Monday, February 15, 2010

Carbohydrates

CARBOHYDRATES

Carbohydrates have the general molecular formula (CH2O)n, and thus were once thought to represent "hydrated carbon". All sugars are very soluble in water because of their many hydroxyl groups. They are called sugars because they are sweet in taste & crystalline in nature. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells. Carbohydrates provide the bulk of the calories (4 kcal/gram)

1)Definition:

a)Hydrates of carbon Cn(H2O)n{exceptions: Acetic acid, C2H4O2 & lactic Acid, C3H6O3 Formaldehyde, CH2Oare not carbohydrates but Rahnose,C6H12O5 is.}

b)carbohydrates are polyhydroxy aldehydes or ketones or compounds, which yields these on hydrolysis.

2)Classification:

a)Monosaccharides:simple sugars that cannot be hydrolyzed to yield Carbohydrates

Name

Aldose

Ketose

Formula

Trioses

Glyceraldehyde

Dihydroxy acetone

C3H6O3

Tetroses

Erythrose

Erytrulose

C4H8O4

Pentoses

Ribose

Ribulose

C5H10O5

Hexoses

Glucose,Mannose, Galactose

Fructose

C6H12O6

Heptoses

Sedoheptulose

C7H14O7

Nonoses

Sialic acid or Neuramic acid

C9H18O9

b)Disaccharides: yields two molecules of Monosaccharides on hydrolysis

Name

Monosaccharides

Formula

Sources

Properties

Maltose

Glucose + Glucose

C3H6O3

Malt sugar, starch

Reducing

Lactose

Glucose + Galactose

C3H6O3

Milk Sugar

Reducing

Sucrose

Glucose + Fructose

C3H6O3

Cane sugar

Non-Reducing

Sucrose: prevalent in sugar cane and sugar beets, is composed of glucose and fructose through an a-(1,2)b-glycosidic bond.

Lactose: is found exclusively in the milk of mammals and consists of galactose and glucose in a b-(1,4) glycosidic bond.

Maltose: the major degradation product of starch, is composed of 2 glucose monomers in an a-(1,4) glycosidic bond.

c)Oligosaccharides: yields 3 to 10 Monosaccharides on Hydrolysis.

Maltotriose

Glucose + Glucose + Glucose

Raffinose

A trisaccharide that occurs in sugar beets and cotton seeds and cereals

Stachyose

A tetrasaccharide found in the tubers of the Chinese artichoke

d)Polisaccharides: yields more than 10 Monosaccharides on hydrolysis. They are of two types.

i)Homopolysaccharides (Homoglycans) Polymer of a single type of Monosaccharide.

Name

Monosacc.

Sources

Importance

Starch

Glucose

Potato,tapioca,rice,wheat,&grains

Plant source of Energy

Amylose

Glucose

Un branched starch similar to cellulose, containing alpha(1->4) linkages

Amylopectin

Glucose

Branched starch containing both 1,4 and 1,6 linkages.

Glycogen

Glucose

5% of liver mass> branched & compact

Animal storage of Glucose

Inulin

Fructose

various bulbs&tubers(onion,garlic)

Renal clearance studies.

Dextrins

Glucose

Product. ofstarch digestion

White dextrin is used in pharmaceutical preparations

Chitin

N-Ac-Glucosamine

Exoskeleton of Insects


 

ii)Hetroplysaccharides (Hetroglycans): polymer of two or more types of sugar residues

Agar

Glu +Gal + other

Obtained from Sea weeds

A colloidal extract of algae; used especially in culture media and as a gelling agent in foods (undigested by bacteria, dissolves at 100 oC and upon cooling sets into a Gel)

iii)MUCOPOLYSACCHARIDES (MPS) or GLYCOSAMINOGLYCANS (GAG)

Are Carbohydrates containing Uronic Acid & Amino Sugars.

Name

Functions

Hyaluronic acid

A viscous mucopolysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humours of the eye; a cementing and protective substance

Heparin

A polysaccharide produced in basophils (especially in the lung and liver) and that inhibit the activity of thrombin in coagulation of the blood.

Chondroitin SO4

Inground substance of connective tissues (cartilage, tendon, bone)

Keratan SO4

In cornea & tendons.


 

e)Isomerism

i)D & L Isomerism

oThe configuration of the asymmetric carbon atom ( carbon atom whose 4 valencies are filled up by 4 different groups)farthest from the aldehydic or ketonic group

oIf –OH group is attached to the right then 'D' series & left then 'L' series.

ii)Pyranose - Furanose ring Structure

oWhen ring structure were suggested by Haworth (projections)

oFive member ring structure is calledFuranose and Six number as Pyranose

iii)Alpha & Beta Anomerism

oThe anomeric carbon (which originally was a part of the aldo or keto group) forms a hemiacetal or hemiketal in the ring structure generating a new asymmetric centre.

oIf hydroxyl gp (-OH) of the anomeric carbon is to the right then it is -form and if to the left it -form.

iv)Epimerism

oEpimers differ in the position of the Hydroxyl gp at only one of their assymetric C-atom.

oD-Glucose & D-Galactsoe are C-4 Epimers

oD-Glucose & D-Mannose are C-2 epimers

v)Enantiomers

oStereo isomers that are mirror images of each other

oEg: D-Glyceraldehyde & L-Glyceraldehyde.

vi)Mutarotation

oWhen D-Glucose is crystallized at RT and fresh solution is prepared, its specific rotation of polarized light is +112 o; but after few hrs it changes to +52.5 o. This change in rotation is called Mutarotation. This b'se D-Glucose has two anomers & forms. (-D-Glucose has specific rotation of +112 o and -D-Glucose has +19 o at equilibrium 1/3 is form and 2/3 is form to get a specific rotation of +52.5 o

3)Other Physiologically important carbohydrates.

Sugar

Present where

Importance&clinical Significance

D-Ribose

Nucleic acids

Nucleic acid co-enzymes.

D-Ribulose

Metabolic Product

Pentose-PO4 pathway intermediate.

D-Arabinose

Gum arabic, Plum & Cherry gum

Constituent of Glycoproteins

D-Xylose

Wood gums, GAG's

Constituent of Glycoproteins

D-Xylulose

Intermediate in Uronic acid pathway

Found in the Urine of patients with Essential pentosuria


4)CHEMISTRY

a)Reactions

i)Benedict's Reaction

Contains Na2CO3, Na-Citrte,CuSO4

In alkaline medium (Na2CO3), the copper remains as cupric hydroxide in presence of sodium citrate

5 ml Benedict's reagent+ 0.5 ml Urine boil for 2 minutes. If sugar present copper is reduced to produce green, yellow, orange, red precipitate depending on the concentration of the sugar.

SugarEnediolCu++(cupric)  CuSO 4(Reagent)

Sugar AcidCu+(cuprous) 2Cu(OH)[Yellow]Cu 2O [Red]

Chemical propertie

s

1

Reactions of aldehyde / keto group

  

Reactions

Reagents

Result

 

a.

Osazone formation

Sugar+Phenyl Hydrazine HCl+ Sodium Acetate (boil+cool)

Glucosazone-needle shapedLactosazone - Powder puff Maltosazone - Sunflower

b.

Reduction → Alcohol

Sodium amalgum / Hydrogen

Glucose - sorbitolFructose - Manitol

c.

Oxidation

  

I.

Mild (CHO → COOH)

Br2 H2O

Glucose - gluconic acid

ii.

Srong (CHO & CH2OH to COOH

HNO3

glucose - Glucosaccharic acid

iii.

Oxdn of CH2OH alone

Enzymatic (living cells only)

Glucose to glucuronic acid

d.

Tautomerisation( Inter conversions)

Weak alkaliBa (OH)2, Ca(OH)2

Glucose ↔ fructose ↔ Mannose

e.

Reduction Reactions

CuSO4, alkaline medium( cupric -> cuprous)

Benedicts, Fehlings & Barfoeds Test( Cuprous oxide- Red ppt)

2

Reactions of Alcoholic group

 

a.

Glycosides

1→4 &1→6 bond in glycosidic bonds in glycogen

  

I.

Cardiac

Derivatives of DIGITALIS,

Drugs in cardiac insufficiency

 

ii.

Oubin

strophanthus sp.

Sodium pump inhibitor (cardiac)

 

iii.

Phlorbizin

Root/bark of apple tree

block transport of sugar

 

iv.

Streptomycine

produced by the actinomycete Streptomyces griseus

Antibioticused to treat tuberculosis

 

b.

Acetilation/Ester formation

Alcohol + acids

phosphates ( glucose -1-phosphate)

 

c.

Dehydratin( Molisch, Selivanoffs, Bials tests)

Strong acids (Con.H2SO4)

Pentose→ Furfurals,Hexose→ Hydroxy methyl Furfural.

TESTS

 

Name of test

Substrate

Reagents

Result

1

Molisch's Test

Sugar

3d Alfa-Naphthol in Methanol +H2SO4 (side)

Violet color in the Junction.

2

Iodine test

Starch

Dil. Iodine

Deep Blue Colour

3

Benedict's

Reducing Sugar(0.5 ml/ 8 dps)

CuSO4+Na2CO3+NaCitrate(5 ml)

Cuprous Oxide (Red Precipitate)

4

Fehling's Test

Reducing Sugar

CuSO4+KOH+NaK Tartrate

Cuprous Oxide (Red Precipitate)

5

Barfoed's test

Monosaccharide.

CuAcetate+Dil.AceticAcid

(Red Precipitate) in < 5'

6

Bial's Test

Pentose

Orcinol Hydrochloride (Boil)

Green

7

SelivanoffsTest

Ketoses

Dil. Resorcinol in Dil.HCl (heat)

Cherry red

8

Tollent's Test

Reducing Sugar

Ammoniacal Silver nitrate

Metalic Siver Mirror

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