Carbohydrate

Carbohydrates

Most abundant organic molecules widely distributed in plants and animals

Primarily composed of Carbon, Hydrogen & Oxygen.

Derived from the French: hydrate de carbone, Hydrates of carbon. Same ratio of H & O as in water.

Empirical formula-(C-H₂O)_n literally “CARBON HYDRATE”; n≥3

Carbohydrates not obeying above: rhamnohexose (C₆H₁₂O₅), deoxyribose (C₅H₁₀O₄)

Therefore, cannot be always considered as carbon hydrates.

Carbohydrates – Basic Structure

Carbohydrates

Functional Group: - OH; -CHO; >C=O

Polyhydroxyaldehydes or polyhydroxyketone or compounds that produce them on hydrolysis.

The term ‘sugar’ is applied to carbohydrates soluble in water and sweet to taste.

Carbohydrates are often referred to as saccharides (Greek: sakcharon :– sugar )

How do we get them?

IN PLANTS:

Glucose synthesized from carbon dioxide and water by PHOTOSYNTHESIS and stored as starch or converted to the cellulose of the plant framework. Each year 100 billion metric tons of CO₂ & H₂O.

IN ANIMALS:

Can SYNTHESIZE carbohydrate from LIPID glycerol and AMINO ACIDS, but most are derived ultimately from plants.

Why do we need them?

Energy (oxidation of carbohydrate is the central energy yielding pathway; 4kcal/gm; 60-80% of total body requirement)

Energy Reserve (eg., starch, glycogen)

Precursor Molecule (eg, (deoxy)ribose, galactose)

Structural Components (Cellulose, Chitin)

Immunoglobulins

Intercellular Communications and Adhesion (Glycolipids and Glycoproteins)

Lubricants in skeletal joints, ETC……….

Some Important Carbohydrates

Glucose

Fructose

Galactose

Ribose

Deoxyribose

Lactose

Starch

Cellulose

Glyceraldehyde

Dihydroxyacetone phosphate

Mannose

Glycosaminoglycans

Hyaluronic acid

Keratan Sufate

Maltose

Sucrose

……………….

Classification of Carbohydrates

Monosaccharide

Disaccharide

Oligosaccharide

Polysaccharide

Classification of Carbohydrates

1. Monosaccharides (and their derivatives):

Those carbohydrates that cannot be hydrolyzed into simpler carbohydrates: aldehyde or ketones that have two or more hydroxyl groups.

Further classified as trioses, tetroses, pentoses, hexoses etc..

Trioses of Physiological Significance:

Both D-glyceraldehyde and dihydroxyacetone ;

(in phosphate esters form) - intermediate in glycolytic pathway.

They are the precursor of glycerol utilized in lipid synthesis

Tetroses of Physiological Significance:

Erythrose-4-P; an intermediate in HMP shunt pathway

Pentoses of Physiological Significance:

Sugar		Biochemical Importance
D-Ribose		Structural elements of Nucleic Acid and Co-enzymes, eg, ATP, NAD, NADP, Flavoproteins. Ribose phosphates are intermediates in Pentose Phosphate Pathway.
D-Ribulose		Ribulose phosphate is an intermediate in Pentose Phosphate Pathway.
D-Arabinose		Constituent of glycoproteins.
D- Xylose		Constituent of glycoproteins.
D-Lyxose		A constituent of lyxoflavin isolated from human heart muscle.
L-Xylulose		An intermediate in Pentose Phosphate Pathway.
Sugar	Source		Importance
D-Glucose	Fruit juices, Hydrolysis of starch, cane sugar, maltose & Lactose.		The ‘sugar’ of the body. The sugar carried by the blood and principal one is used by the tissues.
D-Fructose	Fruit jucies. Honey. Hydrolysis of cane sugar		Can be changed to glucose in the liver and so used in the body.
D-Galactose	Hydrolysis of lactose		Can be changed to glucose in the liver and metabolized. Synthesized in the mammary gland to make the lactose of milk. A constituent of glycolipid and glycoproteins.
D-Mannose	Hydrolysis of plant mannans and gums		A constituent of many glycoproteins.

Classification of Carbohydrates

2. Disaccharides: Condensation products of two monosaccharide units E.g. Lactose, Maltose and Sucrose.

3. Oligosaccharides: Condensation products of 2 to 10 monosaccharides

E.g., raffinose, stachyose

4. Polysaccharides: Condensation products of more than ten monosaccharide units. They are insoluble and tasteless.

Homopolysaccharide, e.g., Glycogen, Starch, Cellulose, Chitin

Heteropolysaccharide, e.g., Peptidoglycan, Glycosaminoglycans

• Reaction of Carbohydrates

• Tautomerization or Enolization

• Oxidation and Reduction Reactions

• Dehydration

• Osazone Formation

• Esterification

• Glycoside Formation

• Reactions of monosaccharides

• Tautomerization or enolization:

– Shifting a H atom from one carbon to another to produce enediol.

– Sugar possessing anomeric carbon undergo tautomerization in alkaline conditions

– The enediols are highly reactive, sugars in alkaline solution are powerful reducing agents.

Reducing Properties of Sugars

• Depends on the FREE aldehyde or keto group on the anomeric carbon.

• Sugars with free C=O groups are capable of reducing ferric or cupric ions and themselves oxidized to form sugar acids, thus called reducing sugars. This property is useful in the analysis of sugar.

OXIDATION

– The terminal aldehyde CHO àCOOH results in the formation of GLUCONIC acid.

– The terminal alcohol group CH₂OH à COOH lead to production of GLUCURONIC acid.

REDUCTION

– When treated with reducing agents as Na-amalgam [Na(Hg)], the aldehyde or keto group of monosaccharide is reduced to corresponding alcohol.

– E.g., GlucoseàSorbitol, GalactoseàDulcitol, Mannoseà Mannitol, RiboseàRibitol

• Dehydration

– When treated with conc. H₂SO₄, undergo dehydration with an elimination of 3H₂O molecules

– Hexosesà hydroxy methyl furfural, pentoses à furfural.

– These can condense with phenolic compounds (α-naphthol, anthrone) to form coloured products.

–

• Osazone Formation

– Phenylhydrazine in acetic acid, when boiled reacts wit carbon #1 and #2 of reducing sugars to form derivatives called osazones

– The distinctive crystalline derivatives formed is useful to compare the structures of sugars.

– Glucose, fructose, mannose give same needle shaped osazones.

– Reducing disaccharides; maltose – sunflower shaped, lactose- powder puff shaped

– Osazone Formation

• Formation of Glycosides

• Condensation between the hydroxyl group of the anomeric carbon of a monosaccharide and a second compound that may be another monosaccharide or aglycone (not a sugar)

• If the second group is also a hydroxyl, then O-glycosidic bond is formed, BUT, if the second group is an amine, then N-glycosidic bond is formed

• The aglycone may be methanol, glycerol, sterol phenol or a base such a adenine

• They are important in medicines because of their action on heart e.g, cardiac glycosides

• Formation of Glycosides

• Formation of Esters

– The alcoholic groups of monosaccharides may be esterified by non-enzymic or enzymic reactions.

• Derivatives of Monosaccharides

SUGAR ACIDS: Sugar with free anomeric carbon are good reducing agent & will reduce H₂O₂, Ferricyanide, Cu²⁺ & such reaction convert sugar into sugar acid.

• Oxidation at C1 & C6 produces aldaric acid (Glucaric acid)

SUGAR ALCOHOLS: Reduction of aldoses or ketoses; usually sweet in taste

• E.g., sorbitol, mannitol, xylitol: sweetener in sugarless gum & candies

• Glycerol and myo-inositol are components of lipids; Ribitol is constituent of flavin enzymes

AMINO SUGARS: When one or more OH group replaced by amino groups.

• Glucosamine, Mannosamine, Galactosamine; C₂-OH substituted by amine. Amino group is nearly always condensed with acetic acid; N-Acetyl glucosamine, Polymer of bacterial cell wall.

9C sugar: N-acetylneuraminic acid (sialic acid) is a derivative of N-acetylmannosamine; component of many glycoprotein and glycolipids in animals.