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 C_n(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