carbohydrates

CarbohydratesDr. Deepak K Gupta

Syllabus

• Definition,

• Biological importance and classification.

• Monosaccharides : Isomerism, anomerism.

• Sugar derivatives,

• Disaccharides.

• Polysaccharides.

• Structure of starch and glycogen

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Introduction• Mostly derived from plant.

• Animals - synthesize carbohydrate from lipidglycerol and amino acids.

• Glucose - major metabolic fuel of mammals and a universal fuel of the fetus.

• Other sugars are converted into glucose in the liver.

• Diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance are some of the disease related to carbohydrate metabolism


Function of Carbohydrate

• Dietary source of energy.• Form the structural framework of RNA and DNA.• Structure of cell membrane and cell wall of plant

cell & microorganism and exoskeleton of insects• Precursor for synthesis of all the other

carbohydrates in the body,– glycogen for storage.– ribose and deoxyribose in nucleic acids. – Galactose - lactose of milk, in glycolipids.– Combination with protein - glycoproteins and

proteoglycans


Structural Component

• They are oxidised polyhydroxy alcohols, having the general formul (CH2O)n where n = number of carbon atom.

• Glycerol can be considered as the parent compound, although its not a carbohydrate.

• It can be further oxidised to get

– Aldehyde (glyceraldhehyde)

– Ketone (dihydroxyacetone).

• This 2 forms the basis of all the carbohydrateswww.facebook.com/notesdental

Stereoisomerism• Important characteristics of

monosaccharide.

• Same structural formulae but differs in their spatial arrangement.

• A carbon is said to assymetricor chiral when its is attached to different atoms or groups.

• Possible number of isomers –2n where n = number of chiralcarbon atoms.


D & L isomerisms

• Arrangement of – OH and –H : adjacent to the terminal alcohol determines the isomerism

• D- isomer has –OH on its right and L-isomer has –OH on its left.


Optical Isomerism

• Optical activity is characteristic principle of asymetric carbon atom.

• Polarized light when passed through solution– turns to left – Levorotatory (-)

– and if it turns to right – Dextrorotatory (+).

• So carbohydrate can be either D(+), D(-), L(+), L(-).

• Racemic mixture : when D and L isomers are present in equal concentration which doesn’t shows any optical activity.


Optical isomerism


Epimers

• Isomers differing as a result of variations in configuration of the -OH and -H on carbon atoms other than functional group or last carbon are known as epimers.

• Ex: epimers of glucose– Mannose epimer at 2nd carbon

– Galactose epimer at 4th carbon


Enantiomers• Chiral molecules that are

mirror images of one another.

• molecules are non-superimposable on one another


Cyclization• Aldehyde reacts with

alcohol to give hemiacetal

• Ketone reacts with alcohol to form hemiketal.

• Hydroxyl group of sugar molecule can react with its own aldehyde/ketone.


Cyclization

• Aldehyde group of glucose at C1 reacts with alcohol at C5 – ß & α cyclic hemiacetals.

• This can be represented either by Fischer formulae or Haworth projection formulae.


Anomers

• Anomers are the cyclic isomers differing from each other in configuration at anomeric carbon i.e at C1. Ex: α and ß form.

• Both this anomers of D-glucose interconvert in aqueous solution by a process called mutarotation


Anomers

• Anomers of D-glucose interconvert in aqueous solution by a process called mutarotation.

• Solution of α-D-glucose and a solution of β-D-glucose - form identical equilibrium mixtures having identical optical properties.

– one-third α -D-glucose

– two-thirds β-D-glucose,

– very small amounts of the linear and five –membered ring (glucofuranose) forms


Pyranose and Furanose Ring Structures

• Pyranoses : 6-membered ring compounds - resemble the 6-membered ring compound pyran.

• Furanoses : cyclic forms having 5 membered rings -resemble the 5 memberedring compound furan.

• Aldopyranose ring is much more stable than the aldofuranose ring


Classification

• Aldehyde or ketone derivatives of polyhydric alcohol.

• They are mainly classified as

– Monosaccharides

– Disaccharides

– Oligosaccharide

– Polysaccharides


Monosaccharide• Cannot be hydrolyzed into simpler

carbohydrates.• Aldoses or ketoses depending upon

whether they have an aldehyde or ketone group.

• Based on the number of carbon atoms, monosaccharide are classified as trioses, tetroses, pentoses, hexoses and heptoses


Monosaccharide

• colorless, crystalline solids

• freely soluble in water but insoluble in nonpolar solvents

• Most have a sweet taste


Monosaccharide• Aldopentose

– D-ribose - component of RNA and of nucleotidecoenzyme

– D-xylose and L-arabinose - constituents of polysaccharides in the walls of plant cells

• Aldohexoses– D-glucose – huge proportion of the biomass is accounted

for by glucose polymers, cellulose and starch– Free D-glucose : plant juices (grape sugar) and as

blood sugar– D - galactose - constituent of lactose (milk sugar)

• Together with D-mannose, galactose is also found inglycolipids and glycoproteins


Monosaccharide


Monosaccharide

• Ketopentose– D-ribulose : intermediates

in the pentose phosphate pathway and photosynthesis

– D-fructose: most widely distributed, fruit juices and in honey, sucrose and inulin (bound sucrose)

• Deoxyaldoses(deoxyribose)– OH group is replaced by a

hydrogen atom from ribose sugar


Oligosaccharides

• Condensation products of two to ten monosaccharides.

• Based on this number this are further subdivided into

– Disaccharide

– Trisaccharide etc

• Ex: maltotriose.


Disaccharides

• consists of two sugars joined by an O-glycosidic bond

• Ex: maltose, sucrose and lactose.• Sucrose

– anomeric carbon atoms of a glucose unit and a fructose unit are joined

– glycosidic linkage is α for glucose and β for fructose

• Lactose• disaccharide of milk• consists of galactose joined to glucose by a

β-1,4-glycosidic linkage• Turns to monosaccharides by lactase in

human beings


Disaccharides

• Maltose– two glucose units - joined by

an α -1,4 glycosidic linkage

– hydrolysis of starch and is in turn hydrolyzed to glucose by maltase

• In humans this are located on the outer surfaces of epithelial cells lining the small intestine


Polysaccharides

• condensation products of more than ten monosaccharide units.

• Linear or branched polymers

• Usually tasteless and forms colloids with wates

• Its again further subdivided into –homopolysachharides and heteropolysaccharides.

• Ex: starches and dextrins


Polysaccharides : Starch• homopolymer of glucose

forming an α- glucosidic chain• abundant dietary carbohydrate

in cereals, potatoes, legumes, and other vegetables

• two main constituents– Amylose (15–20%) : non-

branching helical structure– Amylopectin (80–85%) :

branched chains composed of 24–30 glucose residues united • by 1 → 4 linkages in the chains • by 1 → 6 linkages at the

branch points


Polysaccharides : Glycogen

• storage polysaccharide in animals.

• Glucose gets converted to glycogen by a process called glycogenesis

• more highly branched structure than amylopectin

• chains of 1–4 α-D-glucopyranose residues

• With branching by means of α(1 → 6)-glucosidicbonds


Polysaccharides : Inulin

• polysaccharide of fructose

• found in tubers and roots of dahlias, artichokes and dandelions

• readily soluble in water and is used to determine the glomerular filtration rate


Polysaccharides : Misc

• Dextrins : intermediates in the hydrolysis of starch

• Cellulose

– insoluble and consists of β-D-glucopyranoseunits

– linked by β(1 → 4) bonds to form long, straight chains strengthened by cross-linked hydrogen bonds

– source of “bulk” in the diet


Polysaccharides : Misc

• Chitin

– structural polysaccharide in the exoskeletonof crustaceans and insects and also in mushrooms

– N-acetyl-D-glucosamine units joined byβ (1 → 4)-glycosidiclinkages


Glycosaminoglycans

• Mucopolysaccharides• complex carbohydrates characterized by their content

of amino sugars and uronic acids.• Proteoglycan : when GAG chains are attached to a

protein molecule.• ground or packing substance of connective tissues

– holding large quantities of water - lubricating– occupying space – cushioning– large number of OH groups and negative charges on the

molecules,– keep the carbohydrate chains apart

• Ex: hyaluronic acid, chondroitin sulfate, and heparin


Glycoproteins• Mucoproteins

• different situations in fluids and tissues, including the cell membranes

• containing branched or unbranchedoligosaccharide chains


References

• Color_Atlas_of_Biochemistry_2005

• Harpers_Biochemistry_26th_ed

• Lehninger Principles of Biochemistry, Fourth Edition - David L. Nelson, Michael M. Cox.

• Biochemistry – U. Satyanarayan, U. Chakerpeni


carbohydrates

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