PPT Carbohydrates

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CARBOHYDRATE

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CARBOHYDRATE

•

• Carbohydrates are essential to all living organisms and are Carbohydrates are essential to all living organisms and are thethe

most abundant class of biological molecules. most abundant class of biological molecules.

•

• The metabolic breakdown of monosaccharides provides most of The metabolic breakdown of monosaccharides provides most of

the energy used to power biological processes. the energy used to power biological processes.

•

• Chemically, they are polyhydroxyl aldehydes or ketones.Chemically, they are polyhydroxyl aldehydes or ketones. •

• Contain three elements - C, H, O, many accordContain three elements - C, H, O, many according to theing to the

formula (CH

formula (CH₂₂O)O)_n_n; ; where where nn

≥

3.3.

•

• Greek Greek saccharon saccharon means sugar means sugar •

• Carbohydrates function:Carbohydrates function:



Energy sources (glucose/glycogen)Energy sources (glucose/glycogen)



Structural elements:Structural elements:

–

– cell wall (plants, bacteria)cell wall (plants, bacteria) –

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Monosaccharides

•

• Monosaccharide is another term for a simple Monosaccharide is another term for a simple sugar, which is not sugar, which is not linked to any other sugars.

linked to any other sugars. •

• Common monosaccharides include: glucose, mannose, fructose,Common monosaccharides include: glucose, mannose, fructose, ribose, and galactose

ribose, and galactose •

• Monosaccharides can be classified asMonosaccharides can be classified as or

or depending upon the number of carbon atoms.depending upon the number of carbon atoms. trioses, tetroses, pentoses,

trioses, tetroses, pentoses, hexoses,

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Some Examples of Monosaccharides

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•

• It is commonly found in fruits.It is commonly found in fruits. •

• It is known asIt is known as dextrose dextrose , a name that derives, a name that derives from the fact that the predominant natural from the fact that the predominant natural form of the sugar is dextrorotatory.

form of the sugar is dextrorotatory. •

• It is made by the It is made by the hydrolysis of starch.hydrolysis of starch. •

• It is only 75% as sweet as sucrose.It is only 75% as sweet as sucrose.

•

• It is found in fruits and honey.It is found in fruits and honey. •

• It also referred to asIt also referred to as levulose levulose because it has anbecause it has an optical rotation that is strongly levorotary (- 92 optical rotation that is strongly levorotary (- 9200₎₎

•

• It exists in two forms, pyranose (free state) or It exists in two forms, pyranose (free state) or furanose (combined form) .

furanose (combined form) . •

• It is the sweetest sugar.It is the sweetest sugar.

Glucose Glucose

Fructose Fructose

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• It is more commonly found in the disaccharide, lactose or milk sugar. • It does not occur in nature in the

uncombined state.

• It is formed by the hydrolysis of lactose. • It is needed by the human body for the

synthesis of lactose (in the mammary

glands) is obtained by the conversion of D-glucose into D-galactose.

• It is an important constituent of the

glycolipids that occur in the brain and the myelin sheath of nerve cells.

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•

Monosaccharides can be classified as as

or

depending upon they have an aldehyde or ketone group.

Example:

Glyceraldehyde and dihydroxyacetone have the

same

atomic

composition,

but differ only

in the position of

the hydrogens and double bonds.

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Aldose-ketose intercoversion via an enediol intermediate

When the structures of molecules are related in those ways,

the molecules are called

tautomers.

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Fischer Projection of Monosaccharide

•

The structure of monosaccharide can be drawn by the

Fischer projection formula. The carbon chain was composed

vertical and the aldehyde carbon at the top.

•

If the position of hydroxyl group of the highest numbered

stereogenic center was at:



Right

_

D

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D- and L-isomerism of glyceraldehyde and glucose

O H The OH group is on the right The OH group is on the left L - s u g a r s a r e m u c h l es s a b u n d a n t in n a t u r e .

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Haworth Projection of

Monosaccharide

• Haworth projection is a conventional planar representation of a cyclized monosaccharide molecule. It is useful to show whether the OH

groups on the ring are cis or trans to each other.

• The OH groups that are represented on the right in a Fischer projection are down in a Haworth projection, and vice versa .

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Epimers

•

Epimers are diastereomers that contain more than one

chiral center but differ from each other in the absolute

configuration at only one chiral center.

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•

For a carbon with 'm' chiral carbons, the number of possible

stereoisomers is

Example: An aldohexose (D-glucose) has 4 chiral carbons,

thus it has 16 stereoisomers.

How many are the

of D-glucose? How many are the

of D-glucose?

.

enantiomers

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•

Naturally occuring ketoses have the ketone group in the

2-position. A ketose has

one fewer

chiral carbon than does an

aldose with the same number of carbon atom.

•

Therefore,

with the same number of carbon atoms.

Example: A ketohexose (D-fructose)

has 3 chiral carbons, thus it

has 8 stereoisomers.

How many are the of D-fructose? How many are the of D-fructose?

a ketose has only half as many stereoisomers as

an aldose

enantiomers

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Monosaccharide Ring Structures

Alcohol groups can react with the aldehydes or ketones to form hemiacetals and hemiketals.

A new chiral center is born!

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Formation of the two cyclic forms of D-glucose

Sugars with

≥

5 C mostly exist in their cyclized form

(intramolecular hemiacetal formation)

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Monosaccharide can form the cyclic form as:



=> five-membered ring



=> six-membered ring

Furanose form

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The Chair and Boat Conformation of Monosaccharide

Both conformations (chair and boat) exist, though the is thermodynamically more stable .

chair form

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Mutarotation

•

Mutarotation is a process whereby the configuration of an

anomeric carbon converts from

α

to

β

and vice-versa (the

gradual change of optical rotation), which continues until

equilibrium is established.

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•

The reducing end: the sugar with the free anomeric carbon that can be oxidized.

•

Oxidation occurs only with the linear form.

•

Maltose and lactose are reducing sugars, while sucrose is not.

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•

Monosaccharides act as reducing agents, because the aldehyde group that is present can be oxidized by oxidizing agents such as Fe3+ or Cu2+ ions to form a carboxylic acid group, or in the

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Disaccharides

•

Disaccharides are compounds consisting of two

monosaccharide subunits linked together by an glycosidic

linkage.

•

Disaccharides are taken apart by hydrolysis and put together

by condensation

•

Common disaccharides include:



Sucrose => glucose + fructose



Maltose => glucose + glucose



Lactose => glucose + galactose

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Glu( 1 4)Glu

Disaccharides arise through the formation of O-glycosidic bonds: condensation of anomeric carbon hydroxyl group with an alcohol Glycosidic Bonds

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Some Examples of Disaccharides

• It is known as beet sugar, cane sugar, tablet sugar, or simply as sugar. • It is incapable of mutarotation and said to be a nonreducing sugar .

This is because of the presence of the 1,2-glycosidic linkage makes it impossible for it to exist in the

α

- or

β

- configuration or in the open-chain form.

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• It occurs in animals as the principal sugar formed by the enzymic (ptyalin) hydrolysis of starch.

• It is a reducing sugar, and it exhibits mutarotation. This is because of the presence of the 1,4-glycosidic linkage makes it possible for it to exist in the

α

- or

β

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• It is a stereoisomers of maltose. It is also composed of two glucose units, but in this case the two sugar moieties are joined by

β

-1,4-glycosidic linkage.

• It is a reducing sugar, and it undergoes mutarotation. This is because of the presence of the 1,4-glycosidic linkage makes it possible for it to exist in the

α

- or

β

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•

It known as milk sugar because it occurs in the milk of humans,

cows, and other mammals.

•

It is a reducing sugar, and it exhibits mutarotation.

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Polysaccharides

• The polysaccharides are the most abundant of the

carbohydrates found in nature. • They serve as reserve food

substances and as structural components of plant cell. • They are high molar mass

(25,000-15,000,000) polymers of monosaccharides joined

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• There are two types of polysaccharides:



polysaccharide are the same. • Storage:

starch (storage in plants) and glycogen (storage in animals). • Structural elements:

cellulose (plants cell wall) and chitin (animal exoskeleton).



units of the polysaccharide are different.

ex: hyaluronic acid, keratan sulfate and agarase

Homopolysaccharides: when all the monosaccharide units of a

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Some Examples of Homopolysaccharides

• It is a polymer of glucose used

for energy storage in plants such as potatoes, rice, beans, and corn.

• Starch is a mixture of two

polymers, amylose (10-30%) and amylopectin (70-90%).

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amylopectin

occurs every 24 to 30 residues

Amylose is a straight-chain polysaccharide composed entirely of D-glucose units join by an

α

Amylopectin is a

branched-chain polysaccharide composed of glucose units join by

α

-1,4-glycosidic linkage with

α

amylose

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•It is specially abundant in the liver, 4-8% (per weight of tissue), and

in muscle cell, 0.5-1.0%

•It is quite similar to amylopectin, but it is more highly branched and

its branches are shorter (8-12 glucose units in length).

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•It is the major structural component of the shells of crustaceans

(e.g., lobsters, crabs, and shrimps) and the exoskeletons of insects.

•It is structurally similar to cellulose, but in that it has an

N -acetylamino group instead of an OH group at the C-2 position.

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•It is the major structural polysaccharide in

woody and fibrous plants and is the most abundant single polymer in the biosphere.

•Many microorganisms and herbivorous

animals can digest cellulose because their digestive tracts contain enzymes (cellulase ) that can hydrolyze

_

(1

_

4) linkage.

Cellulose breakdown by wood fungi

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A comparison between starch and cellulose

starch

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Some Examples of Heteropolysaccharides

•

It is a type of polysaccharide called a glycosaminoglycan. Also

known as hyaluronan or hyaluronate.

•

It occurs naturally in the human body and is central to

regulating cell growth and renewal. In fact, it is found

extensively in connective, epithelial, and neural cells.

Hyaluronic Acid

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•It is any of several sulfated glycosaminoglycans that have been

found especially in the cornea, cartilage, and bone.

•It is also synthesized in the central nervous system where it

participates both in development and in the glial scar formation following an injury.

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•It is an enzyme found in agarolytic bacteria and is the first

enzyme in the agar catabolic pathway

• Agarases are classified as either

α

-agarases or

β

-agarases based

upon whether they degrade

α

or

β

linkages in agarose, breaking them into oligosaccharide.

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Derivatives of Monosaccharides

• Monosaccharides can be chemically altered in several ways, in

which a hydroxyl group in the parent compound is replaced with another substituent (–NH-(C=O)-CH₃, –NH₂,

‒

(C=O)H



‒

COO-, –O-H



–O-PO₃2- ) or a carbon atom is oxidized

to a carboxyl group to provide new classes of compounds.

• Derivatives of monosaccharides include:

 Alditols, ex: erythritol, sorbitol, and D-mannitol

 Amino sugars, ex:

β

-D-glucosamine, N-acetyl-

β

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Some Examples of Derivatives of Monosaccharides

•

They are made by reducing the carbonyl group of a

sugar to a hydroxyl.

•

They are used in chewable tablets as sweetening agents

to mask the unpleasant taste of vitamins and minerals and to improve texture.

•

These natural sweeteners are extracted and purified from plant sources, particularly from fruits.

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It is made by reducing a hydroxyl of sugar with an amine group.

Amino sugars

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