Carbohydrate chemistry

Dr. Herat D. Soni

Assistant professor

Rural medical college

Loni

Definition

Carbohydrates may be defined as

polyhydroxy aldehydes or

ketones or compounds which

produce them on hydrolysis.

Formula = (C.H2O)n

Biomedical Importance

Most abundant dietary source of energy. Brain cells

and RBCs are almost wholly dependent on

carbohydrates as the energy source.

Also serve as storage form of energy –Glycogen.

Carbohydrates are precursors for many organic

compounds (fats, amino acids).

Participate in the structure of cell membrane &

cellular functions (cell growth, adhesion and

fertilization).

Certain carbohydrate derivatives are used as

drugs, like cardiac glycosides / antibiotics.

DM (diabetes mellitus)

Sources

CLASSIFICATION OF CARBOHYDRATE

Classification

1• Monosaccharide

2• Oligosaccharide

3• Polysaccharide

Monosaccharide

Cannot further Hydrolyzed

Oligosaccharide• Oligosaccharides(Greek: oligo-few) contain 2-1O

monosaccharide molecules

• Joined by glycosidic bond

Polysaccharides

Contain more than 10 monosaccharide

units.

Polysaccharides

Homopolysaccharides

Starch

Glycogen

Cellulose

Inulin

Dextrans

Chitin

Heteropolysacchrides

Agar

Mucopolysaccharide

Isomers

Same chemical formula but different

structural formula

Example = Glucose and fructose

C6H12O6

Stereoisomer

Same chemical and structural formula but

differ in spatial configuration.

Asymmetric carbon atom

Asymmetric carbon means that four different groups are attached to the same carbon.

The reference molecule is glyceraldehyde which has a single asymmetric carbon atom.

The number of possible stereoisomer depends on the number of asymmetric carbon atoms by the formula 2

nwhere n is

the number of asymmetric carbon atoms.

Reference Carbon Atom of

Sugars

All monosaccharide can be considered as molecules

derived from glyceraldehyde by successive addition of

carbon atoms. Therefore, penultimate carbon atom is the

reference carbon atom for naming the mirror images

D and L isomerism(Enantiomers)

• D-sugars are naturally occurring sugars and

body can metabolize only D-sugars.

• D-glucose is dextrorotatory. In clinical practice,

it is often called as dextrose

Optical isomerism(d and l)

The presence of asymmetrical carbon atom causes optical activity. When a beam of plane-polarized light is passed through a solution of carbohydrates, it will rotate the light either to right or to left.

Right = dextrorotatory (+) (d)

Left = levorotatory (-) (l)

D-glucose is dextrorotatory but D-fructose is levorotatory

Equimolecular mixture of optical isomers has no net rotation (racemic mixture)

Epimers

When sugars are different from one

another, only in configuration with

regard to a single carbon atom,

other than the reference carbon

atom, they are called Epimers.

Epimers

Anomers

To understand this we first understand

Three Representations of Glucose

Structure

open chain projection formula

Fischer's formula

Haworth’s formula

The 1st carbon, aldehyde group is

condensed with the hydroxyl group of the

5th carbon to form a ring. Ring structure

represents hemi acetal form.

Glucose exists in biological systems not

as a rectangle, but as a pyranose ring.

b-D-glucopyranose is the predominant

form (63%).

D-glucose has two anomers, alpha and

beta varieties.

These anomers are produced by the

spatial configuration with reference to the

first carbon atom in aldoses and second

carbon atom in ketoses.

These carbon atoms are known as

anomeric carbon atoms.

Fischer's formula

Haworth formula

Anomeric carbon atom

Mutarotation

When D glucose is crystallized at

room temperature, and a fresh

solution is prepared, its specific

rotation of polarized light is +112o;

but after 12–18 hours it changes

to +52.5o.

This change in rotation with time

is called mutarotation.

ɑ-D-glucopyranose

Rotation of 112O

ɓ-D-glucopyranose

Rotation of 190

1/3 are alpha type and 2/3rd are beta

variety to get the specific rotation of +52.5o

Mutarotation

DISACCHARIDES

Sucrose

Maltose

Isomaltose

Lactose.

Sucrose

• It is the sweetening agent known as cane

sugar.

• It is present in sugarcane and various fruits.

Hydrolysis of sucrose (optical rotation +66.5°) will produce one molecule of glucose (+52.5°) and one molecule of fructose (–92°).

Therefore, the products will change the dextrorotation to levorotation, or the plane of rotation is inverted.

Equimolecular mixture of glucose and fructose thus formed is called invert sugar.

The enzyme producing hydrolysis of sucrose is called sucrase or invertase.

Honey contains invert sugar.

Invert sugar is sweeter than sucrose.

Lactose

It is the sugar present in milk

Lactose intolerance

Maltose

Isomaltose

REACTIONS OF CARBOHYDRATE

Benedict’s test

Principle

The principle of Benedict's test is that

when reducing sugars are heated in the

presence of an alkali(pH 10.6), they get

converted to powerful reducing

compounds known as enediols.

Enediols reduce the cupric ions (Cu2+)

present in the Benedict's reagent to

cuprous ions (Cu+) which get precipitated

as insoluble red copper oxide.

Detect the presence of glucose in urine

(glucosuria).

It is a standard laboratory test employed

for follow-up of diabetes mellitus in PHC.

Benedict's reagent contains sodium

carbonate, copper sulfate and sodium

citrate

Any sugar with free aldehyde/keto group

will reduce the Benedict's reagent.

Therefore, this is not specific for glucose.

Carbohydrates giving positive

Benedict ’ s test:

Glucose, Fructose, Galactose

Lactose, Maltose

Sucrose ???????

Starches do not react or react very poorly

with Benedict's reagent, due to the

relatively small number of reducing sugar

moieties, which occur only at the ends of

carbohydrate chains.

Non-Carbohydrates giving

positive Benedict ’ s test High concentration of Uric acid and

Ketones

Homogentisic acid (solution turns black

due to black colored oxidized

homogentisic acid)

Vitamin C (even without Boiling)

Certain drugs like aspirin, cephalosporins

Glucose oxidase test

Glucose + O2

Gluconolactone + H2O2

H2O2 + (reduced colourless dye)

(Oxidized colored dye)

Glucose Oxidase

Peroxidase

Reagents for this test are present on a strip

of paper in solid form.

When the paper is wet with urine, the

reagents dissolve in urine on paper and

react with glucose in urine.

The darkness of color can be correlated

with amount of glucose present in urine.

Because Glucose oxidase enzyme can act

only on beta-D Glucose, other reducing

substances do not give this test positive.

Thus, compounds like Vitamin C,

Aspirin utilize H2O2 produced in the

reaction.

Due to lack of H2O2, Peroxidase can

not oxidize dye. Thus, glucose may

not be detected even if present, if

urine contain Vitamin C or Aspirin in

large amount. This phenomenon is

called false negative result.

Osazone Formation

All reducing sugars will form

osazones with excess of phenyl

hydrazine when kept at boiling

temperature.

Glucose, Galactose and Fructose will

produce the same needle-shaped

crystals. Why?

Molisch’s test

All carbohydrates when treated with conc.

sulphuric acid undergo dehydration to

give fufural compounds. These

compounds condense with Alpha-napthol

to form colored compounds.

Molish test is given by sugars with at

least five carbons because it involves

furfural derivatives, which are five carbon

compounds.

Purple ring at junction

Fehling’s test

Same principle as benedicts test

Fehling’s A contains 7% copper

sulphate and Fehling’s B contains

sodium potassium tartarate.

Barfoed’s test

This test is based on the same principle as

Benedict’s test.

But, the test medium is acidic.

In acidic medium (pH 4.6)

monosaccharides react faster than

disaccharide.

Barfoed’s reagent contains copper acetate

in glacial acetic acid.

Scanty Red precipitate at

bottom of tube

Seliwanoff’s test

Seliwanoff’s test is a chemical test

which distinguishes between aldose

and ketose sugars.

Ketohexoses like fructose on

treatment with HCl form 5-

hydroxymethylfurfural, which on

condensation with resorcinol gives a

cherry red complex.

Cherry

red color

Oxidation

The glucuronic acid is used by the body for

conjugation with insoluble molecules to make them

soluble in water for detoxification purpose and also

for synthesis of heteropolysaccharides.

Reduction to Form Alcohols

When treated with reducing agents

hydrogen can reduce sugars. Aldose yields

corresponding alcohol.

Glucose is reduced to sorbitol

mannose to mannitol

fructose becomes sorbitol and mannitol

Galactose is reduced to dulcitol and

ribose to ribitol.

Significance of reduction

Sorbitol, mannitol and dulcitol are

used to identify bacterial colonies.

Mannitol is also used to reduce

intracranial tension by forced

diuresis.

The osmotic effect of sorbitol produces

changes in tissues when they

accumulate in abnormal amounts, e.g.

cataract of lens.

Cataract

Retinopathy

Nephropathy

Neuropathy

Lactulose

• Lactulose is also known as beta-D-

galactopyranosyl-D-fructofuranose.

• Used in constipation

Glycosides The hydroxyl group of anomeric carbon of a

carbohydrate can join with a hydroxyl group

of another carbohydrate or some other

compound to form a glycoside and the bond

so formed is known as glycosidic bond.

eg. R-OH + HO-R’ R-O-R' + H2O

The non-carbohydrate moiety is known as

aglycone –phenol, sterol, glycerol and

methanol.

Glycosidic bond can be N-linked or, O-

linked.

Biomedical importance of

glycosides

Cardiac Glycosides –Digoxin, Digitoxin

◦ Used in cardiac insufficiency.

◦ Stimulate cardiac muscle contraction.

◦ Contain steroids as aglycone

component.

Ouabain –Sodium pump inhibitor.

Streptomycin

◦ Antibiotic

◦ Given in Tuberculosis

Phloridzin

◦ cause renal damage, glycosuria.

◦ Blocks the transport of sugar across the

mucosal cells of small intestine & also

renal tubular epithelium.

Formation of Esters

Esterification of alcoholic groups of

monosaccharides with phosphoric

acid is a common reaction in

metabolism.

Examples :Glucose-6-phosphate, and

Glucose-1-phosphate.

ATP donates the phosphate moiety.

Amino sugars

Amino groups may be substituted for hydroxyl

groups of sugars to give rise to amino sugars

Importance

Amino sugars Found in

Glucosamine Hyaluronic acid, heparin and

blood group substances

Galactosamine Chondroitin sulphate of

cartilage, bone and tendons.

Mannosamine constituent of glycoproteins

N-acetylglucosamine

(GluNac) and N-

acetyl galactosamine

(GalNac)

constituents of

glycoproteins,

Mucopolysaccharide and cell

membrane antigens.

Deoxy Sugars

Oxygen of the hydroxyl group may be removed to

form Deoxy sugars

• 2-deoxy D-ribose is important

part in DNA.

Homopolysaccharides

Starch

Glycogen

Cellulose

Inulin

Dextrans

Chitin

Starch

It is the reserve carbohydrate of plant

kingdom

Sources: Potatoes, cereals (rice,

wheat) and other food grains.

Starch is composed of amylose and

amylopectin.

Amylose is made up of glucose units

with alpha-1,4 glycosidic linkages to

form an unbranched long chain. Water

soluble.

The insoluble part absorbs water and

forms paste like gel; this is called

amylopectin.

Amylopectin is also made up of

glucose units, but is highly branched.

The branching points are made by

alpha-1,6 linkage

Iodine test for starch

Starch will form a blue colored

complex with iodine; this color

disappears on heating and reappears

when cooled. This is a sensitive test

for starch.

Starch is nonreducing because the

free sugar groups are negligible in

number.

Hydrolysis of starch

Amylodextrin = violet color with

iodine and is non-reducing.

Erythrodextrin = red color with

iodine and mildly reduce the

Benedict's solution.

Achrodextrins = no color with

iodine, reducing)

Maltose = (no color with iodine, but

powerfully reducing)

Short

time to

long time

Glycogen

It is the reserve carbohydrate in animals.

It is stored in liver and muscle.

Liver glycogen stores increase during the

well-fed state , and are depleted during a

fast.

Glycogen is composed of glucose units

joined by alpha-1,4 links in straight

chains. It also has alpha-1,6 glycosidic

linkages at the branching points.

Glycogen is more branched and more

compact than amylopectin.

Liver and muscle glycogen

Cellulose

It is made up of glucose units combined

with beta-1,4 linkages.

It has a straight line structure, with no

branching points.

Beta-1,4 bridges are hydrolyzed by the

enzyme cellobiase. But this enzyme is

absent in animal and human digestive

system, and hence cellulose cannot be

digested.

Importance

Fiber can absorb 10–15 times its own

weight in water, drawing fluid into

the lumen of the intestine

Increasing bowel motility

1.Decrease the risk for constipation

It is a major constituent of fiber, the

nondigestable carbohydrate.

Intestine

Bile salt Fibers

Excreted

Cholesterol

Decreases

serum

cholesterol

level

.

2. Lower LDL cholesterol levels

Delays gastric emptying and can result in a

sensation of fullness

4. Reduced peaks of blood glucose

following a meal

Can bind various toxic substances

including carcinogens & eliminate them in

faecal matter

3.Decreases chances of some cancers

Inulin

It is a long chain homoglycan composed

of D-fructose units with repeating beta-1,2

linkages.

It is the reserve carbohydrate present in

various bulbs and tubers, such as onion,

garlic.

It is clinically used to find renal

clearance value and glomerular

filtration rate.

Dextrans

These are highly branched homopolymers

of glucose units with 1-6, 1-4 and 1-3

linkages. They are produced by micro-

organisms.

Since they will not easily go out of

vascular compartment, they are used for

intravenous infusion as plasma volume

expander for treatment of hypovolemic

shock.

Dextrose, Dextrin and Dextran

are different

D-glucose is otherwise called Dextrose, a

term often used in bed-side medicine, e.g.

dextrose drip.

Dextrin is the partially digested product of

starch.

Dextran is high molecular weight

carbohydrate, synthesized by bacteria.

Chitin

It is present in exoskeletons of

insects.

It is composed of units of N-

acetylglucosamine with beta-1,4

glycosidic linkages.

Heteropolysaccharides

• Agar

• Mucopolysaccharide

Agar

Agar = The linear polysaccharide Agarose

+ agaropectin

It is dissolved in water at 100ºC, which

upon cooling sets into a gel. Agarose is

used as matrix for electrophoresis.

Agar cannot be digested by bacteria and

hence used widely as a supporting agent to

culture bacterial colonies.

Mucopolysaccharide

i.e. glycosaminoglycans(GAGs)

[acidic sugar–amino sugar]n.

Because of their large number of negative

charges, these heteropolysaccharide chains

tend to be extended in solution. They repel

each other, and are surrounded by a shell

of water molecules. When brought

together, they “slip” past each other.

This produces the “slippery” consistency

of mucous secretions and synovial fluid.

This property contributes to

the resilience of synovial fluid

and the vitreous humor of the

eye

GAGs Composition Tissue distribution Functions

Hyaluronic

acid

D-glucuronic acid

and N-acetyl D-

glucosamine

Connective tissue

Synovial fluid

Vitreous humor

Gel around ovum

lubricant and shock

absorbant in joints

Chondroitin

sulphate

D-glucuronic acid and N-

acetyl D-galactosamine

4-sulfate.

bone, cartilage,

Tendons,heart

valves and skin.

Helps to maintain

the structure And

shapes of tissues

Dermatan

sulfate

D-Iduronic acid and

N-acetyl D-galactosamine

4 –sulfate.

Skin Helps to maintain

shapes of tissues

Keratan

sulphate

galactose and N-acetyl

glucosamine

cornea

tendons

Keeps cornea

Transparent

Heparin sulphated glucosamine

and glucuronic acid or

iduronic acid

blood, lung, liver

,kidney, spleen

Anticoagulant

Clearing factor

Hyaluronic acid

N-Acetyl-glucosamine → beta-1, 4-

Glucuronic acid → beta-1-3-N-Acetyl

glucosamine and so on.

Hyaluronidase

Breaks b(1-4 linkages) in hyaluronic acid.

Present in high concentration in testes,

seminal fluid, and in certain snake and insect

venoms.

Hyaluronidase of semen clears the gel

(hyaluronic acid) around the ovum allowing a

better penetration of sperm into the ovum.

Serves important role in fertilization

Hyaluronidase of bacteria helps their invasion

into the animal tissues.

Chondroitin sulphate

glucuronic acid → beta-1,3-N-acetyl

galactosamine sulfate → beta-1, 4

and so on

Dermatan sulfateDermatan sulfate

L-iduronic acid and N-acetylgalactosamine

in beta-1, 3 linkages

Keratan sulphate

Only GAG not having Uronic acid.

Heparin

It contains repeating units of sulphated

glucosamine → alpha-1, 4-L-iduronic acid

or glucuronic acid → and so on

Heparin is an anticoagulant( prevents

blood clotting).

Heparin helps in the release of the enzyme

lipoprotein lipase which helps in clearing

the turbidity of lipemic plasma.

Lipoprotein lipase breaks TG in glycerol

and FFA.

HSEN

Heparan

sulphate

Capillary

On capillary

endothelial

wall surface

Lipoprotein

lipase

Heparin displaces

lipoprotein lipase from

heparan sulphate binding

site hence clearing factor

Proteoglycans and

Glycoproteins

Proteoglycans: When carbohydrate

chains are attached to a polypeptide chain.

Glycoproteins: Carbohydrate content ≤

10%.

Proteoglycans

Figure showing Proteoglycans aggregate

Glycoprotein Major function

Glycophorin glycoprotein of

erythrocytes cell

membrane

Collagen Structure of cartilage

and bone

Ceruloplasmin Transport protein

Immunoglobulin Defense against

infection

Intrinsic factor Absorption of vitamin

B12

Fibrinogen Blood clotting

Thank

you

Any questions?