Carbohydrates: Structure and Functions

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Carbohydrates

Carbohydrates: Structure and Functions

Introduction of Carbohydrates

Carbohydrates are the most abundantly found biomolecules on the earth. Each year by photosynthesis plants and algae converts more than 100 billion metric tons of carbon-dioxide and water into cellulose and other plant products.

  • the staple of human diet (starcch) in most part of the world and their oxidation yields energy in them.
  • Insoluble carbohydrate polymers serves as structural and protective elements in cell wall of bacteria (chitin) and plants (cellulose).
  • other carbohydrate polymers lubricate skeltal joint (Hyaluranic acid, chondiotin sulphate), and provide adhesion between cells.
  • complex carbohydrates polymers, covalently attaches to proteins, or lipids, act as signal that determine the intercellular location or metabolic fate of the glycoconjugates.

Definition of carbohydrates

Carbohydrates are polyhydroxy aldehyde or ketone or substances that yield such compounds on hydrolysis. They have empirical formula C1H2O1 i.e. 1:2:1. For example glucose in C6H12O6. Although many common carbohydrates conform to the empirical formula, otherr do not, as some contain nitrogen, phosphorus and sulfur, whereas acetic acid CH3COOH, empirical formula (C.H2O)2 is not a carbohydrate.

Classification of Carbohydrates

There are three major classes of carbohydrates: monosaccharide. oligosaccharide and polysaccharides (Saccaharide means sugar)

Monosaccharides: or simple sugar consists of a single polyhydroxy aldehyde or ketone unit. The most abundant being in nature is six carbon sugar i.e. D-glucose.

Olligosaccharide:– Short chain of monosaccharide units joined by glyosidic bond. The most abundant are the disaccharide i.e. two monosaccharide units. Most common are table sugar (Sucrose), Milk sugar (Lactose), Malt sugar (Maltose). The oligosaccharides having three or more units do not occur as free entitles but are joined to non sugar molecules (Lipids or proteins).

Polysaccharides: Consists of long chains having hundreds or thousands of monosaccharides units. Some polysaccharides, such as cellulose, occur in linear chain, whereas others, such as glycogen, have branched chain. Starch is storage polysaccharide of plants and glycogen that of the animals.

Monosaccharides (carbohydrates 🙂

Monosaccharides are colorless, crystaline solids that are freely soluble in water but insoluble in nonpolar solvents. Most have sweet taste. The backbone is unbranched an all carbons are attached by single bond. One of the carbon double bonded to oxygen atom to form carbonyl group, each of the other carbon cotains a hydroxyl group. No carbon contains more than one hydroxyl group on carbohydrates. If the carbonyl group is at the end of the chain the sugar is an aldose. If the carbonyl carbon is at any other position, the monosaccharide is a ketose. The simplest sugar as per definition is three carbon containing compounds as one carbon is carbonyl carbon and other two contains hydroxyl groups. An aldehyde group containing is Glyceraldehyde and a keto group containing is the Dihydroxyactone as shown below.

Glyceraldehyde
3d model of Glyceraldehyde Source: MDC.edu

Monosaccharide with four, five, six and seven carbons are called tetrose, pentose, hexose and heptose respectively. If they are aldoses, the aldo and if ketoses the keto is added as prefix to them, for example aldopentose or ketopentose. In nature the most common hexoses are D-Glucose, D-galactose and D-mannose, which are aldohexoses, whereas D-fructose is a ketohexose. The aldopentoses the most common are D-ribose and 2-deoxy-D-ribose, which are components of nucleic acids.

D-Glucose
D-Glucose
2-deoxy -Ribose
2-doxy -Ribose
D-ribose
D-Ribose
D-Fructose
D-Fructose

 

 

 

 

 

Mono Saccharides have asymmetric carbon atom

All monosaccharides except dihydroxyacetone contain one or more assymmetric carbon atom and thus occur optically active isomeric forms. The simplest aldose i.e. glyceraldehyde, contain one asymmetric (chiral) carbon and hence have two optical isomes or entatiomers. By convention, one of these two forms is designated the D-isomer and the other L-isomer

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D and L Sugar

The stereisomers of monosaccharides of carbon chain length can be divided into two groups, which differ in the configuration about the chiral carbon atom most distant from the carboxyl carbon. Those with the same configuration at this reference carbon as that of D-glyceraldehyde are designated D-isomers and those with L-configuration are termed as L-sugars. It is by this convention that if hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is D-isomer, when on the left, the L-isomer. Of the sixteen possible aldohexoses, 8 are D sugars and other 8 are L sugars.

D and l sugars

Those sugars rote the plane polarized light to right in polarimeter are termed dextrororatory and termed as d sugar or in (+) sign, whereas the sugars that rotate the plane polarized light to the left are termed as levororatory or l or (-) sugars. The D-glucose is dextrorotatory and some time written as D(+) Glucose. On the other hand the fructose rotate the light to the left is written as D (-) fructose. Hence the sugars that are D can be either d o l type, similarly the L sugar can be either d or l type.

Monosaccharides are Aldoses or ketone

Monosaccharides are aldehyde or ketone derivatives of straight-chain polyhydroxy
alcohols containing at least three carbon atoms. They are classified according to
the chemical nature of their carbonyl group and the number of their C atoms.
If the carbonyl group is an aldehyde, the sugar is an aldose and if the carbony group is ketone, the sugar is a Ketose. The smallest monosaccharides, those with three carbon atoms, are trioses. Those with four, five, six, seven, etc. C atoms are, respectively, tetroses, pentoses, hexoses, heptoses

D-glyceraldehyde L-glyceraldehyde

The most common aldoses include the six-carbon sugars glucose, mannose, and galactose. The pentose ribose is a component of the ribonucleotide residues of RNA. The triose glyceraldehyde occurs in several metabolic pathways.

Glyceraldehyde is the simplest aldose. It is three crbon aldose (triose). With one asymmetric center there are only two stereoisomers. This pair of stereoisomers are the enantionmers D– and L-glyceraldehyde. The D/L designation of all other carboydrates is based upon its similarity to the stereocenter in glyceraldehyde. If the highest numbered asymmetric carbon of a monosaccharide matches that of D-glyceraldehyde, then the saccharide is also of the D– configuration.

D-Aldose Structures

D-Tetroses
D-erythose D-threose
D-Pentoses
f
D-ribose D-arabinose D-xylose D-lyxose
D-Hexoses
D-allose D-altrose D-glucose D-mannose D-gulose D-idose D-galactose D-talose

Ketoses are the isomers of aldoses except that, with very few exceptions, the keto group appears at position 2. As a result, there is one less asymmetric center than in an aldose with the same number of carbons.

D-glycerotetrulose
D-erythropentulose D-threopentulose
D-psicose D-fructose D-sorbose D-tagatose

 

Epimers

When two sugars differ only in the configuration around one carbon atom, they are called epimers.

If you see carefully here the different sugars, you will find that D-glucose and D-galactose are differing in their configuration around the carbon number four i.e. D-glucose and D-galactose are epimer at position C-4. Similarly the D-Glucose and D-mannose differ at C-2 and are epimers at C-2. The sugars differing at two carbon atoms will not be epimers as D-mannose is not an epimer of D-galactose.

Diasteromers

Two sugars having the same molecular formulae but not the mirror images of each other are known as diastereoisomers eg. D-glucose and D-mannose as shown above. These sugars are not mirror images of each other.

Sugar in cyclic form

Monosaccharides with five or more carbon atoms usually occur in aqueous solution as cyclic (ring) structure. The carbonyl group form a covalent bond with oxygen of hydroxyl group along the chain. As alcohol reacts with carbonyl group of aldehydes and ketones to form hemiacetal and hemiketals, respectively.

The hydroxyl and either the aldehyde or the ketone functions of monosaccharides can likewise react intramolecularly to form cyclic hemiacetal or hemiketals. The configuration of each carbon after forming hemiacetal or hemiketal can be represented by Haworth’s projection formula.

A sugar with six membered ring is known as an pyranose in analogy with pyran, the simplest compount containing a ring. Similarly, sugars with five membered ring are designated in analogy with furan ring as shown below.

The cyclic forms of glucose and fructose with six and five membered rings are glucopyranose and fructofuranose, respectively.

Anomeric forms and Mutarotation

When the monosaccharide cyclizes the carbonyl carbon, called the anomeric carbon becomes a chiral centre with possible configurations. The pair of stereoisomers that differ in configuration at the anomeric carbon are called anomers. In α anomer, the OH substituent of the anomeric carbon is on the opposite side of teh sugar ring from the CH2OH group at the chiral centre that designate the D or L configuratoin (C5 in hexoses) the other anomer is known as β form as shown below.

The two anomers have slight different physical and chemical properties, including different optical rotation. The anomers freely interconverts in solution. This fact came to light when glucose crystalized from water (α-D-glucopyransoe) had (+) 112.2 rotation, where as rotation crystallized from pyridine (beta D -glucopyranose) had (+)18.7o. Any of the two anomer when dissolved in water freely interconverts to equilibrium with rotation to (+) 52.7o. This is only possible if there is ring closer and opening taking place. It is observed at equilibrium that D-glucose is a mixture of the anomer (63.6%) and the alfa anomver (35.4%). The linear form is normally present in only minute amount. This process of inter conversion of two anomers in solution is termed as Mutarotation.

Sugar Conformations

Six membered ring can exist in a number of conformations, of which chair and boat form are conformationally stable. Among the two, chair form is more stable the equatorial bonds can with stand the bulky groups, whereas the axial bonds can not. Hence out of total 16 Stereoisomers of hexoses, glucose is the most stable as its bulky groups OH is on the equatorial positions. Out of the two anomers the beta is more stable. This is more clear from the diagram shown below.

 

Sugar Derivatives

As the cyclic and linear forms of aldose and ketose do interconverts, these sugars undergo reactions typical of aldehydes and ketones.

  1. Mid chemical or enzymatic oxidation of an aldose converts its aldehyde group to a carboxylic group, thereby yielding an aldonic acid, such an gluconic acid. Aldonic acids are named by appending the suffic-onic acid to the root name of the parent aldose.
  2. The specific oxidation of the primary alcohal group of aldoses yield uronic acid, which are named by appending -uronic acid to the root name of parent aldose, for example, D-glucuronic acid. Aldoses and ketoses can be reduced under mild conditions, for example, treatment wit hNaBH4, yields acyclic polyhydroxy alcohols known as alditol, which are named by appending the suffix-itol to the root name of the alodose. Ribitol is a component of flavin coenzyme, and glycerol and the cyclic polyhydroxy myo-inositol are important components of lipids, Xylitol is sweetener that is used in sugarless gum and candies.
  3. Monosaccharides units in which an OH group is replaced by H are known as deoxy sugars, Biological, the most important of such sugar is beta D-2-deoxyribose, which is a sugar. component of DNA’s sugar phosphate back bone. L-fucose is one of the few L-sugars and is component of polysaccharides.
  4. In amino sugars, one or more OH group is replaced by an amino group, which is often acetylated. D-glucosamine and D-galactosamine are the most common. N-acetyleneuraminic acid which is derived from N-acetylmanosamine and pyruvic acid is an important constituent of glycoprotein and glycolipids. N-acetylneuraminic acid and derivatives are often termed as sialic acid.
  5. The anomeric hydroxyl group of sugars can condense with an alcohol to form alfa and beta glycosides. The bond connecting anomeric carbon to alcohol oxygen is termed as a glycosidic bond. N-glycosidic linkage is between the anomeric carbon atom and an amine, the bond mostly found in between pentose sugar i.e. ribose and purine/pyrimidine ring in nucleic acids. The bond hydrolyzes extremely slowly under the normal physiological conditoins.

Disaccharides:

The disaccharides are the carbohydrates which on hydrolysis yields two monosaccharides. The most common dissacharides in nature are table sugar (Sucrose), Milk sugar (lactose) and Malt sugar (Maltose)

Lactose

Lactose in milk ranges between 0-7% depending upon the species and it is also called as milk sugar. It contains galactose-glucose joined by beta (1->4) glycosidic linkage. Technically it is also called as O-beta D-galactopyranosyl (1->4) D-glucopyranoside. As it contains free anomeric carbon atom on its glucose unit, it is a reducing sugar. Sugars bearing anomeric carbon atom that have not formed gyceride are termed reducing sugars because it can be readily reduced by mild oxidizing agents. As you might have observed in practicals that fehling solution and Barfoed’s test are positive with lactose. This dissacharide is found only in animals or their products.

Lactose is hydrolyzed by the enzyme lactose (beta-glactosidase) into its constituents galactose and glucose. This enzyme is secreted inthe intestine of human beings. If this enzyme is absent, the lactose in the milk can not be digested and that leads to production of convulsions in the stomach. This is called as lactose intolerance.

Sucrose

This is the most abundant disaccharide in nature and commonly known as table sugar. This is the major form in which it is transported in plants. It is also given the name O alfa-D-glucopyranosyl (1>2) beta-D-fructofuronoside. Both anomeric carbons participate in glycosidic bond formation hence it is Non-reducing sugar. It does not exist in either alfa or beta form and there is no mutarotation. It is also termed as invert sugar. Sucrose it self has rotation (+) 66.5o but on hydrolysis changes to (-) 19.8o. That is dextro sugar changes to levo on hydrolysis. Due to this inversion it is termed as invert sugar. Its major sources are cane sugar and beet root.

Maltose

It is also known as malt sugar which is one of the products of amylase action on amylose. It is glucose-glucose joined by alfa (1->4). It is also produced in seeds when they are germinating. The starch is broken down to shorter fragment viz, glucose, maltose, maltotriose and dextrin. The malt extract of germinating jawar is used to produce beer.

Isomaltose

This is very similar to maltose as far as monosaccharides are concerned. Glucose is joined to glucose by alfa (1->) linkage.

Cellobiose

It is also a disaccharide, constituting lucose joined to glucose by beta (1.>) glycosidic bond as shown lbelow. it is a product of cellulose degradation by cellulase enzyme.

Trehalose

This is one of the most important disaccharide of insects and serves as energy storage compound. it also contains glucose linked to glucose but bonding in Glu alfa. As both the anomeric carbons atoms are forming glycosidic linkage, it is a non reducing sugar.

Trisaccharides

The carbohydrates on hydrolysis yield three monosaccharides. One of the most important trisaccharides is raffinose. It is found in sugar beet, coffee and plant seeds (Black gram).

Polysaccharides

Most of carbohydrates present in nature are polysaccharides. These polymers of high molecular weight and are also called as glycans. They on hydrolysis yield large number of monosaccharides. Function as

a. Storage form of cellular fuel.

b. Structural component in living organisms.

They are classified as

  • Homopolysaccharides
  • Hetropolysaccharides

i.e. they contain one or more than one type of monosaccharides respectively. Polysaccharides on contrast with proteins and nucleic acids can form either linear or branched chains as shown below.

Structural Polysaccharides

The most important structural polysaccharides are cellulose, chitin and peptidoglycans. Cellulose, a fibrous, tough, water insoluble substance, is found in the cell wall of plants, particularly in stalk, stem, trunk and all the woody portion of plant tissues. Cotton is almost purely cellulose. Being a component of cell wall, it provides strength, which can withstand osmotic pressure even upto 20 atm, between extracellular and intracellular space. Cellulose accounts for more than half of the carbon in the hemisphere. Approximately 10^5 kg of cellulose is estimated to be synthesized and degraded annually. It is a polymer of up to 15,000 D-glucose residues linked by beta (1->4) glycosidic bond.

Cellulose fiber consists of approximately 40 parallel, extended glycan chains. The highly cohesive, hydrogen bonded structure gives cellulose fibre exceptional strength and makes them water insoluble despite being their hydrophilicity. In cell wall of plants the cellulose fibers are embedded in and are crosslinked by a matrix containing other polysaccharides and lignins (phenolic polymer).

Vertebrates do not conatin any enzyme to digest them. It is by the grace of some microorganisms that harbor the enzyme cellulaes, help the ruminants and termites to digest cellulose.

Chitin

Chitin is linear homoppolysaccharide composed N-acetyl glucosamine residues linked by beta linkage. The only chemical difference between cellulose and chitin is that the C-2 hydroxyl group is replaced by acetylated amino group. It is like cellulose and is indigestible by vererbrates. It is a principla component of exoskeleton of nearly a million species of arthropods, insects, lobsters, and crabs probably second largest molecule in nature than cellulose.

Peptidoglycans:

The cell wall of bacteria consits of covalently linked polysaccharide and polypeptide chain. It is a heteropolysaccharide consisting of repeating dimmer of N-acetyl glucosamine and N-acety neuraminic acid. Neighbouring parallel chains are covalently cross linked through their tetrapeptide side chain by pentaglycine. Lysozyme of tears can break this peptidoglycans of cell wall of bacteria. Pencillin inhibits the crosslinkage of cell wall peptidoglycans and hence lyses the cell.

Storage Polysaccharides

The most important storage polysaccharides in nature are starch in plant cells and glycogen in animal cells. Both occur as large clustes or granules, intracellularly. They are heavily hydrated i.e. water makes hydrogen bonding with the hydroxyl groups on them

Starch

It contains two types of polymers, amylose and amylopectin. The former contains long, unbranched chain of D-glucose units connected by alfa (1->) linkage. Such chains vary in molecular weight from few hundred to 500,000. A amylose molecule varies from cellulose only in the bonding pattern, but the basic unit in both of them is D-glucose. Due to this the packing of amylose is different from the cellulose. It adopts an irregularly aggregating coiled configuration, i.e. left handed helix.

Amylopectin

Amylopectin also has high molecular weight up to one million and is highly branched homopolysaccharide. The basic glycosidic bond is similar to amylose. i.e. alfa (1-> 4) but the branch point occurs every 24-30 residues by alfa (1–>6) glycosidic bond.

Storage of glucose as starch in the cells reduces the osmotic pressure. Digestion of the starch begins in the mouth itself by the presence of enzyme amylase in the alivia. It leads to breakdown to maltose, maltotriose, dextrin (contains branch point). alfa glucosidase cuts to the level of glucose and debranching enzyme cleaves the alfa (1->6) bond.

Glycogen

It is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of alfa (1->4) linked unit of glucose, with alfa (1–>) linked branch points, but glycogen is more extensively branched (branches occur every 8 to 10 residues) and more compoact than starch.

In amylopectin and glycogen, each branch ends with a non-reducingsugar. These polymers have as many non-reducing ends ads the number of branches, but only one reducing end.

Glycosaminoglycans and Proteoglycans

Glycosaminoglycans and proteoglycans are components of the Extracellular Matrix:

The extracellular space in animal tissues is filled with a gel like material, known as the extacellular matrix. It is also called ground substance, which holds the cells of a tissue together and provides a porous medium for diffusion of nutrients and oxygen to individual cells. This medium contains hetropolysaccharides and fibrous protein. These heteropolysaccharides are called glycoaminoglycans. They are family of linear polymer compounds of repeating dissacharides units, e.g. Hyaluroniac acid (fluid of joints) and vitrous humor of eyes. It contains 250 to 25,000 beta (1-à4) linked dissacharide units i.e. consisting of glucuronic acid and N-acetly glucosamine linked by beta (1à3) bond.

The hyaluronic acid of extracellular matrix of animal tissue contains alternating unit of D-glucuronic acid and N-acetyl gucosamine, Hyaluronates have high molecular weight, even greater than one million, they form clear, highly viscous solutions, which serve as lubricants in the synovial fluids of joints, and gives vitreous humor of the vertebrates eye its jelly like consistency. Other examples of glycosaminogycans are heparin, keratin sulfate and dermatin sulfate.

Glycoproteins:

Many classes of membrane proteins and lipids have more or less array of covalently attached oligosaccharides. These are termed as glycoproteins and glycolipids.

 

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