LECTURE 3

 

Classification, structure and chemical properties of carbohydrates.

 

Introduction: Carbohydrates are the most abundant class of bioorganic molecules on planet Earth. Although their abundance in the human body is relatively low, carbohydrates constitute about 75% by mass of dry plant materials.

Green (chlorophyll-containing) plants produce carbohydrates via photosynthesis. In this process, carbon dioxide from the air and water from the soil are the reactants, and sunlight absorbed by chlorophyll is the energy source.

Plants have two main uses for the carbohydrates they produce. In the form of cellulose, carbohydrates serve as structural elements, and in the form of starch, they provide energy reserves for the plants.

Dietary intake of plant materials is the major carbohydrate source for humans and animals. The average human diet should ideally be about two-thirds carbohydrate by mass.

Carbohydrates have the following functions in humans:

1. Carbohydrate oxidation provides energy.

2. Carbohydrate storage, in the form of glycogen, provides short- term energy reserve.

3. Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids).

4. Carbohydrates form part of the structural framework of DNA and RNA molecules.

5. Carbohydrate "markers" on cell surfaces play key roles in cell -cell recognition processes.

Carbohydrate classifications

Most simple carbohydrates have empirical formulas that fit the general formula n2nn. An early observation by scientists that this general formula can also be written as n(2)n is the basis for the term carbohydrate - that is, "hydrate of carbon." It is now known that this hydrate viewpoint is not correct, but the term carbohydrate still persists. Today the term is used to refer to an entire family of compounds, only some of which have the formula n2nn.

Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield such substances upon hydrolysis. The carbohydrate glucose is polyhydroxy aldehyde, and the carbohydrate fructose is polyhydroxy ketone.

striking structural feature of carbohydrates is the large number of functional groups present. In glucose and fructose there is functional group attached to each carbon atom. Carbohydrates are classified on the basis of molecular size as monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides are carbohydrates that contain a single polyhydroxy aldehyde or polyhydroxy ketone unit. Monosaccharides cannot be broken down into simpler units by hydrolysis reactions. Both glucose and fructose are monosaccharides. Naturally occurring monosaccharides have4rom three to seven carbon atoms; five- and six-carbon species are especially common. Pure monosaccharides are water-soluble, white, crystalline solids.

Oligosaccharides are carbohydrates that contain from two to ten monosaccharide units. Disaccharides are the most common type of oligosaccharide. Disaccharides are carbohydrates composed of two monosaccharide units covalently bonded to each other. Like monosaccharides, disaccharides are crystalline, water-soluble substances. Sucrose (table sugar) and lactose (milk sugar) are disaccharides.

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 disaccharides, are seldom encountered in biological systems.

Complete hydrolysis of an oligosaccharide produces monosaccharides. Upon hydrolysis, disaccharide produces two monosaccharides, trisaccharide three monosaccharides, hexasaccharide six monosaccharides, and so on.

Polysaccharides are carbohydrates made up of many monosaccharide units. Polysaccharides, which are polymers, often consist of tens of thousands of monosaccharide units. Both cellulose and starch are polysaccharides. We encounter these two substances everywhere. The paper on which this book is printed is mainly cellulose, as are the cotton in our clothes and the wood in our houses. Starch is component of many types of foods including bread, pasta, potatoes, rice, corn, beans, and peas.

Stereoisomers are isomers whose atoms are connected in the same way but differ in their arrangement in space. The two nonsuperimposable mirror-image forms of chiral molecule are stereoisomers.

There are two major causes of stereoisomerism: (1) the presence of chiral center in molecule, and (2) the presence of "structural rigidity" in molecule. Structural rigidity is caused by restricted rotation about chemical bonds. It is the basis for cis - trans stereoisomerism, phenomenon found in some substituted cycloalkanes and some alkenes.

Stereoisomers can be subdivided into two types: enantiomers and diastereomers. Enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of each other. Left- and right-handed forms of molecule with single chiral center are enantiomers.

Diastereomers are stereoisomers whose molecules are not mirror images of each other. Cis - trans isomers (of both the alkene and the cycloalkane types) are diastereomers. We will see additional examples of carbohydrate diastereomers in the next section. Stereoisomers that are not enantiomers are diastereomers; they must be one or the other.

Enantiomers Diastereomers

Fischer Projections. Drawing three-dimensional representations of chiral molecules, can be both time-consuming and awkward. Fischer projections represent method for giving molecular chirality specifications in two dimensions. Fischer projection is two-dimensional notation showing the spatial arrangement of groups about chiral centers in molecules.

In Fischer projection, chiral center is represented as the intersection of vertical and horizontal lines. The atom at the chiral center, which is almost always carbon, is not explicitly shown.

The tetrahedral arrangement of the four groups attached to the atom at the chiral center is governed by the following conventions: (1) Vertical lines from the chiral center represent bonds to groups directed into the printed page. (2) Horizontal lines from the chiral center represent bonds to groups directed out of the printed page.

Fischer

projection

Our immediate concern is Fischer projections for monosaccharides. Such projections have the monosaccharide carbon chain positioned vertically with the carbonyl group (aldehyde or ketone) at or near the top.

The smallest monosaccharide that has chiral center is the compound glyceraldehydes (2,3-dihydroxypropanal). The structural formula and Fischer projections for the two enantiomers of glyceraldehyde are

D-glyceraldehyde L-glyceraldehyde

The handedness (right and left) of these two enantiomers is specified by using the designations D and L. The enantiomer with the chiral center - group on the right in the Fischer projection is by definition the right-handed isomer (-glyceraldehyde), and the enantiomer with the chiral center - group on the left in the Fischer projection is by definition the left-handed isomer (L-glyceraldehyde).

We now consider Fischer projections for the compound 2,3,4-trihydroxybutanal, monosaccharide with four carbons and two chiral centers.

There are four stereoisomers for this compound - two pairs of enantiomers.

A B C D

First enantiomeric pair Second enantiomeric pair

In the first enantiomeric pair, both chiral center - groups are on the same side of the Fischer projection, and in the second enantiomeric pair, the chiral center - groups are on opposite sides of the Fischer projection. These are the only - group arrangements possible.

The D, L system used to designate the handedness of glyceraldehyde enantiomers is extended to monosaccharides with more than one chiral center in the following manner.

The carbon chain is numbered, starting at the carbonyl group end of the molecule, and the highest-numbered chiral center is used to determine D or L configuration.

A B C D

D- isomer L-isomer D-isomer L- isomer

The D, L nomenclature gives the configuration (handedness) only at the highest-numbered chiral center. The configuration at other chiral centers in molecule is accounted for by assigning different common name to each pair of D, L enantiomers. In our present example, compounds and (the first enantiomeric pair) are D-erythrose and L-erythrose; compounds and D (the second enantiomeric pair) are D-threose and L-threose.

and are diastereomers, stereoisomers that are not mirror images of each other. Other diastereomeric pairs in our example are and D, and , and and D. These four pairs are epimers. Epimers are diastereomers that differ only in the configuration at one chiral center.

In general, compound that has n chiral centers may exist in maximum of 2n stereoisomeric forms. For example, when three chiral centers are present, at most eight stereoisomers (23 = 8) are possible (four pairs of enantiomers).

Stereoisomers. 1. Isomers in which the atoms have the same connectivity but differ in spatial arrangement.

2. Stereoisomerism results either from the presence of chrial center or from structural rigidity caused by restricted rotation about chemical bonds.

Enantiomers. 1. Stereoisomers that are nonsuperimposable mirror images of each other.

2.Handedness (D and L configuration) is determined by the configuration at the highest-numbered chiral center.

3. Enantiomers rotate plane-polarized light in different directions. (+) Enantiomers are dextrorotatory (clockwise), and (-) enantiomers are levorotatory (counterclockwise).

Diastereomers. 1. Stereoisomers that are not mirror images of each other.

2. Epimers are diastereomers whose configurations differ only at one chiral center.

 

Classification of Monosaccharides. Now that we have considered molecular chirality and its consequences, we return to the subject of carbohydrates by considering further details about monosaccharides, the simplest carbohydrates.

Although there is no limit to the number of carbon atoms that can be present in monosaccharide, only monosaccharides with three to seven carbon atoms are commonly found in nature. three-carbon monosaccharide is called triose, and those that contain four, five, and six carbon atoms are called tetroses, pentoses, and hexoses, respectively.

Monosaccharides are classified as aldoses or ketoses on the basis of type of carbonyl group present. Aldoses are monosaccharides that contain an aldehyde group. Ketoses are monosaccharides that contain ketone group.

Monosaccharides are often classified by both their number of carbon atoms and their functional group. six-carbon monosaccharide with an aldehyde functional group is an aldohexose; five-carbon monosaccharide with ketone functional group is ketopentose.

Monosaccharides are also often called sugars. Hexoses are six-carbon sugars, pentoses five-carbon sugars, and so on. The word sugar is associated with "sweetness," and most (but not all) monosaccharides have sweet taste. The designation sugar is also applied to disaccharides, many of which also have sweet taste. Thus sugar is general designation for either monosaccharide or disaccharide.

The simplest aldose and ketose are the trioses glyceraldehyde and dihydroxyacetone.

glyceraldehydes dihydroxycaetone

The D and L designations specify the configuration at the highest-numbered chiral center in monosaccharide. The configurations about other chiral centers are accounted for by assigning different common name to each pair of D and L enantiomers. This naming system, for simple aldoses, is given

 

The L forms are mirror images of the molecules shown.

major difference between glyceraldehyde and dihydroxyacetone is that the latter does not possess chiral carbon atom. Thus, D and L forms are not possible for dihydroxy acetone. This reduces by half (compared with aldoses) the number of stereoisomers possible for ketotetroses, ketopentoses, and ketohexoses. An aldohexose has four chiral carbon atoms, but ketohexose has only three. atoins.

Cyclic forms of monosaccharides. So far in this chapter, the structures of monosaccharides have been depicted as open-chain polyhydroxy aldehydes or ketones. However, experimental evidence indicates that for monosaccharides containing five or more carbon atoms, such open-chain structures are actually in equilibrium with two cyclic structures, and the cyclic structures are the dominant forms at equilibrium.

The cyclic forms of monosaccharides result from the ability of their carbonyl group to react intramolecularly with hydroxyl group. The result is cyclic hemiacetal or cyclic hemiketal. Such an intramolecular cyclization reaction for D-glucose is shown:

Structure 2 is rearrangement of the projection formula for D-glucose in which the carbon atoms have locations similar to those found for carbon atoms in six-membered ring. All hydroxyl groups drawn to the right in the original projection formula appear below the ring. Those to the left in the projection formula appear above the ring.

Structure 3 is obtained by rotating the groups attached to carbon-5 in counterclockwise direction so that they are in the positions where it is easiest to visualize intramolecular hemiacetal formation. The intramolecular reaction occurs between the hydroxyl group on carbon-5 and the carbonyl group (carbon-1). The - group adds across the carbon - oxygen double bond, producing heterocyclic ring that contains five carbon atoms and one oxygen atom.

Intramolecular cyclic hemiacetal formation and the equilibrium between forms associated with it is not restricted to glucose. All aldoses with five or more carbon atoms establish similar equilibria, but with different percentages of the alpha, beta, and open-chain forms. Fructose and other ketoses with sufficient number of carbon atoms also cyclize; here, cyclic hemiketal formation occurs.

Galactose, like glucose, forms six-membered ring, but both D-fructose and D-ribose form five-membered ring.

D-fructose D-ribose

D-Fructose cyclization involves carbon-2 (the keto group) and carbon-5, which results in two CH2OH groups being outside the ring (carbons 1 and 6). D-Ribose cyclization involves carbon-1 (the aldehyde group) and carbon-4.

Haworth Projection Formulas. The structural representations of the cyclic forms of monosaccharides found in the previous section are examples of Haworth projection formulas. Haworth projection is of carbohydrate.

In Haworth projection, the hemiacetal ring system is viewed "edge on" with the oxygen ring atom at the upper right (six-membered ring) or at the top (five-membered ring).

The D or L form of monosaccharide is determined by the position of the terminal 2 group on the highest-numbered ring carbon atom. In the form, this group is positioned above the ring. In the form, which is not usually encountered in biological systems, the terminal CH2OH group is positioned below the ring.

a or b configuration is determined by the position of the - group on carbon-1 relative to the CH2OH group that determines D or L series. In b configuration, both of these groups point in the same direction; in an a configuration, the two groups point in opposite directions.

b-D-Monosaccharide a-D-Monosaccharide b-L-Monosaccharide

Where a or b configuration does not matter, the - group on carbon-1 is placed in a horizontal position, and wavy line is used as the bond that connects it to the ring.

The specific identity of monosaccharide is determined by the positioning of the other: - groups in the Haworth projection. Any - group at chiral center that is to the right in Fischer projection formula points down in the Haworth projection. Any group to the left in Fischer projection points up in the Haworth projection. The following is a matchup between Haworth projection and Fischer projection.

b-form a-form

Reactions of monosaccharides. Five important reactions of monosaccharides are oxidation, reduction, glycoside formation, phosphate ester formation, and amino sugar formation. In considering these reactions, we will use glucose as the monosaccharide reactant. Remember, however, that other aldoses as well as ketoses undergo similar reactions.

Oxidation. Monosaccharide oxidation can yield three different types of oxidation products. The oxidizing agent used determines the product.

Weak oxidizing agents, such as Tollens, Fehling's, and Benedict's solutions, oxidize the carbonyl group end of monosaccharide to give an -onic acid. Oxidation of the aldehyde end of glucose produces gluconic acid, and oxidation of the aldehyde end of galactose produces galactonic acid. The structures involved in the glucose reaction are

D-Glucose D-Gluconic acid

Because monosaccharides act as reducing agents in such reactions, they are called reducing sugars. With Tollens solution, glucose reduces Ag+ ion to Ag, and with Benedict's and Fehling's solutions, glucose reduces Cu2+ ion to Cu+ ion. reducing sugar is a carbohydrate that gives a positive test with Tollens, Benedict's and Fehling's solutions. All monosaccharides are reducing sugars.

Tollens, Fehling's, and Benedict's solutions can be used to test for glucose in urine, symptom of diabetes. For example, using Benedict's solution, we observe that if no glucose is present in the urine ( normal condition), the Benedict's solution remains blue.

The presence of glucose is indicated by the formation of red precipitate. Testing for the presence of glucose in urine is such common laboratory procedure that much effort has been put into the development of easy-to-use test methods.

Strong oxidizing agents can oxidize both ends of monosaccharide at the same time (the carbonyl group and the terminal primary alcohol group) to produce dicarboxylic acid. Such polyhydroxy dicarboxylic acids are known as -aric acids. For glucose, such an oxidation produces glucaric acid.

D-Glucose D-Glucaric acid

Although it is difficult to do in the laboratory, in biological systems enzymes can oxidize the primary alcohol end of an aldose such as glucose, without oxidation of the aldehyde group, to produce -uronic acid. For glucose, such an oxidation produces D-glucuronic acid.

D-Glucose D-Glucuronic acid

Mutarotation. a- and b-forms of monosaccharides are readily interconverted when dissolved in water. This spontaneous process, called mutarotation, results in an equilibrium mixture of a- and b-furanose forms and a- and b-pyranose forms. The open chain that is formed during muterotation can participate in oxidation-reduction reactions.

Glycoside Formation. As you known, that hemiacetals and hemiketals can react with alcohols in acid solution to produce acetals and ketals. Because the cyclic forms of monosaccharides are hemiacetals and hemiketals, they react with alcohols to form acetals and ketals, as is illustrated for the reaction of b-D-glucose with methyl alcohol.

b-D-glucose Methyl b-D-glucoside

The general name for monosaccharide acetals and ketals is glycoside. glycoside is an acetal or ketal forpined p cyclic monosaccharide. More specifically, glycoside produced from glucose is glucoside, from galactose galactoside, and so on. Glycosides, like the hemiacetals and hemiketals from which they are formed, can exist in both a and b forms. Glycosides are named by listing the alkyl or aryl group attached to the oxygen, followed by the name of the monosaccharide involved, with the suffix ide appended to it.

Methyl-a-D-glucoside Methyl-b-D-glucoside

Phosphate ester formation. The hydroxyl groups of monosaccharide can react with inorganic oxyacids to form inorganic esters. Phosphate esters, formed from phosphoric acid and various monosaccharides, are commonly encountered in biological systems. For example, specific enzymes in the human body catalyze the esterification of the carbonyl group (carbon-1) and the primary alcohol group (carbon-6) in glucose to produce the compounds glucose 1-phosphate and glucose -phosphate, respectively.

a-D-Glucose-1-phosphate a-D-Glucose-6-phosphate

These phosphate esters of glucose are stable in aqueous solution and play important roles in the metabolism of carbohydrates.

Amino Sugar Formation. Amino sugars of glucose, mannose, and galactose are common in nature. Such sugars are produced by replacing the hydroxyl group on carbon-2 on the monosaccharide with an amino group. Amino sugars and their N-acetyl derivatives are important building blocks of polysaccharides found in cartilage.

a-D-Glucosamine a-D-Glalactosamine N-acety1a-D-glucosanune

The N-acetyl derivatives of D-glucosamine and D-galactosamine are present in the biochemical markers on red blood cells, which distinguish the various blood types.

Isomerization. Monosaccharides undergo several types of isomerization, for example, after several hours an alkaline solution of D-glucose will also contain D- mannose and D- fructose. Both isomerizations involve an intramolecular shift of a hydrogen atom and a charge in the location of a double bond.

The intermediate that is formed during this process is called an enediol. The reversible transformation of glucose to fructose is an example of an aldose-ketose interconversion. Because there is a change in the conversion of glucose to mannose is referred to as an epimerization. Several enzyme-catalyzed reactions involving enediols occur in carbohydrate metabolism.

 

Oligosaccharides

The term oligosaccharide is often used for carbohydrates that consist of between two and ten monosaccharide units. Oligosaccharides are carbohydrates that contain from two to ten monosaccharide units. Disaccharides are the most common type of oligosaccharide. Disaccharides are carbohydrates composed of two monosaccharide units covalently bonded to each other. Like monosaccharides, disaccharides are crystalline, water-soluble substances. Sucrose (table sugar) and lactose (milk sugar) are disaccharides.

Carbohydrates have the following functions in humans:

1. Carbohydrate oxidation provides energy.

2. Carbohydrate storage, in the form of glycogen, provides short- term energy reserve.

3. Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids).

4. Carbohydrates form part of the structural framework of DNA and RNA molecules.

5. Carbohydrate "markers" on cell surfaces play key roles in cell -cell recognition processes.

As mentioned earlier, disaccharides are those sugars which on hydrolysis give two moles of monosaccharides general these are sweet-testing crystalline, water-soluble substances, easily hydrolysed by enzymes and dilute mineral acids. The common disaccharides have the general formula C12H22O11 which during hydrolysis take un one molecule of water to form two hexoses.

Disaccharides are formed by intermolecular dehydration between two monosaccharide molecules, e.g.

In the formation of disaccharides, at least one monosaccharide unit is linked to the other through the glycosidic carbon. In other words we can say that in the formation of disaccharide, reducing property of at least one hexose unit is lost. Hence disaccharides may be considered .as glycosides in which both components of the molecules are sugars. Disaccharides may be of two types, namely non-reducing and reducing depending on the fact that 1 of one hexose is linked to the carbonyl carbon at in or any other carbon atom of other hexose.

Weak oxidizing agents, such as Tollens, Fehling's, and Benedict's solutions, oxidize the carbonyl group end of monosaccharide to give an -onic acid.

(1) Nn-reducing disaccharides. In these disaccharides the two hexose units are linked together through their reducing (i . aldehydic or ketonic) groups which is , in aldoses and , in ketoses. Now in such cases since the reducing groups of both the hexoses are lost, the resulting compound (disaccharide) will be non-reducing. Hence such disaccharides do not form osazone do not show mutarotation and do not react with reagents like Fehling solution, Tollens reagent, etc. Important example of non-reducing disaccharides is sucrose.

(2) Reducing disaccharides. In these disaccharides, one hexose unit is linked through its reducing carbon to the non-reducing carbon (C4 or 6) of the other Now since the reducing group of one of the hexoses is not involved, the resulting disaccharide will be reducing sugar. Maltose and lactose are examples of reducing disaccharides

Disaccharides. monosaccharide that has cyclic forms (hemiacetal or hemiketal) can react with an alcoho1 to form glycoside (acetal or ketal). This same type of reaction can be used to produce disaccharide, carbohydrate in which two monosaccharides are bonded together. In disaccharide formation, one of the monosaccharide reactants functions as hemiacetal or hemiketal, and the other functions as an alcohol.

Glycosidic linkage

Maltose, often called malt sugar, is produced whenever the polysaccharide starch breaks down, as happens in plants when seeds germinate and in human beings during starch digestion. It is common ingredient in baby foods and is found in malted milk. Malt (germinated barley that has been baked and ground) contains maltose; hence the name malt sugar.

Structurally, maltose is made up of two D-glucose units, one of which must be a-D-glucose. The formation of maltose from two glucose molecules is as follows:

a-D-Glucose a-D-Glucose a-(1-4)-linkage

The glycosidic linkage between the two glucose units is called an a(1 - 4) linkage. The two - groups that form the linkage are attached, respectively, to carbon-1 of the first glucose unit (in an a configuration) and to carbon-4 of the second.

Cellobiose is produced as an intermediate in the hydrolysis of the polysaccharide cellulose. Like maltose, cellobiose contains two D-glucose monosaccharide units. It differs from maltose in that one of D-glucose units - the one functioning as hemiacetal - must have b configuration instead of the configuration for maltose. This change in configuration results in b(1 - 4) glycosidic linkage.

b-D-Glucose b(1 - 4)-linkage

Like maltose, cellobiose is a reducing sugar, has three isomeric forms in aqueous solution, and upon hydrolysis produces two D-glucose molecules.

Lactose is made up of b-D-galactose unit and D-glucose unit joined by b-(1 - 4) glycosidic linkage.

b-D-galactose a-D-Glucose b(1 - 4)-linkage

The glucose hemiacetal center is unaffected when galactose bonds to glucose in the formation of lactose, so lactose is reducing sugar (the glucose ring can open to give an aldehyde).

Sucrose, common table sugar, is the most abundant of all disaccharides and occurs throughout the plant kingdom. It is produced commercially from the juice of sugar cane and sugar beets. Sugar cane contains up to 20 % by mass sucrose, and sugar beets contain up to 17 % by mass sucrose. The two monosaccharide units present in a-D-sucrose molecule are a-D-glucose and b-D-fructose. The glycosidic linkage is not (1 - 4) linkage, as was the case for maltose, cellobiose, and lactose. It is instead an a,b(1 - 2) glycosidic linkage. The - group on carbon-2 of D-fructose (the hemiketal carbon) reacts with the - group on carbon-l of D-glucose (the hemiacetal carbon).

Sucrose, unlike maltose, cellobiose, and lactose, is nonreducing sugar. No helmiacetal or hemiketal center is present in the molecule, because the glycosidic linkage involves the reducing ends of both monosaccharides. Sucrose, in the solid state and in solution, exists in only one form - there are no a and b isomers, and an open-chain form is not possible. Sucrase, the enzyme needed to break the a,b(1 - 2) linkage in sucrose, is present in the human body. Hence sucrose is an easily digested substance. Sucrose hydrolysis (digestion) produces an equimolar mixture of glucose and fructose called invert sugar.

dextrorotatcry laevorotatory

This reaction is also as inversion of sugar because the dextrorotatory case sugar is converted into laevorotatory product due to hydrolysis. The mixture of glucose and fructose is called invert sugar.

 

Polysaccharide

polysaccharide (glucans) contains many monosaccharide units bonded to each other by glycosidic linkages. The number of monosaccharide units varies with the polysaccharide from few hundred to hundreds of thousands. Polysaccharides are polymers. In some, the monosaccharides are bonded together in linear (unbranched) chain. In others, there is extensive branching of the chains.

Although there are many naturally occurring polysaccharides, in this section we will focus on only four of them: cellulose, starch, glycogen, and chitin. All play vital roles in living systems - cellulose and starch in plants, glycogen in humans and other animals, and chitin in arthropods.

Polysaccharides may be divided into two classes: homopolysaccharides, which are composed of one type of monosaccharide units, and heteropolysaccharides, which contain two or more different types of monosaccharide units.

Starch, glycogen and cellulose are homoglycans as they are made of only glucose and are called glucans or glucosans. On the other hand, mucopolysaccharides like hyaluronic acid and chondroitin sulphates are heteroglycans as they are made up of different monosaccharide units.

Cellulose is the most abundant polysaccharide. It is the structural component of the cell walls of plants. Approximately half of all the carbon atoms in the plant kingdom are contained in cellulose molecules. Structurally, cellulose is linear (unbranched) D-glucose polymer in which the glucose units are linked by b(1-4) glycosidic bonds.

Even though it is glucose polymer, cellulose is not source of nutrition for human beings. Humans lack the enzymes capable of catalyzing the hydrolysis of b (1- 4) linkages in cellulose. Even grazing animals lack the enzymes necessary for cellulose digestion. However, the intestinal tracts of animals such as horses, cows, and sheep contain bacteria that produce cellulose, an enzyme that can hydrolyze b (1- 4) linkages and produce free glucose from cellulose. Thus grasses and other plant materials are source of nutrition for grazing animals. The intestinal tracts of termites contain the same microorganisms, which enable termites to use wood as their source of food. Microorganisms in the soil can also metabolize cellulose, which makes possible the biodegradation of dead plants.

Starch, like cellulose, is polysaccharide containing only glucose units. It is the storage polysaccharide in plants. If excess glucose enters plant cell, it is converted to starch and stored for later use. When the cell cannot get enough glucose from outside the cell, it hydrolyzes starch to release glucose.

Iodine is often used to test for the presence of starch in solution. Starch-containing solutions turn dark blue-black when iodine is added. As starch is broken down through acid or enzymatic hydrolysis to glucose monomers, the blue-black color disappears.

Two different polyglucose polysaccharides can be isolated from most starches: amylose and amylopectin. Amylose, straight-chain glucose polymer, usually accounts for 15% 20% of the starch; arnylopectin, highly branched glucose polymer, accounts for the remaining 80% 85% of the starch.

In amylose's structure, the glucose units are connected by a(1- 4) glycosidic linkages.

Starch (amylose)

The number of glucose units present in an amylose chain depends on the source of the starch; 300 500 monomer units are usually present.

Amylopectin, the other polysaccharide in starch, is similar to amylose in that all linkages are linkages. It is different in that there is high degree of branching in the polymer. branch occurs about once every 25 - 30 glucose units. The branch points involve a(1 6) linkages:

Starch (amylopectin)

Note that all of the glycosidic linkages in starch (both amylose and amylopectin) are of the a type. In amylose, they are all a(1 - 4); in amylopectin, both a(1 -4) and a(1 -6) linkages are present. Because linkages can be broken through hydrolysis within the human digestive tract (with the help of the enzyme amylase), starch has nutritional value for humans. The starches present in potatoes and cereal grains (wheat, rice, corn, etc.) account for approximately two-thirds of the world' s food consumption.

Glycogen, like cellulose and starch, is polysaccharide containing only glucose units. It is the glucose storage polysaccharide in humans and animals. Its function is thus similar to that of starch in plants, and it is sometimes referred to as animal starch. Liver cells and muscle cells are the storage sites for glycogen in humans.

Glycogen has structure similar to that of amylopectin; all glycosidic linkages are of the a type, and both (1 - 4) and (1 - 6) linkages are present. Glycogen and amylopectin differ in the number of glucose units between branches and the total number of glucose units present in molecule. Glycogen is about three times more highly branched than amylopectin, and it is much larger, with molar mass of up to 3,000,000 amu.

Chitin is polysaccharide that is similar to cellulose in both function and structure. Its function is to give rigidity to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods. It also occurs in the cell walls of fungi.

Structurally, chitin is linear polymer (no branching) with all b(1- 4) glycosidic linkages, as is cellulose. Chitin differs from cellulose in that the monosaccharide present is an N-acetyl amino derivative D-glucose.

Mucopolysaccharides are compounds that occur in connective tissue associated with joints in animals and humans. Their function is primarily that of lubrication, necessary requirement if movement is to occur. The name mucopolysaccharide comes from the highly viscous, gelatinous (mucus-like) consistency of these substances in aqueous solution.

Unlike all the polysaccharides we have discussed up to this point, mucopolysaccharides are heteropolysaccharides rather than homopolysaccharides.

heteropolysaccharide is polysaccharide in which more than one (usually two) type of monosaccharide unit is present.

One of the most common mucopolysaccharides is hyaluronic acid, heteropolysaccharide in which the following two glucose derivatives alternate in the structure.

It is highly viscous substance and has molecular weight in several hundred millions. Hyaluronic acid is principal component of the ground substance of connective tissue. Among other places it is found in skin, synovial fluid, vitreous hemour of the eye, and umbilical cord. It exercises cementing function in the tissues and capillary walls, and forms coating gel round the ovum. It accounts for about 80% of the viscosity of synovial fluid which contains about 0. 02 0.05% of hyaluronate.

(1,4)-O-b-D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-b-D-glucopyranose.

Chondroitin sulphate. It has similar structure as hyaluronic acid with the difference that the N-acetyl glucosamine unit of the latter is replaced by N-acetyl galactosamine 6 sulphate unit. The two other chondriotin sulphates are and ; the type nas sulphate group in position 4 while the type has L-iduronate ( stereoisomer f D-glucuronic acid) in place of D-glucuronic acid. Chondroitin sulphates are found in cartilage, bone, heart valves, tendons and cornea.

(1,4)-O-b-D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-6-O-sulfo-b-D-galactopyranose.

Dermatan sulfate. (Varying amounts of D-glucuronic acid may be present. Concentration increases during aging process.)

(1,4)-O-a-L-Idopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-4-O-sulfo-b-D-galactopyranose.

Heparin. It is naturally occurring anticoagulant found mainly in the liver, and also in lung, spleen, kidney and iatestinal mucosa. It prevents blood clotting by inhibiting the prothrombin-thrombin conversion and thus eliminating the thrombin effect on fibrinogen. This polysaccharide is composed of glucosamiae-N-sulphate aad sulphate ester of glucuronic acid linked via 1 4 1 4 linkages (difference from hyaluronic acid and chondroitin sulphates).

(1,4)-O-a-D-Glucopyranosyluronic acid-2-sulfo-(1,4)-2-sulfamindo-2-deoxy-6-O-sulfo-a-D-glucopyranose.

Glycolipids and glycoproteins. Before 1960, the biochemistry of carbohydrates was thought to be rather simple. These compounds served (1) as energy sources for plants, humans, and animals and (2) as structural materials for plants and arthropods.

Research since that time has shown that carbohydrates (oligosaccharides) attached through glycosidic linkages to lipids and proteins, which are called glycolipids and glycoproteins, respectively, have wide variety of cellular functions.

It is now known that carbohydrate units on cell surfaces (glycolipids and glycoproteins) govern how individual cells interact with other cells, with invading bacteria, and with viruses. In the human reproductive process, fertilization begins with the binding of sperm to specific oligosaccharide on the surface of an egg.

 

 

References:

1.     Andrew Streltwieser, Jr. Clayton H. Hcathcocr. Introduction to Organic Chemistry. - New York: Macmillan Publishinc Co., 1996. - 1508 p.

2.     David Gutsche C., Baniel J. Pasto. Fundamentals of Organic Chemistry. - New Jersey: Prentice -Hall, Inc.Englewood Cliffs, 1995. -1346 p.

3.     Lewis D.E. Organic chemistry. A modern perspective. Copyright, 1996. 1138 p.