Carbohydrate chemistry

Dr.S.Sethupathy,M.D.,Ph.D.,Division of Biochemistry,

Rajah Muthiah Medical College,Annamalai University

Carbohydrates are polyhydroxy aldehydes (-CHO) or ketones(-C-CO-C-) or compounds that yield them on hydrolysis.

Glucose- aldohexoseFructose-ketohexose

Definition

Reactive groups of carbohydrates

Ring structure represents Hemi acetal form which is the condensation of an aldehyde or keto with a hydroxyl group.

Glucose exists as a six membered pyranose ring.

Fructose exists as five membered furanose ring structure.

Ring form

Glucose- Aldohexose

Compounds having same structural formula but different spatial configuration and it is due to asymmetric carbon atom.

Asymmetric carbon (Chiral carbon ) means four different groups are attached to that carbon.

The possible stereoisomers depend on the number of asymmetric carbons

The formula 2n where n is the number of asymmetric carbons present in the molecule.

Stereoisomers

D,L- isomers

Due to the presence of asymmetric carbon, when plane polarized light is passed through the sugar solution, it will be either rotated to right or left.

If it is rotated to right, it is called dextrorotatory (+) and if to left, it is levorotatory(-).

Racemic mixtures contain equal amounts of (+) and (-) isomers and so NO rotation .

Optical Isomers

Enantiomers are chiral molecules that are mirror images of one another. An example of an enantiomer is the D and L isomers of glucose.

Diastereomers are stereoisomers are not mirror images of one another and are non-superimposable on one another. Stereoisomers with two or more stereocenters can be diastereomers. Eg: D-Glucose and D-Altrose.

Epimers: Another type of isomer , carbohydrates can take on are epimers.

Epimers are diastereomers that differ in configuration of only one stereogenic center. ie : two diastereomers that differ only at one stereocenter.

EPIMERS

When a monosaccharide gets cyclized from chain structure , its carbon 1 becomes asymmetric and forms α and β anomers.

Eg α ,D-Glucose and β D-glucose. If hydroxyl group is below the plane of the

ring, it is α , D-glucose and if it is above, it is β anomer.

ANOMERS

Mutarotation : It is the change in the specific optical rotation due to inter conversion of α, β anomers.

α ,D-glucose – +α112 0 – β ,D glucose + 190 At equilibrium + 52.5 0 when α and β anomers will be in the ratio of approximately 1:2.

Mutarotation

Sugars due to their free aldehyde or keto group reduce cupric ions into cuprous ions and give red precipitate.

Sucrose is non reducing sugar due to absence of free aldehyde or keto group.

Starch is also a non reducing sugar due to insufficient number of reducing groups.

Molish test is the general test for carbohydrates. Sugars when treated with concentrated .sulfuric acid, they get dehydrated to form furfural derivatives which on condensation with phenolic compounds give colored products.

Test for reducing sugars (Benedicts test)

All reducing sugars will form osazone with excess of phenyl hydrazine on boiling.

Osazones are insoluble and different sugars give characteristic form of crystals. Because it involves Carbon 1 and 2 of glucose, fructose and mannose form needle shaped crystals as the difference is masked.

Maltaosazone will give sunflower shaped crystals and lactosazone will give hedge-hog shaped or badminton ball shaped crystals.

Osazone formation

Osazone crystals of Glucose - Maltose- Lactose

Sugar acids: Glucose on mild oxidation by Bromine water, the aldehyde group is oxidized to form aldonic acid.

In system, aldehyde group is protected and the last hydroxyl group is oxidized to form glucuronic acid.

Under strong oxidation, both aldehyde and last hydroxyl groups are oxidized to form dicarboxylic saccharic acids.

Sugar acids

Sugar acids

On reduction of aldehyde group of glucose , it forms sorbitol.

In diabetes mellitus, sorbitol accumulates in the lens causing early cataract.

Fructose on reduction can form both sorbitol and mannitol.

Mannose can form mannitol. Mannitol is an osmotic diuretic and used to reduce

intracranial tension. Galactose can form galactitol or dulcitol and its

accumulation in the lens causes early development of cataract in galactosemia condition.

Sugar alcohols

Sugar alcohols

Hydroxyl groups of sugars (Usually the second carbon of hexoses) are substituted with amino group to give amino sugars.

Amino sugars do not show reducing property and do not form osazones also.

Glucosamine is present in hyaluronic acid, heparin and blood group substances.

Galactosamine is present in chondroitin sulfate of cartilage , bone and tendons.

Mannosamine is present in glycoproteins.

Amino sugars

Removal of oxygen from hydroxyl group results in the formation of deoxy sugar.

2-Deoxy ribose is formed by removal of oxygen from hydroxyl of second carbon in ribose

It is the sugar present in Deoxy ribo nucleic acid (DNA) .

Feulgen staining using Schiff’s reagent is specific for 2-Deoxy ribose (DNA) in tissues.

Deoxy sugars

Ribose is present in RNA ,ATP ,NAD and 2-deoxy ribose is present in DNA.

Ribulose is an intermediate in HMP shunt pathway.

Xylulose is an intermediate in uronic acid pathway.

Pentoses

Monosaccharides Further cannot be broken in to simpler

sugars Eg: glucose, ribose, fructose, xylulose Oligosaccharides- 2- 10 monosccharides on

hydrolysis 2- disaccharides Maltose- glucose + glucose Lactose- glucose + galactose Sucrose- glucose + fructose

Classification of carbohydrates

Homopolysccharides – single type of monosccharides- Starch, glycogen, cellulose- glucose

Heteropolysaccharides- more than one type of monosaccharides

Heparin. Chondroitin SO4, dermatan SO4, Hyaluronic acid- Mucopolysccharides

Polysaccharides

It is cane sugar. It is a non reducing sugar Aldehyde glucose and keto group of fructose are involved

in the linkage and are not free to reduce the Benedict’s reagent.

But its hydolysate products reduce the reagent. Hydrolysis of sucrose : The products of hydrolysis have

reducing property. The optical rotation of sucrose is +66.5 and after hydrolysis

it changes to – 39. This change is called inversion and is due to the fact that on hydrolysis, it yields glucose (+52.5 ) and fructose (- 92) which is greater than glucose.

The hydrolysate is called invert sugar and is sweeter than sucrose. Honey contains invert sugar.

Sucrose

Lactose: It is the milk sugar composed of glucose and galactose in beta glycosidic linkage. It is a reducing disaccharide.

Maltose and iso maltose

Maltose is composed of two glucose molecules linked by alpha-1,4linkage. It is a reducing disaccharide. It forms sunflower shaped osazone crystals.

Isomaltose is composed of two glucose molecules linked by alpha-1,6 linkage.

Intestinal enzyme isomaltase hydrolyzes isomaltose. It is a reducing sugar.

Starch is of plant origin, composed of amylase and amylopectin.

Amylose is unbranched and contains only α1,4 linkage. Amylopectin has both α -1, 4 and α -1, 6 linkages.

Starch forms blue colored complex with iodine. It is non reducing .

Amylase enzyme acts on starch and finally hydrolyzes them to maltose and isomaltose.

Starch

Inulin is a polyfructosan in beta-1,2 linkage and it is used for Renal Glomerular filtration rate assessment .

Cellulose contains β 1, 4 linkage and so it is not acted upon by amylase .

It is hydrolyzed by cellulase enzyme. It is absent in human digestive system. So it is indigestible and forms roughage in the feces.

Cellulose

Dextrans are branched homoglycans of glucose units with α 1,4, α 1,6, α 1, 3 linkages , produced by microorganism

It is used as plasma volume expander to treat hypovolemic shock.

It is composed of (beta 1,4 linkage)D-glucuronic acid and (beta -1,3 linkage) N-acetyl-D-glucosamine repeating units.

It does not contain sulfate. It is present in connective tissue, synovial

fluid and vitreous humor of the eye. It serves as a lubricant in joint cavities.

Hyaluronic acid (HA) :

It contains repeating units of D-Iduronate-2 SO4 and D-glucosamine-6-SO4 in alph-1,4 linkages.

It is present in liver, spleen, lungs and monocytes.

It activates antithrombin III and inhibits blood clotting.

Heparin

It is present in ground substance of connective tissues

it is composed of (beta -1,4 Linkage) D-glucuronic acid and beta-1,3 linkage N-acetyl galactosamine SO4 repeating units.

It is widely distributed in cartilage, bone, tendons, cornea and skin.

Chondroitin sulfate(C4S & C6S)

Dermatan sulfate(DS) : It contains repeating units of L- iduronic acid and N acetyl galactosamine SO4 in beta-1,3 linkages.

It is found in skin, blood vessels and heart values.

Heparan sulfate(HS) : It contains repeating units of D-glucuronic acid sulfate or Iduronic acid sulfate and N-acetyl glucosamine sulfate.

It has more amount of N-acetyl glucosamine residues than heparin.

It is present in the basement membrane of renal glomerulus and cell surfaces.

It helps in glomerular filtration and cell to cell communications.

Keratan Sulfate (KS): It is composed of repeating units of D-galactose and N-acetyl galactosamine sulfate. It does not contain uronic acid. It is present in cornea and tendons as a structural component.

Glycoproteins and mucoproteins : If the carbohydrate content is less than 10% , it is called as glycoprotein and if carbohydrate content is more than 10% ,it is called as mucoprotein.

In glycoproteins, protein component is predominant

The oligosaccharide chains are covalently bound to proteins which may be either O- linked or N-linked.

Eg: Immunoglobulins, blood group substances, Insulin receptor etc.

They are also involved in cell to cell recognition.

Proteoglycans: They contain glycosaminoglycans as a major component with lesser protein.

It occurs in extracellular matrix of connective tissue , synovial fluid of joints, cartilage ,mucous secretion.

Peptidoglycans: They are complex of polysaccharides chains and polypeptides in bacterial cell wall.

Penicillin , an antibiotic inhibits the peptidoglycan cross linking and kills the bacteria.

β , L- fucose is 6-deoxy β ,L - galactose. It is the fundamental subunit of the

fucoidan polysaccharide. α(1→3) linked to core .

Fucose is a suspected carbohydrate antigen for IgE-mediated allergy.

L-Fucose is claimed to have application in cosmetics, pharmaceuticals, and dietary supplements.

Fucose

They are the indigestible portion of plant foods that move food through the digestive system, absorbing water and easing defecation.

Insoluble fibers Make the feces bulk, softer and shorter the transit

time. Sources: Whole wheat, tomato peel, green beans Soluble fibers They are the edible carbohydrates resistant to

digestion and absorption in the human intestine which undergoes complete or partial fermentation in the large intestine .

Dietary Fiber

Sources: skin, pulp, seeds, stems leaves, roots legumes, onion, carrots, potatoes, oats etc.

On fermentation, butyric acid is formed which increases insulin release , decreases cholesterol synthesis, Low density lipoprotein and triglycerides levels, increases mineral absorption and improves immunity.

Adult : Fiber intake 20 – 35 gm/day . In children, the requirement is = Children age in years +5 gm/day

A sugar substitute is a food additive that gives the taste of sugar with less or no calories and they may be natural or synthetic compounds.

Those that are synthesized are called artificial sweeteners which are chemicals that give the sweetness of sugar with no or less calories and usually are sweeter than sugar.

Eg. Aspartame is heat unstable and is a methyl ester of the aspartic acid/phenylalanine dipeptide.

Saccharin, sucralose (chlorinated sucrose), sorbitol are also sugar substitutes.

Sugar substitutes

Mucopolysaccharidosis : They are inborn errors characterized by accumulation glycosaminoglycans (mucopolysaccharides) in various tissues causing skeletal and extracellular matrix deformities due to inherited enzyme defects.

In MPS disorders, Skeletal changes, mental retardation, visceral involvement and corneal clouding are manifested due to particular MPS deposits and its excretion in urine.

That can be detected in urine by Cetyl Trimethyl Ammonim Bromide (CTAB) test or O-tolidine blue test.

Mucopolysaccharidosis

Mucopolysaccharidosis- Hurler syndrome

Glycome and Glycomics Glycome is the total complement of all the

carbohydrates in an organism. Glycomics is the study of structure and function of glycome.

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