CARBOHYDRATES

Dr. Vidya.DAsst. Professor,

College of Pharmacy,

Prince Sattam Bin Abdul Aziz University,

Kingdom of Saudi Arabia

PHL – 213

Biochemistry - I

Objectives

Identify the functions of carbohydrates.Name the primary sources of carbohydrates.Describe the classification of carbohydrates.

Facts

Primary source of energy for the bodyLeast expensive and most abundant of the energy nutrientsNamed for the chemical elements they are composed of—carbon, hydrogen, and oxygen

Functions

Provide energyProtein-sparing actionNormal fat metabolismProvide fiber

Providing Energy

Each gram of carbohydrate provides 4 calories.

A body needs a constant energy supply.A half day’s supply of carbohydrates is stored

in the liver and muscles for use as needed.Stored form is called glycogen.

Protein-Sparing Action

The primary function of proteins is to build and repair tissues.When enough carbohydrates (at least 50–100 g/day) are ingested, proteins are spared to be used for their primary function.

Normal Fat Metabolism

Without an adequate supply of carbohydrates, fat is not metabolized to meet energy requirements.Ketones are produced as a byproduct of fat metabolism.Ketosis may result.

Providing Fiber

Dietary fiber is found in grains, vegetables, and fruits.

Recommended intake is 20–35 g/day.Fiber lowers blood glucose levels; may

prevent some colon cancers; and helps prevent constipation, hemorrhoids, and diverticular disease by softening stool.

Food Sources

Principal sources of carbohydrates are plant foods:

e.g. Cereal grains, Vegetables, Fruits, Nuts, Sugars

The only substantial animal source is milk.

Aldehyde groupH-C=O

Monosaccharides

Enantiomers

if they are

Mirror images of each other

can link to form

Disaccharides

e.g.,sucrose = glucose + fructoseLactose = galactose + glucoseMaltose = glucose + glucose

Oligosaccharides Polysaccharides

can be

Homo-e.g.,

Starch, glycogen,cellulose

Hetero-e.g.,

GAGs

Epimers

Differ in configuration around one specific carbon atom

Isomers

if they contain

Same chemical formula

Ketoses

Keto groupC=O

Can be classified as

if theyif they containif they contain

Aldoses

Carb

oh

ydra

tes

Sim

ple

(or

) small

sug

ars

Com

ple

x (or )

larg

e su

gars

Monosaccharides

Sweet In tasteNot hydrolysableHave three to seven carbons3 carbons=Triose4 carbons=Tetrose5 carbons=Pentose6 carbons=Hexose7 carbons=Heptose

Structure of monosaccharide

Fisher projection• The straight

chain structural formula

Haworth projection• Cyclic

formula or ring structure

X-ray diffraction analysis• Boat and

chair form

Straight chain

Ring structure

Chair form

Isomerism

The compounds possessing identical molecular formula but different structures are called isomers.

Various types of isomerism1. Structural isomerism2. Stereoisomerism

Structural isomerism

Same molecular formulae but differ from each other by having different structures.

Stereoisomerism

Same molecular formula and same structure but they differ in configuration.

That is arrangement of their atoms in space.

Presence of asymmetric carbon atoms allow the formation of stereoisomerism

Stereoisomerism

The important types of stereoisomerism associated with glucose are

D and L isomerism

Optical isomerism

Epimerism

α and βanomerism

D and L isomerism

Optical isomerism

Optical activity is the capacity of a substance to rotate the plane polarized light passing through it.

Clockwise direction

• Dextrorotatory(d) or (+)

Counterclockwise direction

• Levorotatory(l)or (-)

Optical isomerism

Chiral compounds rotate polarized light clockwise or counter clockwise through certain angle

Epimerism

Epimerism is the stereoisomerism if two monosaccharides differ from each other in their configuration around a single specific carbon(other than anomeric) atom.

Epimerism

Anomerism

These are isomers obtained from the change of position of hydroxyl group attached to the anomeric carbon e.g. and glucose are 2 anomers.

Also and fructose are 2 anomers.

Anomerism

Mutarotaion is defined as the change in the specific optical rotation by the interconversion of α and β forms of D glucose to an equilibrium mixture

Mutarotaion

Types:1. Aldoses2. KetosesAldoses contain Aldehydic group –CHOKetoses contain Ketonic group –CO-

Name of the sugar

Role IN the body Example

Triose Important in cellular respiration, in the glycolysis step.

D-glyceraldehydeL- glyceraldehyeDihydroxyacetone

Pentoses They form the backbone of polysaccharides, Proteins, Lipids & nucleic acids

RiboseRibulose

Hexoses Glucose - Primary energy molecule Fructose - energy molecule in semenGalactose - dairy products, sugar beets, gums and mucilage

Mannose - it forms part of glycolipids & glycoproteins in several tissues.

Glucose, FructoseGalactose, MannoseTalose, Allose, Idose

Derivatives of monosaccharides 1) Sugar phosphates

Metabolized as phosphate esters2) Deoxy sugars

Hydrogen atoms replaces -OH group on C-2.Important to structure of nucleic acids.

3) Amino sugarsAmino group (NH-) substituted for -OH group in monosaccharide.

4) Sugar alcoholsReplace carbonyl oxygen to form polyhydroxy alcoholse.g. glycerol --> glyceraldehydeReplace “-ose” with “-itol”.Ribose --> ribitol

5) Sugar acidsOxidation of carbonyl carbon or highest carbon.glucose --> gluconate or glucuronateImportant in many polysaccharides.

6) Ascorbic acidDerived from D-glucuronate.Primates cannot do the conversion, so must be supplied in the diet.

Glucose

Polyhydroxy aldehydeDextrose=DextrorotatoryGrape sugarBlood Sugar - 110mg/1000mL in bloodEnergy source for the bodyCombines with others to form disaccharides

Importance of Glucose

Most widely used HexoseAn energy sourceA precursor forms: Cellulose, Glycogen,

Starch etc.Hypoglycemia and Hyperglycemia

Importance of Fructose

Found in foods and Drinks1 to 2 times sweeter than table sugarUsed as artificial sweetenerAnaerobic fermentation raw material for

bacteria and yeasts.Apricots, apples, grapes etc.

Fructose in body(from Sucrose)

Structure of Oligosaccharides

Disaccharides

Disaccharides

Reducing

MaltoseLactose

Isomaltose

Non-reducing

Sucrose

DISACCHARIDESThese are glycosides formed by the

condensation of 2 simple sugars.

If the glycosidic linkage involves the carbonyl groups of both sugars (as in sucrose) the resulting disaccharide is non-reducing.

On the other hand, if the glycosidic linkage involves the carbonyl group of only one of the 2 sugars (as in maltose and lactose) the resulting disaccharide is reducing.

POLYSACCHARIDES

These are formed by the condensation of n molecules of monosaccharides with the removal of n-1 molecules of water.

Since condensation involves the carbonyl groups of the sugars, leaving only one free carbonyl group at the end of a big molecule, polysaccharides are non-reducing.

They are of 2 types:1. Homopolysaccharides (or) Homoglycans - composed of one

type of monosaccharide(e.g. Starch, Glycogen, cellulose).2. Heteropolysaccharides (or ) Heteroglycans – composed of

more than one type of monosaccharide (e.g. glycosaminoglycans, glycoproteins)

POLYSACCHARIDES

Often classified according to their biological role:

1) starch and glycogen - storage polysaccharidesBoth are homoglycans.Starch is storage form in plants and fungi.Glycogen is storage form in animals.Bacteria contain both.

Starch

Starch - mixture of amylose and amylopectinamylose is an unbranched polymer of 100-1000 D-glucose in an a-(1 --> 4) glycosidic linkage.

amylopectin is a branched polymer a-(1--> 6) branches of residues in an a-(1 --> 4) linkage; overall between 300-6000 glucose residues, with branches once every 25 residues; side chains are 15-25 residues long

α-amylase is an endoglycosidase found in human saliva but also plants that randomly hydrolyzes the a (1--> 4) bond of amylose and amylopectin.

β-amylase is an exoglycosidase found in higher plants that hydrolyzes maltose residues from non-reducing ends of amylopectin.

- 1,4 linkage between two glucose units

-1,6 linkage between two glucose units

Glycogen

Glycogen - branched polymer of glucose residues with branches every 8-12 residues with branches containing as many as 50,000 glucose residues

Cellulose & Chitin - structural polysaccharides

Cellulose - straight chain homoglycan of glucose with b-(1--> 4) linkages with alternating glucose molecules; ranges in size from 300-15,000 glucose residues

Extensive H-bonding within and between cellulose chains.

Makes bundles or fibrils ---> rigid.

Chitin - linear polymer of N-acetylglucosamine residues

Alternating 180o with b - (1 --> 4) linkage.Lots of H-bonding between adjacent strands.

The ability to digest cellulose is found only in microorganisms that contain the enzyme

Cellulase.

Certain animal species (e.g. Cow) utilize such organisms in their digestive tracts to digest cellulose

Heteroglycans (or) Hetropolysaccharides (or) Glycoconjugates

Heteroglycans

Proteoglycans

Peptidoglycans

Glycoproteins

Proteoglycans

Complexes of polysaccharides called glycosaminoglycans & core proteins.

Found in extracellular matrix of connective tissues.

Glycosaminoglycans are unbranched heteroglycans made of disaccharide units (amino sugar, D-

galactosamine or D-glucosamine & alduronic acid). e.g. hyaluronic acid Found in cartilage and synovial fluid.

Proteoglycan cartilage

Peptidoglycans

Found in cell wall of bacteria.Composed of alternating residues of N-

acetylglucosamine and N-acetylmuramic acid joined by b- (1--> 4) linkages.

Glycoproteins

Proteins with oligosaccharides attached.Carbohydrate chains are from 1-30 residues in

length.Examples: enzymes, hormones, structural

proteins, transport proteins.Found in eucaryotic cells.Can be attached to proteins with one of two

configurations:O-linked - carbohydrate bonded to -OH of serine or threonineN-linked - carbohydrate (usually N-acetylglucosamine) linked to asparagines

ROLES OF CARBOHYDRATES IN BIOLOGY

Carbohydrates serve as information-rich molecules that guide many biological processes.

Examples include:1) Asialoglycoprotein receptorPresent in liver cells; binds to asialoglycoproteins

to remove them from circulationPresence of sialoglycoprotein prevents

glycoproteins such as antibodies and peptide hormones from being internalized

Presence of sialic acid on terminal galactose on these proteins mark the passage of time; when they are removed (usually by the protein itself), the glycoproteins are removed from circulation.

2) Lectins

Carbohydrate-binding proteins of plant origin.

Contain 2 or more binding sites for carbohydrate units ->

cross-link or agglutinate erythrocytes and other cells.

3) Many viruses and bacteria can gain entry into host cells via

carbohydrates displayed on cell surface.

Influenza virus contains a hemagglutinin protein that

recognizes sialic acid residues on cells lining respiratory tract.

Neisseria gonorrhoeae infects human genital or oral epithelial

cells because of recognition of cell surface carbohydrates;

other cells lack these carbohydrates.

Contd..4) Interaction of sperm with ovulated eggs

Ovulated eggs contain zona pellucida, an extracellular coat made of O-linked oligosaccharides.

Sperm cells have receptor for these carbohydrates. Binding of sperm to egg causes release of proteases and hyaluronidase,

which dissolve zona pellucida to allow sperm entry.

5) Selectins

Carbohydrate-binding adhesion proteins that mediate binding of neutrophils and other leukocytes to sites of injury in the inflammatory response.

6) Homing receptor of lymphocytes

Homing is phenomenon in which lymphocytes tend to migrate to lymphoid sites from which they were originally derived.

Mediated by carbohydrates on lymphocyte surface and endothelial lining of lymph nodes.