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II. CARBOHYDRATES

Carbohydrates are polyhydroxyaldehydes or polyhydroxyketones, or compounds that yield such substances upon hydrolysis. They are produced by the photosynthetic activity of the green plants.

- among the most abundant constituent of the plant and animal world and serve many vital functions: a) storehouse of chemical energy (glucose, starch, glycogen)

a gram of digested carbohydrate gives about 4 kcal of energy complex carbohydrates are best for diet USFDA recommends about 58% daily calories from carbohydrates

b) supportive structural components in plants and some animals (cellulose, chitin) c) essential components in the mechanisms of genetic control of growth and development of living

cells (ribose and deoxyribose) d) carbohydrate markers on cell surfaces play key roles in cell-cell recognition processes. e) Ligands and receptors in cellular communication (lectins)

- also referred to as saccharides(Latin, saccharum, meaning sugar)

CLASSES OF CARBOHYDRATES (based on the products of acid hydrolysis)

1. Monosaccharides

because of the sweet taste of many carbohydrates

- the simple sugars; contain a single polyhydroxy aldehyde or polyhydroxy ketone unit; cannot be degraded into simpler products by hydrolysis reactions; pure monosaccharides are water-soluble, white, crystalline solids.

2. Oligosaccharides - those consisting of more than one monosaccharide unit and which on degradation, yield 2 to 10

monosaccharide units; within the human body, oligosaccharides are often found associated with proteins and lipids in complexes that have both structural and regulatory functions; free oligosaccharides, other than the disaccharides, are seldom encountered in biological systems.

o disaccharides (maltose) o trisaccharides (raffinose) o tetrasaccharides (stachyose)

3. Polysaccharides - consist of tens of thousands of monosaccharide units; colloidal in size a) homopolysacchrides polymers of a single monosaccharide (glycogen, cellulose, starch) b) heteropolysaccharides contain more than one kind of monosaccharide (hyaluronic acid, heparin,

chondroitin sulfate) c) hybrid macromolecules proteoglycans, glycoproteins, glycolipids

4. Derived carbohydrates - those where carbohydrate moieties have undergone some reactions converting them into other

products like sugar acids, sugar alcohols, deoxysugars, and sugar amines

- mono- and disaccharides are sweet-tasting, crystalline solids, readily soluble in water MONOSACCHARIDES

- carbohydrates that have the general formula CnH2nOn where n varies from 3 8. They are grouped together according to the number of carbons they contain as:

C3H6O3 triose C6H12O6 hexose C4H8O4 tetrose C7H14O7 heptose C5H10O5 pentose C8H16O8 octose

- while examples of heptoses and octoses do occur in nature, they are far less abundant than the smaller monosaccharides

- may either be: a) an aldose contains aldehyde group b) a ketose contains ketone group

Gen. Formula: Aldoses H- C=O CH2OH

(CHOH)m C=O CH2OH (CHOH)n

CH2OH (m = 1,2,3,4,5) (n = 0,1,2,3,4)

Ketoses

- presence of a ketone group is usually indicated by using the ending ___ulose in naming the sugar

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- the parent member of the family of monosaccharides; from them emanates the other members of the monosaccharide family.

Trioses

- e.g. H H-C=O H-C-OH H-C-OH C=O H-C-OH H-C-OH H H D(+)- glyceraldehyde Dihydroxyacetone (an aldotriose) (a ketotriose)

- the final form of carbohydrate into which all

carbohydrates, regardless of their complexity, are degraded in the body during carbohydrate metabolism.

- 4 aldopentoses: D-(-)-lyxose - 2 ketopentoses: D-ribulose D-(-)-xylose D-xylulose D-(-)-arabinose D-(-)-ribose Physiological function: * lyxose a constituent of the heart muscle * ribulose formed during metabolism of carbohydrates * ribose and 2-deoxyribose present as intermediates in metabolic pathways and are important building blocks of RNA and DNA

Pentoses

- the most common of all the monosaccharides Hexoses

- 3 aldoses: D-(+)-mannose - 1 ketose D-(-)-fructose D-(+)-glucose D-(+)-galactose

1) D-mannose - found in certain bacteria, fungi, and plants - converted to usable glucose in the body, but has no real physiological significance 2) D-glucose - ripe fruits, particularly ripe grapes (20-30% glucose by mass), are a good source of glucose. - also known as grape sugar, dextrose, blood sugar; found in blood at concns. as high as 0.1% - the most abundant sugar in nature; the most important sugar in the human diet - during the process of digestion, all ingested carbohydrates are ultimately degraded to glucose. It is

the raw material for carbohydrate metabolism that produces energy for the body processes. For this reason, patients who are unable to ingest carbohydrates but must maintain an adequate blood level of glucose are given glucose by injection directly into the bloodstream (IV).

3) D-galactose - a hydrolysis product of lactose (with glucose); seldom encountered as a free monosaccharide - not used as such in the body; must first be converted to glucose by specific enzymes - also called brain sugar because it is a component of glycoproteins found in brain and nervous

tissues; N-acetyl--D-galactosamine are components of the blood group antigens, which are chemical markers that distinguish various types of blood groups

- When milk is used as food for infants, the lactose is broken down to produce galactose and glucose. The galactose in turn is then converted to the usable D-glucose. Galactosemia is a result of genetic deficiency in the infant the gene responsible for this particular enzyme that converts D-galactose to D-glucose. Such infants cannot metabolize galactose and it builds up in the blood and tissue.

4) D-fructose - also known as levulose (from levorotatory) - found in high concentrations in fruit juices and in honey - sweetest of all the sugars, it is very easily converted to glucose in the liver and intestine for use in the

body.

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Hemiacetals and hemiketals are formed from the reaction between two functional groups: aldehyde and alcohol. Hemiacetal and hemiketal formation may take place either intermolecularly or intramolecularly as in the case of sugars, provided there are sufficient number of carbons between the aldehyde and the alcohol group to permit a stable ring formation (five- or six-membered hemiacetal rings are stable) Two types of ring structures are possible:

Cyclic Forms of Sugars

a) five-membered ring, or furanose ring, derived from parent compound furan b) six-membered ring, or pyranose ring, derived from parent compound pyran

* In the cyclic hemiacetals of glucose, C1*, is now a chiral center (an anomeric carbon a chiral center created by cyclic hemiacetal formation. Because of this new chiral center, we have two isomers 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 MUTAROTATION. Only sugars that form hemiacetal or hemiketal structure mutarotate.

1) Fischer Projection 2) Haworth Projection 3) Strain-free chair conformation

Rules: 1) In drawing the Haworth projection and chair conformation from Fischer projection, any group of atoms written to the right in the Fischer projection appears below the plane of the ring, any group written to the left appears above the plane. 2) In Haworth & chair stereorepresentations, the OH group on the anomeric carbon is drawn above the plane of the ring in the -isomer, and below the plane of the ring in the -isomer. For the D-family the terminal CH2OH is written above the plane of the ring; for the L-family, below the plane.

Stereorepresentations of Sugars

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Other examples: D-ribose D-fructose H-C=O H-C-OH H-C-OH H-C-OH H-C-OH H-C-OH O H-C-OH H-C CH2OH CH2OH D-ribose -D-ribose -D-ribose (open chain) (w/ hemiacetal bond) -D-ribofuranose

Acetal formation; glucoside (a double ether) The cyclic hemiacetals are capable of forming acetals known as glycosides

(e.g., glucosides, mannosides, etc.)

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- name derived from the fact that it occurs mainly in malt liquors, the juice obtained from sprouted barley and other cereal grains

DISACCHARIDES The two monosaccharides are linked together by acetal formation. Condensation of the hydroxyl function

of the hemiacetal group of one monosaccharide with the hydroxyl group of another monosaccharide forms the bond, called a glycosidic bond, joining the 2 saccharide units.

e.g. maltose lactose cellobiose sucrose 1) Maltose (reducing disaccharide)

- found in corn syrup, malt, and germinating seeds. - consists of two molecules of glucose joined together by -1,4-glycosidic bond

Interpretation of the linkage: -1,4-glycosidic bond means that the first sugar is in -configuration and its C#1 is linked to C#4 of the second sugar component, which may either be an - or a -anomer. More specifically, the linkage is called

-1,4-glucosidic linkage

2) Cellobiose (reducing disaccharide)

- one of the major fragments isolated after extensive hydrolysis of cellulose - differs from maltose only in that the 2 glucose units are joined by a -1,4-glycosidic linkage

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3) Lactose (reducing disaccharide) - found only in milk of mammals - consists of -galactose with a -1,4-glycosidic linkage to -glucose (or -glucose)

- Lactose intolerance

bacteria that release organic acids & CO2 gas into the intestine causing further discomfort. Lactose intolerance is unpleasant, but its effects can be avoided by a diet that rigorously excluded milk and milk products.

: many adults and children are unable to hydrolyze lactose because they do not make the enzyme lactase. Undigested lactose remains in the intestinal tract, causing cramping and diarrhea that can eventually lead to dehydration. Some of the lactose is metabolized by intestinal

- Galactosemia

4) Sucrose (non-reducing disaccharide)

: the genetic disease caused by the absence of one or more of the enzymes needed for the conversion of galactose to phosphorylated glucose. A reduced form of galactose, called dulcitol, is produced in the body. This toxic metabolite then accumulates. If galactosemia is not treated, it leads to severe mental retardation, cataracts, and early deaths

- the common table sugar and is readily obtained from the juice of the sugar cane and the sugar beet - the -anomeric carbon 1 of glucose joins the -anomeric carbon 2 of fructose (-1,2-glycosidic

bond)

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H+ or

Sucrose D-glucose + D-fructose invertase +66.5 (sucrase) +53 -92 (d-) -39 (l-) invert sugar: equimolar mixture of D-glucose & D-fructose - invert sugar has a much greater tendency to remain in solution. In the manufacture of jelly and candy

and in the canning of fruit, crystallization of the sugar is undesirable, therefore conditions leading to the hydrolysis of sucrose are employed in these processes; in addition, fructose is sweeter than sucrose

- reserve food substances (starch, glycogen)

POLYSACCHARIDES They are the most abundant of the carbohydrates found in nature.

- structural materials (cellulose) - tasteless, insoluble in water, amorphous compounds with high molar masses

1) Starch A.Homopolysaccharides

- the chief caloric distributor in the diet; the reserve carbohydrates for plants; found in all plant seeds and tubers

- a mixture of 2 polymers which can be separated from each other by physical and/or chemical means a) amylose

- water-soluble fraction; about 15-20% of the starch; soluble in hot water - composed of 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

b) amylopectin - the water-insoluble fraction; about 80-85% of the starch - composed of 300 6000 glucose units joined primarily by -1,4-glucosidic bonds and occasionally by -1,6-glucosidic bonds (responsible for branching which occurs about once every 25-30 units.

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2) 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 and a more highly branched structure, its branches are shorter - liver glycogen is important in the regulation of blood glucose levels. After a meal the liver cells take

up the excess glucose from the blood and convert it to glycogen for storage, thereby lowering the blood glucose concentration. Between meals, when the blood glucose levels fall, the liver cells break glycogen down to glucose, which is released into the bloodstream ; gives red-brown color with I2

3) Cellulose - a linear polymer of glucose units joined by -1,4-glucosidic bonds. The 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

- a fibrous carbohydrate found in all plants where it serves as the structural component of the plants cell wall

- yields D-glucose upon hydrolysis yet man & the carnivorous animals cant utilize cellulose as a source of glucose. Our digestive juices lack the enzyme cellulase that hydrolyze -1,4-glucosidic linkages. Ruminants (cows, goats) and termites have, within their digestive tracts, microorganisms that produce cellulase

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- some alterations of monosaccharide units give modified sugars that are important in cellular structure, connective tissues, anticoagulants, and antibiotics.

Modified Sugars Have Important Roles

- Two modified sugars have structures similar to glucose. In glucuronic acid, carbon 6 has been oxidized to a carboxylic acid. In glucosamine and N-acetylglucosamine, a hydroxyl group has been replaced by an amine.

4) Chitin - a structural polysaccharide found in insect exoskeleton and shells of crabs, shrimps and lobsters; also

in cell wall of certain fungi - consists of a polymer of N-acetyl-D-glucosamine bound by -1 4 glycosidic linkages (has a linear

extended structure like cellulose) - itself is inert and practically insoluble in most solvents. Its derivative, chitosan, can be prepared by

simple alkali-catalyzed deacylation. Chitosan derivatives are commercially used as films, fibers, surface coatings and ultrafiltration membranes.

5) Carageenan - occurs as hydrocolloid extracted from selected species of red algae; locally obtained from Eucheuma striatum, Eucheuma spinosum and Acanthapora - sulphated polysaccharides, consisting of polymers of D-galactopyranose bonded through alternating

- 13 and -14 glycosidic linkages; cell wall material - widely used in food industry; its gelling property is used in enhancing the texture of various dairy

products and in preventing oiling off in caramel and toffee during hot weather; also serve as coating to retard moisture loss from foods and fresh produce like fruits and vegetables; as food additive it prevents protein denaturation in emulsion prior to cooking; in pharmaceutical industry, for making stable suspensions of inorganic compounds as ingredient of edible molded toys.

- mucopolysaccharides (mucoprotein, mucin) are polysaccharides composed of alternating units, often uronic acid, alternating with an amino sugar; serves as lubricants, for example as a coating on the esophagus

B.Heteropolysaccharides

1) Hyaluronic acid

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- repeating unit is a disaccharide composed of -D-glucuronic acid and N-acetyl-D-glucosamine in a -(13)-linkage. Each disaccharide is attached to the next by -(14)-linkage; alternating -(13) and -(14)-linkages

- in the connective tissues, in the fluid of the eye interior; cements tissues together; conc. viscous solutions of hyaluronic acid are found at movable joints such as knees & elbows to serve as lubricant and shock absorber; in umbilical cord; best commercial source is cockscomb and bacterial fermentation

- when some insects sting, they inject an enzyme called hyaluronidase, which breaks hyaluronic acid linkages and facilitates the spread of the venom

2) Chondroitin sulfate - structural role in cartilage, bone, and cornea of the eye - bind calcium through coordinate binding and when the amount of calcium that is bound is low some

cells escape into the circulation - consists of repeating units of D-glucuronic acid-2-sulfate-D-glucosamine sulfate 3) Heparin - an anticoagulant in blood that inhibits blood clot formation; used in open-heart surgery - consists of repeating units of D-glucuronic acid and D-glucosamine

4) Alginic acid - usually obtained from brown seaweeds; locally extracted from Sargassum seaweeds; cell wall material - consist of repeating units of -14 bonded mannuronic and -14 bonded L-guluronic acid; cell

wall material - used in the preparation of clear candy gels which when prepared at 0.1 0.7% produces a tender to

chewy structures; serves as base coatings in meats and fish which reduces moisture loss and fat absorption; serves as aqueous phase stabilizer in low fat spreads like butter and margarine; commercially available as salts of sodium, potassium, ammonium and calcium and as propylene glycol alginate; forms gels in the presence of calcium ions

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in type A, the nonreducing end is NAGal in type B it is Gal in type AB, both types are present in Type O, neither of these terminal residues is present

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- these are those not hydrolyzed by digestive enzymes UNAVAILABLE CARBOHYDRATES

- include the hemicelluloses, cellulose, and pectin. When mixed with lignin, they constitute the dietary fiber; food sources of dietary fiber include whole grains, bran, fruits, and vegetables.

- Fiber in the diet aids in the formation of bulk in the intestinal tract, which increases the absorption of water along the tract. Dietary fiber, as it reaches the gut, is intact in structure where they form a meshwork. The meshwork has spaces where fecal matter and water are trapped. The effect is soft fecal matter which can be easily removed. If absent fecal matter is hard, and has a long transit time. Long sojourn will dehydrate it and will make it harder to remove. It also increases the rate at which digestive wastes move through the intestinal tract, which lessens the time the intestine comes in contact with any ingested carcinogens. Some forms of diverticulitis (inflammation of the colon) have been relieved by increasing the quantity of fiber in the diet.

- Straining at stool because of lack of dietary fiber can lead to hemorrhoidsand nervous disorders. - Lack of dietary fiber may also lead to overnutrition. When one does not masticate, the secretion of

digestive hormones (such as gastrin and cholecystokinin) is not induced. Without these hormones, it takes longer to reach the feeling of satiety. Dietary fiber may also be beneficial in weight maintenance. Fiber increases the bulk in the stomach and intestines without contributing to the caloric intake.

- There is also a correlation between ischaemic heart disease and gallstone formation with the lack of dietary fiber. Cholesterol can be trapped in the meshwork reducing the concentration of blood cholesterol. With dietary fiber, bile will not be supersaturated with cholesterol. In its absence, bile will be supersaturated with cholesterol and gallstone formation results. When cholesterol is trapped plaque formation will be reduced. In the absence of dietary fiber, excess cholesterol can lead to plaque formation leading to ischaemic heart disorders.

- About 20-35 grams of dietary fiber daily is a desirable intake.

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II. CARBOHYDRATESUMONOSACCHARIDESUAldosesU UKetosesUTriosesUPentosesUHexosesUCyclic Forms of SugarsUStereorepresentations of SugarsUA.Homopolysaccharides