage Learning. All rights reserved 39  Section 18.17 Structural Polysaccharides Chitin Similar to cellulose in both itself is inert and practically function and structure insoluble in most solvents. Its polymer of N-acetyl-D- derivative, chitosan, can be glucosamine bound by β-1 → 4 prepared by simple alkali- glycosidic linkages (has a linear catalyzed deacylation. Chitosan extended structure like cellulose) derivatives are commercially used Function is to give rigidity to the as films, fibers, surface coatings exoskeletons of crabs, lobsters, and ultrafiltration membranes shrimp, insects, and other arthropods Copyright © Cengage Learning. All rights reserved 40  Section 18.17 Structural Polysaccharides widely used in food industry Carageenan its gelling property is used in enhancing the texture of various occurs as hydrocolloid extracted dairy products and in preventing from selected species of red algae oiling off in caramel and toffee locally obtained from Eucheuma during hot weather striatum, Eucheuma spinosum also serve as coating to retard and Acanthapora moisture loss from foods and fresh sulphated polysaccharides, produce like fruits and vegetables consisting of polymers of sulphated D-galactopyranose bonded through alternating α- 1→3 and β-1→4 glycosidic linkages Copyright © Cengage Learning. All rights reserved 41  Section 18.18 Acidic Polysaccharides Hyaluronic acid highly viscous - serve as lubricants in the fluid of joints and repeating unit is a disaccharide part of vitreous humor of the eye composed of -D-glucuronic acid when some insects sting, they and N-acetyl-D-glucosamine in a inject an enzyme called -(1→3)-linkage. hyaluronidase, which breaks each disaccharide is attached to hyaluronic acid linkages and the next by -(1→4)-linkage facilitates the spread of the venom alternating -(1→3) and -(1→4)- linkages Copyright © Cengage Learning. All rights reserved 42  Section 18.18 Acidic Polysaccharides Heparin Alginic acid locally extracted from Sargassum consists of repeating units of D- seaweeds glucuronic acid and D- glucosamine consist of repeating units of β- 1→4 bonded mannuronic and α- an anticoagulant in blood that 1→4 bonded L-guluronic acid; cell inhibits blood clot formation wall material used in open-heart surgery serves as base coatings in meats and fish which reduces moisture loss and fat absorption Copyright © Cengage Learning. All rights reserved 43  Section 18.18 Acidic Polysaccharides Chondroitin sulfate consists of repeating units of D- glucuronic acid-D-glucosamine sulfate structural role in cartilage, bone, and cornea of the eye Copyright © Cengage Learning. All rights reserved 44  Section 18.19 Dietary Considerations and Carbohydrates Glycemic Foods A developing concern about intake of carbohydrates involves how fast the given dietary carbohydrates are broken down to glucose within the human body Glycemic index refers to: – how quickly carbohydrates are digested – how high blood glucose rises – how quickly blood glucose levels return to normal Glycemic index (GI) has been developed for rating foods Low-GI foods are desirable Copyright © Cengage Learning. All rights reserved 45  Section 18.19 Dietary Considerations and Carbohydrates Glycemic Foods Copyright © Cengage Learning. All rights reserved 46  Section 18.19 Dietary Considerations and Carbohydrates If absent fecal matter is hard, and has Unavailable carbohydrates a long transit time. Long sojourn will dehydrate it and will those not hydrolyzed by digestive make it harder to remove. enzymes It also increases the rate at which they constitute the dietary fiber digestive wastes move through the Fiber in the diet aids in the formation intestinal tract, which lessens the time of bulk in the intestinal tract, which the intestine comes in contact with any increases the absorption of water ingested carcinogens. along the tract. Some forms of diverticulitis Dietary fiber, as it reaches the gut, is (inflammation of the colon) have been intact in structure where they form a relieved by increasing the quantity of meshwork fiber in the diet. The meshwork has spaces where fecal Straining at stool because of lack of matter and water are trapped dietary fiber can lead to The effect is soft fecal matter which hemorrhoidsand nervous disorders. can be easily removed. Copyright © Cengage Learning. All rights reserved 47  Section 18.19 Dietary Considerations and Carbohydrates There is also a correlation between Dietary fiber ischaemic heart disease and gallstone Lack of dietary fiber may also lead to formation with the lack of dietary fiber. overnutrition. Cholesterol can be trapped in the When one does not masticate, the meshwork reducing the concentration secretion of digestive hormones (such of blood cholesterol. as gastrin and cholecystokinin) is not With dietary fiber, bile will not be induced. supersaturated with cholesterol. Without these hormones, it takes In its absence, bile will be longer to reach the feeling of satiety. supersaturated with cholesterol and Dietary fiber may also be beneficial in gallstone formation results. weight maintenance. When cholesterol is trapped plaque Fiber increases the bulk in the formation will be reduced. stomach and intestines without In the absence of dietary fiber, excess contributing to the caloric intake. cholesterol can lead to plaque formation leading to ischaemic heart disorders. About 20-35 grams of dietary fiber daily is a desirable intake. Copyright © Cengage Learning. All rights reserved 48  Section 18.19 Dietary Considerations and Carbohydrates Copyright © Cengage Learning. All rights reserved 49