Carbohydrates

I. DEFINITION & ROLES

II. STRUCTURE OF CARBOHYDRATES

A. Linear structure
1. Nomenclature
2. Stereoisomerism - Chirality
3. D & L series of saccharides

B. Cyclic structure
1. Hemiacetal and mutarotation
2. Cyclization mechanism and representation of Haworth
3. Conformation space

III. CHEMICAL PROPERTIES OF SACCHARIDES
A. Properties related to aldehyde group
1. Oxidation
2. Reduction
3. Condensation reactions

B. Properties related to alcoholic functions
1. Complexes with boron
2. Methylation

IV. STUDY OF SOME SACCHARIDES AND DERIVATIVES
A. Simple saccharides
1. Glucose (aldohexose - pyranose)
2. Arabinose (aldopentose - pyranose)
3. Fructose (ketohexose - furanose)
4. Galactose and mannose: aldohexose / pyranose

B. Osamines or amino sugars
C. Sialic acids
D. Muramic acids
E. Phosphorylated saccharides

V. STUDY OF SOME GLYCOSIDES AND DERIVATIVES
A. Definitions

B. Determination of the structure of glycoside

C. Some very important disaccharides

1. Maltose
2. Lactose
3. Sucrose
4. Cellobiose

D. Two triholosides : gentianose and raffinose

E. Heterosides

F. The homopolyosides

1. Starch (amylose and amylopectin)
2. Cellulose
3. Industrial applications
4. Glycogen
5. Glucan degrading enzymes

G. Heteropolyosides

1. Mucopolysaccharides
2. Glycoproteins

VI. References and Internet links

 

I. Definition and roles

Carbohydrates are a group of substances whose basic units are simple sugars called monosaccharides.

Carbohydrates were defined as polyhydroxy aldehydes or ketones. They are water-soluble compounds and reducing agents.

Carbohydrates are present everywhere in the biosphere and represent the prominent weight class among organic molecules. The largest part of carbohydrates comes from photosynthesis, a process that incorporates CO2 into carbohydrates.

Carbohydrates play many critical roles in cells :

  • they serve as energy reserve polymers : starch and glycogen. Starch is the main form of photosynthetic energy storage in the biosphere.
  • they are structural element of the cell mucopolysaccharides in higher animals and cellulose in plants.
  • they act as recognition elements and communication between cells : blood group polysaccharides, antigenic polysaccharides of bacteria.
  • finally, they are an integral part of the structure of many fundamental biological macromolecules such as glycoproteins, nucleic acids (ribose and deoxyribose), coenzymes and antibiotics.
Carbohydrates are classified according to their degree of polymerization :
  • oligosaccharides are polymers of 2 to 20 saccharide residues, being the most common disaccharides
  • the polysaccharides are composed of more than 20 units

Polymerized form of carbohydrates are called saccharides. They can be made of :

  • carbohydrates only and they are called holosides or homosaccharides
  • carbohydrates and a non-carbohydrate part (or aglycone) and and they are called glycosides or heterosaccharides.

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II. Structure

A. Linear structure

1. Nomenclature.

  • their empirical formula is Cn(H20)p.
  • they are characterized by the presence in the same molecule of a reducing function (aldehyde or ketone) and at least one alccol function.
  • carbohydrates who have aldehyde function are called aldoses and those with a ketone function are called ketoses.
  • nomenclature of the carbon atoms of aldoses assigns the number 1 to the one bearing the aldehyde function. In the case of ketoses, the carbon bearing the ketone function is number 2.
The smallest compound defined as sugars is glycolic aldehyde, but this compound has no chiral carbon and no biochemical role in its free state.

The first carbohydrates with biological roles are C3 sugars or trioses : glyceraldehyde and dihydroxyacetone.

It should be noted that in their phosphorylated form, these two compounds represent an important step in the glycolytic pathway , since it is the cleavage of a C6 sugar (fructose 1, 6 diphosphate) to 2 C3 sugars.

2. Stereoisomerism - Chirality

Glyceraldehyde has a carbon with four different substituents, so it is an asymmetric or chiral carbon.

Glyceraldehyde can therefore exist in two different forms (image of one another in a mirror, therefore non-superimposable) which correspond to opposite configurations around the chiral carbon: the two compounds are called enantiomers.

In 1906, Emil Fischer and Rosanoff chose glyceraldehyde as a reference compound to study sugars configuration.

Emil Fischer arbitrarily chose the symbol D for the dextrorotatory enantiomer, that is to say the compound which deflects the plane of polarized light to the right or rather in the direction of clockwise.

It was not until 1954 that Bijvoet showed by crystallographic studies that the arbitrary Fischer corresponded to the absolute configuration of monosaccharides.

All monosaccharides derived from dextrorotatory glyceraldehyde were said to belong to the D series and those from the glyceraldehyde levorotatory were said to belong to the L series.

D & L series of saccharides

All aldoses can be synthesized from glyceraldehyde.

In the Fischer projection , all monosaccharides whose hydroxyl carried by the penultimate carbon is on the right belong to D series.

When moving from one carbohydrates to a higher one, H-C-OH group is added between the chiral carbon bearing the terminal primary alcohol and the adjacent carbonyl carbon.

With each addition, there are two possibilities:

  • for an aldose with n carbons there are thus 2n-2 stereoisomers
  • in the case of ketoses, which can be linked to dihydroxyacetone which does not have a chiral carbon, there are 2n-3 stereoisomers.

Example of glucose : it is a carbohydrate with 6 carbons or hexose. There are therefore 16 stereoisomers, 8 of D series and 8 of L series.

The natural sugars are mostly of D series.

Diastereoisomers are non enantiomeric stereoisomers : they have several chiral carbons with different configurations.

Epimers are stereoisomers that differ in the configuration of only one chiral carbon.

Example: D-mannose and D-galactose are epimers of D-glucose but are not epimers among them.

Serie aldohexose

Ascending series of D-ketoses and epimers of ketohexoses.

Serie ketohexose

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B. Cyclic structure

1. Hemiacetal and mutarotation.

The linear structure or open-chain structure of carbohydrates do not realize all their properties as soon as the number of atoms is greater than 4.

First reducing properties that are not quite those of aldehydes and ketones:

  • for example, if glucose is treated with methanol, it does not bind two molecules of alcohol to form an acetal with an aldehyde such as, but not fixed a single molecule of methanol to form a hemiacetal
  • This is a first indication that the aldehyde function of carbohydrates is not as simplistic as true aldehydes
Secondly, according to the mode of solubilization, glucose gives two solutions respectively called α and β-glucose :
  • these two solutions deviate polarized light but are distinguished by their specific optical rotation [α]20D measured on fresh solutions
  • however, if we let these solution, their rotatory power operates to stabilize at the same value of + 52.5°

This phenomenon has been called mutarotation by Lowry (1889).

The specific rotation [α]20D is measured with a device called a polarimeter.

It is defined by specifying the temperature, the wavelength at which the measurement is made (it is usually the sodium D line 589 nm).

In addition, the concentration is expressed in g/ml and the length of the polarimeter tube is expressed in decimeters.

Knowing the specific rotation of a compound, BIOT's law determines the concentration of a solution of that compound.

This law is additive, that is to say that the rotatory power of a mixture is the sum of the optical rotations of compounds that constitute the mixture.

2. Cyclization mechanism and representation of Norman HAWORTH.

The mutarotation phenomenon implies the existence of an additional asymmetric carbon.

In addition, the formation of hemiacetal implies that the reducing function has already established a connection with an alcohol.

It was in 1884 that Bernhard Tollens provided explanation of the cyclic structure of monosaccharides:
  • the bond angles of the tetrahedral carbon 109.3° make it possible to the carbon skeleton to form cycle
  • the reaction takes place between the aldehyde group and the nearest alcoholic group spatially : that carried by the carbon 5
  • one obtains a 6-membered ring : five carbons and 1 oxygen. A 7 summits cycle would suffer too much tension
  • only 5 and 6 summits cycles are important among natural monosaccharides.
There are several conventions in the representation called cyclic representation of Haworth :
  • it is considered that all the carbon chain is in the same plane, the thick solid line indicates the part of the cycle directed toward the observer
  • in addition, the hydroxyl on the right in the Fischer projection are directed downwards in the ring and those on the left are directed upwards.
The mechanism is as follows :
  • resulting from these conventions, hydroxyl group supported by carbon 5 is found below the cycle
  • it performs a 90° rotation about the bond between carbon 4 and carbon 5 such that the hydroxyl group of carbon 5 approaches the aldehyde group of carbon 1
  • therefore, the carbon 6 rotates and is equivalently located above cycle
  • from that moment one of the lone pairs of the oxygen atom may react on one side or other of the carbon atom : one gets the α-D-glucopyranose if hydroxyl group carried by carbon 1 is below the ring or the β-D-glucopyranose in the opposite case.
The result is a new asymmetric carbon. The two isomers differ only in the position of a group and are called anomers.

The hydroxyl group carried by the carbon 4 can also react and get a 5 vertices or furanose ring.

The names of pyranose and furanose have been adopted by analogy with hydrocarbon containing 6 and 5 summits, respectively.

3. Spatial conformation

Studies of the conformational stability of cyclohexane showed that the spatial arrangements that are not subject to steric constraints adopt a so-called chair conformation and others, less stable, are in the so-called boat conformation.

The position of the hydrogen substituents may be either in an axis perpendicular to the plane defined by the six carbon-carbon bonds, these are so-called axial substituents, or conversely directed outwardly of the ring and they are said equatorial.

In the case of glucopyranose is essentially the chair form exists.

Chair boat form carbohydrate cyclic

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III. Chemical properties

A. Properties related to reductive group

1. Oxidation

Carbohydrates are lower seducers than true aldehydes or ketones. The result of oxidation depends on the conditions of oxidation.

a) aldonic acids are obtained by mild oxidation of aldoses using Br2 or I2 in alkaline condition :

  • glucose gives the gluconic acid
  • mannose gives mannonic acid
  • galactose gives galactonic acid
The reaction is stoichiometric and allows the specific dosage of aldoses since ketoses are not oxidized under these conditions.

b) further oxidation with nitric acid leads to aldaric acids, diacids having a carboxylic function on carbon 1 and carbon 6:

  • glucose gives glucaric acid
  • galactose gives galactaric acid
ketoses are degraded under these conditions. The chain is broken at the ketone function giving a mixture of carboxylic acids.

c) Finally, if the aldehyde function is protected during oxidation, uronic acids are obtained which are oxidized only on the primary alcohol function:

These compounds are involved in cell recognition in bacteria.

The glucuronic acid is the precursor of the synthetic pathway of vitamin C or L-ascorbic acid.

Vitamin C is an enediol form of a lactone derived from an aldonic acid . The pKa of the C3 hydroxyl group of Vitamin C is relatively low due to the resonance stabilization of its conjugate base.

Vitamin C

2. Reduction

The reactions are reduced by catalytic hydrogenation or by the action of an alkali metal borohydride such as NaBH4 or LiBH4.

  • the polyhydric alcohol corresponding to thestarting aldose is obtained
  • regarding ketoses, there are 2 polyalcohols epimers.

It is worth mentioning other reduction reactions used for the determination of sugars and their characterization, including:

Reaction Fehling solution

3. Condensation reactions

a) with cyanide and hydroxylamine

Condensation reactions include the Kiliani's synthesis reaction and the Wohl-Zemplen degradation' reaction. These two paths transform a carbohydrate to the upper one and the lower one, respectively.

They both helped establishing the parentage of carbohydrates with glyceraldehyde.

Kiliani's synthesis : glucose reacts with cyanhidric acid to form a cyanhidrine (2 stereoisomers) which, after hydrolysis, gives a hexahydroxylated acid.

It is reduced by IH (in the presence of red phosphorus) and gives heptanoic acid.

The same reaction from fructose acid gives methyl-2-hexanoic acid.

Synthesis Kiliani

b) with alcohols and phenols

This reaction is particularly important : the substances obtained are saccharides or glycosides. The bond joining the carbohydrate to the alcohol or the phenol is the O-glycoside bond or glycosidic bond.

It is important to note that this bond formation is accompanied by the loss of the reducing power of the carbohydrate and locks the configuration of the cycle.

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B. Properties related to alcoholic functions

1. Complexes with boron

They are used to perform electrophoresis of carbohydrates, which otherwise would not be possible since carbohydrates are not naturally charged.

These complexes were used to demonstrate that the hydroxyl group of the anomeric carbon of α-D-glucose is in the cis position relative to the hydroxyl group carried by carbon 2, so that it is located below the ring. Indeed, the complex is formed more easily with the cis anomer.

The anomeric sugar affect the formation of complexes with boron and thus their electrophoretic mobility.

2. Methylation

Methylating agents such as methyl sulfate [(SO4(CH3)2] in the presence of sodium hydroxide (Haworth) or methyl iodide (ICH3) with Ag2O (Purdie) act by substituting all hydrogen of hydroxyl groups with a -CH3 group, thereby forming an ether group.

If the reducing group of carbohydrates is free, it will react by forming a O-methylated derivative.

However, this bond is a glycosidic bond which does not have the same stability in an acidic medium where it is easily hydrolyzed.

This bond must be therefore distinguished from ether bond by specifying it in the nomenclature of carbohydrates.

Methylation carbohydrate

Methylation is an important technique that has two main applications:

a) Determination of the structure of cycles

A saccharide ring is completely methylated, then the glycosidic bond is hydrolyzed in a dilute acid medium.

The compound is then oxidized using nitric acid. The oxidation breaks the cycle and eliminates the carbons which are not part of the cycle, carbon 6 in the case of a pyranose and carbons 5 and 6 in the case of a furanose.

The remainder of the cycle becomes a tri-O-methylated diacid in the case of a pyranose and a di-O-methylated diacid in the case of a furanose.

Oxidation nitric acid

b) Determination of the sequence in polysaccharides

A glycoside is completely methylated and then glycosidic bonds are cut in dilute acid medium.

The example of amylose shows that the non-reducing terminal compound (hence the compound whose hemiacetalic hydroxyl group is involved in the glycosidic bond but, conversely, whose carbon 4 is not involved in this bond) leads to a tetra-O-methylated derivative while all other elements leads to a tri-O-methylated derivative.

In the case of a branched structure, one obtains a di-O-methylated derivative for each carbohydrate involved in the connection (in this case, the monosaccharide whose carbon 6 is involved in the glycosidic bond).

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IV. Study of several monosaccharides and derivatives

A. Simple carbohydrates

1. Glucose (aldohexose - pyranose)

It is extremely widespread in the plant kingdom and the animal kingdom under a free state or combined with other saccharides, either phosphorylated or not.

This is the "fuel" of the cell (see glycolysis), set aside in the form of glycogen (animal) or starch (plant kingdom).

From a structural view, concerning monosaccharides, it is necessary to pay attention to the position of the hydroxyl group of the penultimate carbon (C5 for a hexose) in the Fischer projection and the position of the resulting C6 in Haworth's representation : in Haworth's representation, C6 indicates either a D or L series since the hydroxyl group of C5 is engaged to form a hemiacetal and therefore does not further indicates the series of carbohydrates.

Visualization of glucose.

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2. Arabinose (aldopentose - pyranose)

Arabinose is abundant in the plant world. It contributes to the formation of the supporting tissues.

From a structural point of view, it is necessary to pay attention to the position of the hydroxyl group of C4.

3. Fructose (ketohexose - furanose)

This ketose is obtained by interconversion of glucose and mannose, e.g. by epimerization of C2 of glucose.

Fructose is a monosaccharide which emphasizes the importance of the linear form: there is always a high concentration of the linear form and there is also a balance with furan forms, e.g., the most stable forms.

4. Galactose and mannose (aldohexose - pyranose)

These two monosaccharides are much less abundant in cells than glucose but they are found as constituents of glycoproteins and glycolipides.

Arabinose fructofuranose galactopyranose

B. Osamines or amino sugars

Osamines are synthesized from fructose-6-phosphate and are obtained by substitution by a NH2 group of the hydroxyl group of carbon 2. The amino group is most often acetylated.

Osamines are very important carbohydrates : for example, chitin , the polysaccharides constituent of insects exoskeletons is a polymer of N-acetylglucosamine containing β-1, 4 bonds.

C. Sialic acids

These are compounds characteristic of glycoproteins. They are linked by a α-glycosidic bond.

The COOH function is free, which imparts a marked acid character to glycoproteins.

N-acetyl-mannosamine-6-phosphate is the precursor of sialic acids.

Sialic acids are all derived from neuraminic acid, whose the most common is N-acetylneuraminic acid.

The substituents vary according to the animal species (N-acetyl in the case of sheep / N-glycolyl in the case of pigs; ...).

Muramic acid

Their synthesis is obtained from N-acetyl-mannosamine-6-phosphate and phosphoenolpyruvate.

In glycoproteins, the sialic acids are arranged at regular intervals along the chain. They thus form a electronegative cloud that, by electrostatic repulsion, keeps the chain in a elongated stick form. The consequence is a high viscosity.

Mannosamine

D. Muramic acids

The N-acetylated muramic acid is a component of murein, a high moleular weigth polymer type glycopeptide which forms the support core of the bacterial walls.

The N-acetylated muramic acid derives from N-acetyl-glucosamine.

It is also biosynthesized from phosphoenolpyruvate.

Glucosamine

E. Phosphorylated sugars

There is formation of phosphoric esters by the action of kinases which transfer the terminal phosphate group of ATP.

Used as an energy source, it is in their phosphorylated forms that carbohydrates are interconverted and therefore metabolized (glycolytic pathway and pentose phosphate pathway, for example).

The ester-phosphate bond is hydrolyzed by phosphatases. Phosphoric esters of glucose and fructose can be considered as products of photosynthetic assimilation.

The α-D-ribose-5-phosphate and 2-deoxy-D-ribose-5-phosphate are the two monosaccharides constituent of nucleic acids.

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V. Study of some saccharides and derivatives

A. Definition

Osides or glycosides are substances in which the hydroxyl group of the hemiacetal anomeric carbon of a saccharide is condensed with a hydroxyl group (alcoholic or phenolic).

The bond between the carbohydrate and the alcohol or phenol function is called O-osidic or glycoside. Osides hydrolysis yield at least two sugars.

B. Determination of the structure of oside

a) Determination of the nature of the constituent monosaccharides

Osides are hydrolyzed enzymatically or in the presence of acid , in order to break the osidic bonds. In the case of glycosides, the nature of the aglycone part must be identified.

Then they are separated by chromatographic techniques and identified and assayed individually.

b) Determination of the binding mode between the constituent monosaccharides

All free hydroxyl functions are marked by methylation and periodic acid. Further acid hydrolysis differ ether-oxides bonds from osidic bonds.

The determination of the osidic bond anomery needs the use of enzymes specific of each type of bond, or in the simplest case, needs the study of the mutarotation after hydrolysis.

c) Determination of the reducing power

Reduction by Na borohydride is used to characterize the terminal reducing monosaccharide in the case of an oligosaccharide or dioside.

In the case of a polysaccharide, the proportion of the terminal reducing sugar is so low that it is necessary to use borohydride marked by tritium.

d) Determination of the molecular weight and chain length in the case of polysaccharides

This is obtained by usual physical techniques (osmometry, ultracentrifugation, light scattering, viscometry, molecular gel filtration , electrophoresis of complexes with boron...), biochemical techniques (specific enzymes for degradation, ...) and immunochemical techniques.

C. Some very important disaccharides

1. Maltose

This diholoside is released by amylose hydrolysis (see below), which is a polymer of glucose residues : maltose is the α-D-glucopyranosyl-(1,4)-D-glucopyranose.

Glucose residues are released by chemical hydrolysis or by an enzyme : the α-D-glucosidase.

It is a reducing sugar since the hydroxyl group of the anomeric carbon of the second glucose residue is free.

Methylation followed by hydrolysis lead to 2,3,6 tri-O-methyl-glucose and 2,3,4,6 tetra-O-methyl-glucose.

2. Lactose

It is the sugar in milk synthesized in the mammary glands. Lactose is the β-D-galactopyranosyl-(1,4)-D-glucopyranose.

This is the only reducer diholoside found naturally.

3. Sucrose

This is an extremely represented sugar in the plant kingdom and especially in sugar cane and sugar beet.

Sucrose and trehalose are the only non-reducer diholoside found naturally (the hydroxyl group of the anomeric carbon of fructose is involved in the osidic bond with glucose).

Sucrose is the α-D-glucopyranosyl-β-D-or β fructofuranoside-α-D-fructofuranosyl-D-glucopyranoside.

Methylation followed by hydrolysis lead to 3,4,6 tri-O-methyl-fructose and 2,3,4,6 tetra-O-methyl-glucose.

4. Cellobiose

Cellobiose results from cellulose degradation.

Cellobiose is the β-D-glucopyranosyl-(1,4)-D-glucopyranose.

This is an epimer of lactose (C4 epimer of the first glucose residue).

cellobiose

D. Two trisaccharides : gentianose and raffinose

These trisaccharides are sucrose derivatives :

  • gentianose : a β-D-glucopyranosyl derivative, so this is the β-D-glucopyranosyl-(1,6)-α-D-glucopyranosyl-(1,2)-β-D-fructofuranoside
  • raffinose : a α-D-galactopyranosyl derivative, so this is the α-D-galactopyranosyl-(1,6)-α-D-glucopyranosyl-(1,2)-β-D-fructofuranoside

E. Heterosides

Example: thiogalactoside used as substrate analogues or inhibitors of enzymatic reactions.

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F. The homopolyosides

1. Starch

This is the reserve polysaccharide of plants. The starch in food and non food industries is mainly produced from cereals (corn, wheat...).

Starch is actually a mixture of two polysaccharides: amylose and amylopectin. The relative proportions of amylose and amylopectin influences the physical properties of starch. Starch is insoluble in cold water, and form a paste (viscous dispersion) when the mixture is heated.

a. Amylose represents 15 to 30% of the weight of the starch. It is a linear polymer of glucose residues (roughly 500 to 20000 residues - MM = 105 to 106 Da) linked by a α-(1,4)-D-osidic bond.

The ramifications [α-(1,6)-D-osidic bond] represent only about 1%.

Amylose form a long chain form as a single left-handed helix :

  • 6-8 glucose residues per helix turn
  • propeller pitch : 10.6 Å
  • internal diameter : 0.45 nm

Chain is stabilized by hydrogen bonds between the hydroxyl groups and water molecules.

amylose

This structure confers two properties to amylose :

  • clathrate manufacturing : physical inclusion of iodine atom (blue-violet colored starch) or lipid inclusion.
  • the retrogradation of starch : the sudden drop in viscosity after starch heating and water expulsion from amylopectin.

b. The amylopectin : it represents 70 to 85% of the weight of the starch. It is a branched polymer :

  • bond between the glucose residues inside a chain : α-(1,4)-D-glucosidic
  • bond between chains : α-(1,6)-D-glucosidic

Several results have identified the arrangement of amylopectin :

  • methylation followed by hydrolysis gives about 5% of 2,3-di-O-méthylglucose for branch points and also about 5% of 2,3,4,6-tetra-O-méthylglucose at the reducing ends .
  • β-amylase, an enzyme capable of digesting amylopectin, hydorlyses about 55% of amylopectin into maltose.

These results and the study of some models have shown that there is an average branch every 25 residues and branches contain twenty residues. There are more connections on the reducing end of the chain. Finally, some branches are themselves branched.

Visualization of the β-amylase of Glycine max at a resolution of 1.90 Å.

PDB: 1BYB

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2. Cellulose

Cellulose is of vegetable origin only. This is a support substance, since it is the main constituent of the cell wall of young plants.

This is the most important biomolecule in term of biomass on the surface of the earth and it contains half the carbon available on earth.

It consists of long linear chains (roughly 300 to 1500 residues - MM = 5 104 to 2.5 106 Da) of glucose residues linked by a β-(1,4)-D-osidic bond.

The repeating unit is cellobiose (see above).

Cellulose

Cellulose is characterized by a high chemical inertness. It is insoluble in water. However it has a hydrophilic character (it is a hydrocolloid) : attaching many water molecules results in the swelling of cellulose.

Cellulose is not attacked by the digestive juices of omnivores : man is unable to digest cellulose because it is devoid of enzymes active on β-glucosidic bonds. Enzymes that degrade cellulose are called cellulases (ruminants, snails and some bacteria).

3. Industrial applications

Go to the website: " Polysaccharides food "- USTL

The food industry uses polysaccharide with thickening and stabilizing properties (e.g., starch, cellulose, hemicellulose, xanthan).

4. glycogen

This is the reserve polysaccharide of animals. The main stock is located in the liver (200 g for an adult) and muscle (100 to 300 g).

The brain is a major user of glucose : 100 mg / min, but it has a limited supply of glycogen (10 to 20 g).

Glycogen is very similar to amylopectin : it is a chain of glucose residues linked by a α-(1,4)-D-osidic bond and connection between chains is a α-(1,6)-D-osidic bond. However, the chains are much shorter and the glycogen molecule is denser.

Glycogen is degraded by amylases like starch.

See a course on regulation of glycogenolysis.

5. Glucan degrading enzymes

a) Amylases that hydrolyse specifically α-1, 4 bonds :

  • β-amylases with an active -SH group are found in the plant world. They hydrolyse a bond to two from the nonreducing end, releasing maltosyl units. β-amylases do not hydrolyse α-1, 6 bonds, and thus they act only on external chains of polysaccharides.
  • α-amylases are metalloenzymes found in animals and plants. They hydrolyse α-1, 4 bond inside chains forming small oligosaccharides (3 to 8 residues) that may contain one or two connection points. α-amylases do not hydrolyse α-1, 6 bonds.

b) Phosphorylases : they hydrolyse chains from the non-reducing ends, by phosphorolysis of α-1, 4 bonds, with the release of α-D-glucose-1-phosphate.

c) Enzymes catalyzing chain disconnection: they hydrolyse α-1, 6 bonds of branch points in different modes according to their origin.

G. Heteropolyosides

1. Mucopolysaccharides

These are heterogeneous compounds that result from the condensation of a high number of disaccharide subunits. This unit consists of :

  • a molecule of hexosamine, sulfated or not
  • a hexuronic acid

Mucopolysaccharides are very acidic molecules. They are always associated with some proteins. However, in the final compound, carbohydrates represent roughly 95%.

The simplest mucopolysaccharide is hyaluronic acid consisting of one molecule of N-acetyl-glucosamine-β (1,4) and one molecule of glucuronic acid.

Its main function, due to the high viscosity it provides to solutions, is to oppose the spread of foreign substances.

2. Glycoproteins

These compounds consist of a carbohydrate and a protein. The carbohydrate part varies from 1 to 50% of the weight of the assembly. The polysaccharide chains are often branched.

There are O-linked polysaccharides, such as galactose bound to the hydroxyl group of hydroxylysine in collagen. However, the amino acids involved are often serine or threonine.

N-linked polysaccharides are covalently joined to the nitrogen of the peptide bond of some asparagines.

The glycosylation is a posttranslational event that occurs only in Eukaryotes. Glycosylated proteins are intended to be secreted or to be incorporated into the plasma membrane.

Determining the structure of the glycoproteins is currently one of the most difficult jobs. Every carbohydrates has several free hydroxyl groups and any of them can establish a bond with another monosaccharide or another compound. Thus, the number of polysaccharides that can be formed is immense. For example, with only three monosaccharides, there are hundreds of configurations.

 

VI. Internet links and references
"Principles of Biochemistry" Horton, Moran, Ochs, Rawn and Scrimgeour (1994) - Ed DeBoeck Universities - ISBN: 2-8041-1578-X
"The Course of Organic Chemistry General C41" (Exercises and corrected) Go to Site
Online University (RUCA realization): "Carbohydrates" Go to Site
"Chapter 25: Carbohydrates" - University of Calgary Go to Site
"Polysaccharides food" (very good educational site with quiz) Go to Site
"LEA Database" Go to Site

 

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