7 Carbohydrates

CarbohydratesCarbohydrates

Dr Tolunimi Adedeji (MD)Dr Tolunimi Adedeji (MD)

I. OverviewI. Overview

Carbohydrates are the most abundant biomoleculesCarbohydrates are the most abundant biomolecules They provide a significant portion of the energy in the diet They provide a significant portion of the energy in the diet

of most organismsof most organisms They also act as a storage form of energy in the bodyThey also act as a storage form of energy in the body Carbohydrates serve as structural components of many Carbohydrates serve as structural components of many

organismsorganisms General formula is represented as (CHGeneral formula is represented as (CH22O)O)nn – ‘hydrate of – ‘hydrate of

carbon’carbon’

II. Classification and structure of II. Classification and structure of carbohydratescarbohydrates

The simplest The simplest carbohydrates are carbohydrates are monosaccharidesmonosaccharides – can – can be classified based on the be classified based on the number of carbon atomsnumber of carbon atoms

Some common Some common monosaccharides found in monosaccharides found in humans (see figure)humans (see figure)

Carbohydrates with an Carbohydrates with an aldehyde functional group aldehyde functional group are known as are known as aldosesaldoses

Carbohydrates that contain Carbohydrates that contain a ketone moiety are called a ketone moiety are called ketosesketoses (see examples) (see examples)

II. Classification and structure of II. Classification and structure of carbohydratescarbohydrates

MonosaccharidesMonosaccharides, when , when linked with each other, form linked with each other, form glycosidic bondsglycosidic bonds to to generate extended structures generate extended structures (see figure)(see figure)

DisaccharidesDisaccharides consist of two consist of two monosaccharide unitsmonosaccharide units

OligosaccharidesOligosaccharides contain contain from three to twelve from three to twelve monosaccharide unitsmonosaccharide units

PolysaccharidesPolysaccharides are are comprised of more than comprised of more than twelve monosaccharide units, twelve monosaccharide units, and can be hundreds of and can be hundreds of monosaccharides longmonosaccharides long

A. Isomers and epimersA. Isomers and epimers

IsomersIsomers – compounds having – compounds having the same chemical formula but the same chemical formula but different structural formulasdifferent structural formulas

Examples of isomers – fructose, Examples of isomers – fructose, glucose, mannose, galactose – all glucose, mannose, galactose – all have chemical formula Chave chemical formula C66HH1212OO66

EpimersEpimers – monosaccharides – monosaccharides (isomers) that have different (isomers) that have different configurations around one configurations around one specific carbon atomspecific carbon atom

Examples of epimers – glucose Examples of epimers – glucose and galactose are C-4 epimers and galactose are C-4 epimers (see figure)(see figure)

glucose and mannose are C-2 glucose and mannose are C-2 epimersepimers

B. EnantiomersB. Enantiomers

EnantiomersEnantiomers – optically – optically active isomers that are active isomers that are mirror images of each othermirror images of each other

In monosaccharides, the two In monosaccharides, the two enantiomers are each enantiomers are each termed a D- and L-sugar (see termed a D- and L-sugar (see figure)figure)

The vast majority of The vast majority of monosaccharides in humans monosaccharides in humans are D-sugars are D-sugars

C. Cyclization of monosaccharidesC. Cyclization of monosaccharides

The open chain form of The open chain form of monosaccharides exist in solution monosaccharides exist in solution as less than one percent abundantas less than one percent abundant

The ring or cyclized form is the The ring or cyclized form is the more stable form, which occurs more stable form, which occurs when the aldehyde or ketone when the aldehyde or ketone group has reacted with an alcohol group has reacted with an alcohol group on the same group on the same monosaccharidemonosaccharide

Formation of the ring also creates Formation of the ring also creates an an anomeric carbonanomeric carbon at carbon 1 at carbon 1 of an aldose and carbon 2 of a of an aldose and carbon 2 of a ketoseketose

The anomeric carbon gives rise to The anomeric carbon gives rise to two different configurations two different configurations around it - around it - or or configurations configurations (see figure)(see figure)

Interconversion of these anomers Interconversion of these anomers is called is called motarotationmotarotation

C. Cyclization of monosaccharidesC. Cyclization of monosaccharides

If the oxygen atom on the anomeric carbon (carbonyl If the oxygen atom on the anomeric carbon (carbonyl group) of a monosaccharide is not attached to any other group) of a monosaccharide is not attached to any other structure, the sugar is a structure, the sugar is a reducing sugarreducing sugar

A reducing sugar can react with chemical reagents (e.g. A reducing sugar can react with chemical reagents (e.g. Benedict’s solution) upon which the anomeric carbon is Benedict’s solution) upon which the anomeric carbon is oxidized and the chemical reagent reducedoxidized and the chemical reagent reduced

D. Complex carbohydratesD. Complex carbohydrates

Carbohydrates can also be linked to Carbohydrates can also be linked to non-carbohydrate structures via non-carbohydrate structures via glycosidic bonds (e.g. purines and glycosidic bonds (e.g. purines and pyrimidines found in nucleic acids, pyrimidines found in nucleic acids, aromatic rings present in steriods, aromatic rings present in steriods, proteins found in glycoproteins, and proteins found in glycoproteins, and lipids found in glycolipids)lipids found in glycolipids)

The aldose or ketose unit in these The aldose or ketose unit in these structures are called structures are called glycosyl glycosyl residuesresidues

Lactose is, therefore, known as Lactose is, therefore, known as galactosyl-glucose, since it is galactosyl-glucose, since it is comprised of one galactose and one comprised of one galactose and one glucoseglucose

If the moiety on the non-If the moiety on the non-carbohydrate molecule to which the carbohydrate molecule to which the sugar is attached is an –OH group, sugar is attached is an –OH group, the structure is an the structure is an O-glycosideO-glycoside

If the group is an –NHIf the group is an –NH22, the structure , the structure is an is an N-glycosideN-glycoside (see figure) (see figure)

D. Complex carbohydratesD. Complex carbohydrates

Glycosidic bonds between monosaccharide units are Glycosidic bonds between monosaccharide units are named according to the numbers of the connected carbon named according to the numbers of the connected carbon atoms, and also to the position of the anomeric hydroxyl atoms, and also to the position of the anomeric hydroxyl group of the sugar involved in the bondgroup of the sugar involved in the bond

If the anomeric –OH group has an If the anomeric –OH group has an configuration, the configuration, the bond formed is an bond formed is an -bond-bond; if the ; if the configuration is shown, configuration is shown, the bond is a the bond is a -bond-bond

The glycosidic bond in lactose, for instance, is formed The glycosidic bond in lactose, for instance, is formed between carbon 1 of between carbon 1 of -galactose and carbon 4 of glucose - -galactose and carbon 4 of glucose - (1(14) glycosidic bond4) glycosidic bond

Lactose is also a reducing sugarLactose is also a reducing sugar

III. Digestion of carbohydratesIII. Digestion of carbohydrates

Carbohydrate digestion primarily Carbohydrate digestion primarily occurs in the mouth and intestinal occurs in the mouth and intestinal lumenlumen

A mixed animal and plant diet has A mixed animal and plant diet has little monosaccharide present little monosaccharide present (contains disaccharides, (contains disaccharides, oligosaccharides, and oligosaccharides, and polysaccharides)polysaccharides)

Enzymes (disaccharidases and Enzymes (disaccharidases and endoglycosidases) are, therefore, endoglycosidases) are, therefore, required for digestion of required for digestion of carbohydratescarbohydrates

Carbohydrate digestion involves Carbohydrate digestion involves hydrolysis of glycosidic bonds (see hydrolysis of glycosidic bonds (see figure)figure)

These enzymes are usually specific These enzymes are usually specific for the type of glycosyl residue and for the type of glycosyl residue and also for the type of glycosidic bondalso for the type of glycosidic bond


Digestion of carbohydrates begins in the Digestion of carbohydrates begins in the mouthmouth

The principal dietary polysaccharides include The principal dietary polysaccharides include glycogenglycogen (animal derived) and (animal derived) and starchstarch (plant (plant derived, composed of amylose and derived, composed of amylose and amylopectin)amylopectin)

During mastication, salivary During mastication, salivary -amylase -amylase breaks some breaks some (1(14) bonds4) bonds

Humans cannot digest cellulose, due to the Humans cannot digest cellulose, due to the lack of lack of (1(14)-endoglucosidases, which 4)-endoglucosidases, which cleave cleave (1(14) bonds4) bonds

After mastication, the After mastication, the (1(16) bonds present 6) bonds present in glycogen or starch are not broken, and in glycogen or starch are not broken, and smaller branched oligosaccharides remain smaller branched oligosaccharides remain (see figure)(see figure)

Carbohydrate digestion does not continue Carbohydrate digestion does not continue into the stomach (high acidity deactivates into the stomach (high acidity deactivates salivary salivary -amylase)-amylase)


When acidic stomach contents When acidic stomach contents reach the small intestine, they are reach the small intestine, they are neutralized by bicarbonate neutralized by bicarbonate secreted by the pancreassecreted by the pancreas

Pancreatic Pancreatic -amylase continues to -amylase continues to work on carbohydrate digestionwork on carbohydrate digestion

The final digestive stages occur at The final digestive stages occur at the mucosal lining of the upper the mucosal lining of the upper jejunum, and continue into the jejunum, and continue into the small intestine, where several small intestine, where several disaccharidases and disaccharidases and oligosaccharidases act on the oligosaccharidases act on the carbohydrate (see figure)carbohydrate (see figure)

Isomaltase cleaves the Isomaltase cleaves the (1(16) 6) bonds while maltase cleaves bonds while maltase cleaves maltose; sucrase hydrolyzes maltose; sucrase hydrolyzes sucrose; lactase cleaves lactosesucrose; lactase cleaves lactose


Abnormal digestion of Abnormal digestion of carbohydrates can occur when a carbohydrates can occur when a defect in disaccharidase activity defect in disaccharidase activity of the intestinal mucosa causes of the intestinal mucosa causes the passage of undigested the passage of undigested carbohydrate into the large carbohydrate into the large intestine – water from the intestine – water from the mucosa is drawn into the large mucosa is drawn into the large intestine, causing osmotic intestine, causing osmotic diarrheadiarrhea

This can also be accompanied by This can also be accompanied by bacterial fermentation of the bacterial fermentation of the carbohydrate to two- and three-carbohydrate to two- and three-carbon compounds and large carbon compounds and large amounts of COamounts of CO22 and H and H22 gas, gas, which cause abdominal cramps, which cause abdominal cramps, diarrhea, and flatulence (see diarrhea, and flatulence (see figure)figure)


Digestive enzyme deficiencies could also lead to improper Digestive enzyme deficiencies could also lead to improper digestion of carbohydratesdigestion of carbohydrates

Disaccharide intoleranceDisaccharide intolerance has been reported in some has been reported in some infants and children (hereditary)infants and children (hereditary)

This condition may also be manifested as a result of This condition may also be manifested as a result of malnutrition or drug usemalnutrition or drug use

More than one half of the world’s adults are More than one half of the world’s adults are lactose lactose intolerantintolerant; certain races, e.g. African or Asian descent, are ; certain races, e.g. African or Asian descent, are lactase deficient, compared to people of northern European lactase deficient, compared to people of northern European originorigin

Isomaltase-sucrase deficiency also present in about ten Isomaltase-sucrase deficiency also present in about ten percent of Greenland’s Eskimos – inability to digest sucrosepercent of Greenland’s Eskimos – inability to digest sucrose

Diagnosis of an enzyme deficiency can be afforded through Diagnosis of an enzyme deficiency can be afforded through oral tests, e.g. measurement of Horal tests, e.g. measurement of H22S gas in breathS gas in breath

Key concept map for structure of Key concept map for structure of monosaccharidesmonosaccharides

Explain in your own Explain in your own wordswords

glycosidic bondsglycosidic bondsanomeric carbonanomeric carbon

alpha configurationalpha configurationisomersisomers

enatiomersenatiomersglycosyl residuesglycosyl residuesreducing sugarsreducing sugars

draw a flowchart of the digestion of draw a flowchart of the digestion of carbohydratescarbohydrates

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7 Carbohydrates