Carbohydrates

  Carbohydrates                                                                  

                                    

Carbohydrates are the main source of energy in the body. Although whole body depends on the carbohydrates of their energy source. When carbohydrates are not available inadequate amount, the body starts using other main constituents such as Fat & Protein. Brain cells partially & RBC fully depends on the carbohydrates as their energy sources.

           Carbohydrates are organic substances containing carbon (C), hydrogen(H) and oxygen(O) usually in the ratio of 1:2:1 e.g. Ribose, Glucose and Fructose.  

    Energy production from carbohydrates will be 4 kcal per gram.

Any carbohydrate must contain:

1.                      Two hydroxy(OH) groups or more

2.                      Aldehydes or ketone or their derivatives

3.                      Minimum no of carbon atoms must be three

DEFINITION

 “Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis”. Or

Carbohydrates are aldehyde or ketone derivatives of polyhydric alcohol or any substance derived from them.

Sources of carbohydrates:

Plants source: Cellulose, starch.

Animals Source: Glucose, Glycogen

Carbohydrates from a variety of foods are an essential component of the diet.

Classification of Carbohydrates:

Carbohydrates are broadly classified into four major groups-

1.                  Monosaccharides

2.                  Disaccharides

3.                  Oligosaccharides

4.                  Polysaccharides

1.                        Monosaccharides: (contain one sugar unit)

     Monosaccharides are those carbohydrates that cannot be hydrolyzed into simpler carbohydrates. They are the simplest units of carbohydrates. The monosaccharides are divided into different categories,  

 a.  Based on the functional group:

 Aldoses: When the functional group in monosaccharide is an aldehyde (-CHO) group, they are known as aldoses, eg glyceraldehydes, glucose.

Ketoses:  When the functional group is a keto (-CO) group they are refer to as ketoses eg dihydroxyacetone, fructose.

b.  Based on number of carbon atoms. (C₃ to C₇)

 Trioses(C₃), tetroses(C₄), pentoses(C₅), hexoses(C₆) and heptoses(C₇) depending on the number of carbon atoms.

The common monosaccharides are given in the Table:

 Classification of the monosaccharides.

Monosaccharides	Aldoses	Ketose
Trioses(C3H6O3)	Glyceraldehyde	Dihydroxyacetone
Tetroses(C4H8O4)	Erythrose	Erytthrulose
Pentoses(C5H10O5)	Ribose, Xylose	Ribulose, Xylulose
Hexoses(C6H12O6)	Glucose, Galactose	Fructose
Heptoses(C7H14O7)	Glucoheptose	Sedoheptulose

 Glucose is aldohexose while fructose is a ketohexose.

Note: The suffix ulose means keto sugar

 

Ring Structure of Glucose

Glucose can exist in an open-chain or cyclic structures.You will recollect that aldehydes react with alcohols to for

hemiacetals.In case of glucose, however, the hemiacetal formation  is intramolecular as the CHO group and the OH group  are parts of the same molecule.

Formation of the cyclic forms of monosaccharides is  favored because these structures have lower energies

than the straight-chain forms. Cyclic forms of D-glucose are formed by the hemiacetal

linkage between the C1 aldehyde group and the C4 or C5 alcohol group.

If the ring structure is formed between C1 and C4, the resulting five-membered ring structure is named D-

glucofuranose because it resembles the compound furan.

D-glucofuranose

If the ring structure is formed between C1 and C5, theresulting six-membered ring is named D-glucopyranose

because it resembles the compound pyran"

Aldohexoses exist in solutions mainly in six-membered

pyranose ring forms, since these forms are thermodynamically more stable than furanose ring forms.

	Pyranose and furanose forms of glucose

 

Fructose:

                                               

Fructose is often called fruit sugar because it contributes sweetness to ripe fruits, such as these peaches.

Is fructose an aldose or a ketose?

Fructose, also called levulose and fruit sugar, is the

sweetest of all sugars.It is found in large amounts in honey, corn syrup, and sweet fruits.

Fructose is a major constituent (38%) of honey; the

Other constituents are glucose (31%), water (17%),

maltose (a glucose disaccharide, 7%), sucrose (a glucose-fructose disaccharide, 1%), and polysaccharide (1%).

The variability of these sugars in honey from different

sources is quite large.

Ring Structures of Fructose

Hexoses of physiologic importance:

Galactose

Galactose is one of the components of lactose, or milk sugar. Both galactosemia and lactose intolerance are treated by removing milk and milk products from the diet.

Galactosemia:

Galactosemia:

Is an inherited disorder that affects the way the body breaks down certain sugars.Specifically, it affects the way the sugar called galactose is broken down. Galactose can be found in food by itself. A larger sugar called lactose, sometimes called milk sugar, is broken down by the body into galactose and glucose. The body uses glucose for energy.Because of the lack of the enzyme (galactose-1-phosphate uridyl transferase) which helps the body break down the galactose, it then builds up and

becomes toxic. In reaction to this build up of galactose the body makes some abnormal chemicals.

The buildup of galactose and the other chemicals can cause serious health problems like a swollen and inflamed liver, kidney failure, stunted physical and mental growth, and cataracts in the eyes.

If the condition is not treated there is a 70% chance that the child could die.

                                                         

Properties of monosaccharides:

A.                     Physical properties.

1.                       All monosaccharides are soluble in water.

2.                       All monosaccharides show the property of optical activity.

3.                       All monosaccharides can exist in α and β forms.

4.                       All monosaccharides can undergo mutarotation.

B.                     Chemical properties

  

1.                       Oxidation: Oxidation of sugars gives acids.

Sugar acids are produced by oxidation of carbonyl carbon, last hydroxyl carbon or both.

a.                      Aldonic acid: Oxidation of carbonyl carbon to carboxylic group gives aldonic acid e.g. glucose is oxidized to gluconic acid.

b.                       Uronic acids. Oxidation of last hudroxyl carbon gives uronic acid e.g. glucose is oxidized to glucoronic acid.

c.                        Aldaric acids : These are dicarboxylic acids produced by oxidation of both carbonyl carbon and last hydroxyl carbon e.g. glucose is oxidized to glucaric acid.

2.                       Reduction

Reduction of carbonyl group gives the corresponding alcohol e.g. glucose gives sorbitol, ribose gives ribitol, galactose gives galactitol.

3.                      Reducing Sugars:

The aldehyde group of aldoses is readily oxidized by the

Benedict’s reagent.

Benedicts’s test: Reduction under alkaline condition.

When solution containing reducing sugar is boiled with

Benedict’s reagent (blue color) for two minutes, brick

Red colored precipitate appears.

.

                                  2CuOH                Cu₂O + H₂O

The test is useful in detection of glucose in urine.(Glycosuria, complication of diabetes)

Recall that the Benedict’s reagent is a basic buffer

solution that contains Cu²⁺ ions.

The Cu ²⁺ ions are reduced to Cu  ions, which, in basic

solution, precipitate as brick red Cu²⁺o.

The aldehyde group of the aldose is oxidized to a

carboxylic acid, which undergoes an acid-base reaction

to produce a carboxylate anion.

Although ketones generally are not easily oxidized,

ketoses are an exception to that rule.

Because of the  OH group on the carbon next to the

carbonyl group, ketoses can be converted to aldoses,

under basic conditions, via an enediol reaction

The name of the enediol reaction is derived from the structure of the Intermediate through which the ketose

is converted to the aldose: It has a double bond (ene) and it has two hydroxyl groups (diol).

Because of this enediol reaction, ketoses are also able to react with Benedict’s reagent, which is basic.

Because the metal ions in the solution are reduced, the sugars are serving as reducing agents and are called

reducing sugars.

Note:All monosaccharides and all the common disaccharides, except sucrose, are reducing sugars

4.                      Osazone formation:

When reducing sugars are treated with phenyl hydrazine (C₆H₅-NH-NH₂) first phenyl hydrazones is formed followed by the formation of osazones. Osazones can be isolated in the characteristic crystalline shape which are useful in the identification of different reducing sugars.

Glucoosazone crystals are fine yellow neddles in fan shaped aggregates or sheaves or crosses, typically described as “bundle of hay”.

v     Glucose, mannose, and fructose due to similarities of    structure form the same osazones.

v     Lactoosazone : Irregular clusters of fine needles look like a “power puff”.

v     Maltoosazone : Star shaped compared to sunflower petals.

v     Sucrose cannot forms osazone.

Note: All sugars having free carbonyl group can form     osazone crystal.

Disaccharides:(contain two sugar unit joined by glycosidic bond)

Two monosaccharides join covalently by glycosidic bonds with the loss of a molecule of water to form a disaccharide. or Disaccharides are those sugars which gives two molecules of monosaccharide on hydrolysis.

Two monosaccharide units are joined by glycosidic bond.

Examples       Product formed              Sources

                       upon hydrolysis

Maltose          Glucose + Glucose             Malt

Lactose          Glucose + Galactose           Millk

Sucrose          Glucose + Fructose             Sugar cane

Isomaltose      Glucose + Glucose             Amylopectin

Maltose:( Glucose+Glucose)   

Maltose is composed of 2 α-D-glucose units held together by α(1-4) glycosidic bond. The free aldehyde group present on C₁ of second glucose answers the reducing reactions, besides the osazone formations (sunflower shaped). Maltose can be hydrolysed by dilute acid or the enzyme maltase to liberate two molecules of α-D-glucose.

 

Sucrose:( Glucose+Fructose)

                                                                                                                                                       

Sucrose is source of sugar cane and sugar beets. Sucrose is made up of α-D-glucose and β-D-fructose. The two monosaccharides are held together by a glycosidic bond(α₁- β₂), between C₁ of α-glucose and C₂ of β-fructose. The reducing groups of glucose and fructose are involved in glycosidic bond, hence sucrose is a non reducing sugar.

Sucrose is major carbohydrate produced in photosynthesis. It is transported into the storage organs of plants(roots, tubers, seed). Sucrose is the most abundant among the naturally occurring sugars. It has distinct advantages over other sugars as a storage and transport form. This is due to the fact that in sucrose, both the functional groups(aldehyde and ketone) are held together and protected from oxidative attacks.

                       Sucrose is an important source of dietary carbohydrate. It is sweeter than most other common sugars(except fructose) namely glucose, lactose and maltose. Source is employed as a sweeting agent in food industry. Sucrose is also called invert and table sugar.

 

Lactose:( Glucose+Galactose)

                                                                                                                                                                              

                                                                                                                                        

Lactose is also known as milk sugar. Lactose is composed of β-D-glucose and β-D-galactose held tighter by β(1-4) glycosidic bond. The anomeric carbon of C₁ glucose is free, hence lactose exhibits reducing properties.

Lactase Lactose =                                                β-D-glucose + β-D-galactose

             Lactose of milk is the most important carbohydrate in the nutrient of young mammals. It is hydrolysed by the intestinal enzyme lactase to glucose and galactose.

 

         

 

Oligosaccharides:(contain 3 - 6 monosaccharide unit joined by glycosidic bond) 

They yield three to six molecules of monosaccharide Units on hydrolysis.

e.g. Maltotriose (Glucose +Glucose+ Glucose).

                                                                                                                                                                                                     

Polysaccharides :

They yield more than six molecules of monosaccharide

Units on hydrolysis.

They are further classified into homopolysaccharides

and heteropolysaccharides.

Homopolysaccharides:-

They are polymer of same monosaccharide units.

Examples    Monosaccharide unit       Sources

Starch          Glucose                            Plant, rice

Dextrin           do                                   Starch

Glycogen         do                                 Liver, muscle

Cellulose         do                                 Plant fiber

Inulin             Fructose                        Dahlia roots

Chitin          N-acetyl glucosamine    Shells of arthropod

Heteropolysaccharides

They are polymer of different monosaccharide units or

Their derivatives. E.g. Mucopolysaccharides and blood

Group substances.

mucopolysaccharides are :-

hyaluronic acid,  chondroitin sulphate, heparin,  keratan

sulfate, heparan sulfate and dermatan sulfate.

                     Starch: (also called glucosan or glucan)

It is a plant polysaccharides store in grains, tubers, roots  etc. Starch is the major form of stored carbohyhrate in plants cells.

Structure:

Starch granule is form of inner and outer layer i.e. amylase and amylopectin.

(a)                    Amylose(inner layer):

 It constitutes 15-20% of the granule and formed of non branching helical structure or glucose units linked together by a 1-4 glycosidic bonds.

(b)                    Amylopectin(outer layer):

  It constitutes 80-85% of the granule and formed ob branched chain. Each chain is composed of 24-30 glucose units linked together by a 1-6 glucosidic bonds at the branch points.

Sources: It is the most important food source of carbohydrate. It is found in cereals, potatoes, legumes and other vegetables.

Properties:

1.                      Starch gives blue colour with iodine. Amylopectin gives red colour with iodine.

2.                       Partial hydrolysis(digestion) by amylase enzyme gives various forms of dextrins

3.                      Amylase is soluble in cold water and amylopectin  is not soluble in water.

      Fig:Structure of starch  A:Amylose, B: Amylopectin.

                     Glycogen : (also called animal starch)

Glycogen is the storage polysaccharide in animals.

 Structure:

 It is highly branched chain homopolysaccharide. Each branch is composed of 12-14 glucose units, linked together by 1-4 glycosidic bonds and by 1-6 glycosidic bond at branch point(like amylopectin).

Sources:

 Glycogen is the storage form of carbohydrate in human and animals. It is synthesized and stored in liver, muscle and other tissues.

Properties:

It gives red colour with iodine.

It is soluble in water.

Dextrins:

These are hydrolytic products of

Fig: The glycogen molecule

Mucopolysaccharides:

 Mucopolysacchrides are heteroglycans made up of repeating units of sugar derivatives, namely amino sugar and uronic acids. These are more commonly known as Glycosaminoglycans

(GAG).Acetylated amino groups, besides sulfate and carboxyl groups are generally present in GAG structure. The presence of sulfate and carboxyl groups contributes to acidity of the molecules, making them acid mucopolysaccharides.

          Some mucopolysaccharides are found in combination with proteins to form mucoproteins or mucoids or proteoglycans. Mucoproteins may contain up to 95%carbohydrates and 5%protein.

      Mucopolysaccharides are essential components of tissue structure. The extracellular space of tissue (skin, blood vessels, tendons, connective tissue-cartilage) consists of collagen and elastin fibers embedded in a matrix or ground substance. The ground substance is predominantly composed of GAG.

Mucopolysaccharides	Composition	Site of occurence
Hyaluronic acid (sulfate free)	Glucuronic acid and N-acetyl glucosamine	Synovial fluid of joint vitreous humour of eye Umbelical cord, cell membrane, skin
Chondroitin sulfate	Glucuronic acid and N-acetyl galactoseamine	cartilage
Dermatan sulfate	Glucuronic acid and N-acetyl galactoseamine	Widely distributed in animal tissue
Keratan sulfate	Galactose and N-acetyl galactoseamine	Corneal transperancy
Heparin (highly sulphated)	Glucuronic acid and N-acetyl galactoseamine	Blood anticoagulant, Liver, lungs, arterial wall.

GLYCOSIDIC BOND

   It is the linkage formed between OH group of anomeric carbon of one sugar with any OH group of another sugar (or alcohol) resulting in the loss of a water molecule.

    This linkage is involved in the formation of disaccharide and polysaccharides.

Sugar	Source	Importance	Clinical Significance
D-Glucose	Fruit juices. Hydrolysis of starch, cane sugar, maltose and lactose.	The “sugar” of the body. The sugar carried by the blood and the principal one used by the tissues.	Present in the urine (glycosuria) in Diabetes mellitus owing to raised blood glucose (hyperglycemia).
D-Fructose	Fruit juices.Honey, Hydrolysis of cane and of inulin	Can be changed to glucose in the liver and so used in the body	Heredity fructose intolerance leads to fructose accumulation and hypoglycemia.
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.	Failure to metabolize leads to galactosemia and cataract.
Maltose	Digestion by amylase or hydrolysis of starch. Germinating cereals and malt
Lactose	Milk	May occur in urine during pregnancy	In lactase deficiency, malabsorption to diarrhea and flatulence .
Sucrose	Cane and beet sugar. Pineapple, Carrot  roots		In sucrase deficiency , malabsorption leads to diarrhea and flatulence.

 Function of Carbohydrates

1.                They are the most abundant dietary source of energy (4 cal/g) for all organisms.

2.                Carbohydrates are precursors for many organic compounds (fats, aminoacids).

3.                Carbohydrates as glycoproteins and glycolipids participate in the structure of cell membrane and cellular function such as cell growth adhesion and fertilization.

4.                They are structural components of many organisms. These include the fiber (cellulose) of plants, exoskeleton of some insects and cell wall of microorganisms.

5.                Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of body.

6.                Carbohydrates also utilized as raw materials for several industries e.g.paper, plastics,textiles,alcohol etc.

 are those carbohydrates

Carbohydrates are aldehyde or ketone derivative of polyhydric (more than one –OH group) alcohols or compounds that yield these derivatives on hydrolysis. Carbohydrates are often referred to as saccharides (Greek: sachharon -sugar). Carbohydrate are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means hydrates of carbon. It is derived from the observation that the empirical formula for these compounds contain approximately one molecule of water per carbon atom. Thus glucose , C₆H₁₂O₆ and lactose C₁₁H₂₂O₁₁ can be written as C₆(H₂O)₆ and C₁₁(H₂O)₁₁ respectively.These compounds are not hydrates of carbon in the usual chemocal sense. In addition, there are several non-carbohydrate compounds (Eg. Acetic acid C₂(H₂O)₂, lactic acid C₃(H₂O)₃) which also appear as hydrates of carbon. 

Biomedical Importance

  Carbohydrates are widely distributed in plants and animals; they have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize cellulose of the plant framework. Animal can synthesize carbohydrate from lipid and protein, but most animal carbohydrate is derived ultimately from plants.

Glucose is the most important carbohydrate, most dietary carbohydrate is absorbed into the blood stream as glucose and other sugars are converted into glucose in the liver. Glucose is the major metabolic fuel of mammals and a universal fuel of the fetus. Dextrose (glucose solution in dextrorotatory form )is frequently used in medical practice. Glucose is the precursor for the synthesis of all other carbohydrates in the body, including glycogen for storage.; ribose and deoxyribose in nucleic acid and galactose in lactose of milk , in glycolipids and glycoproteins .Disease associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage disease and lactose intolerance.