Dental Biochemistry 1- (5)

Chemistry and digestion of carbohydrates

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DEFINITION

Carbohydrates are organic substances composed of carbon, hydrogen and

oxygen.

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Function of carbohydrates:

• Carbohydrates are the main sources of energy in the body. Brain cells and RBCs are almost wholly dependent on carbohydrates as the energy source. Energy production from carbohydrates will be 4 Kcal/g.

• Storage form of energy (starch and glycogen).

• Excess carbohydrate is converted to fat.

• Glycoproteins and glycolipids are components of cell membranes and receptors,

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• Structural unit of many organisms: Cellulose of plants; exoskeleton of insects, cell wall of microorganisms, mucopolysaccharides as ground substance in higher organisms.

• Important part of nucleic acids and free nucleotides and coenzyme.

• Major antigens are carbohydrates in nature, e.g., blood group substance.

• Has a biological role as a part of hormones and their receptors and enzymes.

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Nomenclature • Molecules having only one actual or potential sugar

group [(CH2O)n] are called monosaccharides (e.g.

C6H12O6); they cannot be further hydrolyzed into smaller units.

• When two monosaccharides are combined together with elimination of a water molecule, it is called a disaccharide (e.g. C12H22O11).

• Trisaccharides contain three sugar groups. Further addition of sugar groups will correspondingly produce tetrasaccharides, pentasaccharides and so on, commonly known as oligosaccharides.

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• When more than 10 sugar units are combined, they are generally named as polysaccharides.

• Polysaccharides having only one type of monosaccharide units are called homopolysaccharides and those having different monosaccharide units are called heteropolysaccharides.

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Monosaccharides

They have the common formula

(CH2O)n, where n = 3 or some

larger number

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CLASSIFICATION OF MONOSACCHARIDES

Can be carried out by one of two methods: 1) According to the number of carbon atoms

: Trioses, Tetroses, Pentoses,

Hexoses, Heptoses, Octoses.

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• 2) According to the characteristic carbonyl group:

Aldehyde group or ketone group

a) Aldo sugars: Aldoses :

Monosaccharides containing aldehyde group e.g. glucose, ribose, erythrose and glyceraldehyde.

b) Keto sugars: Ketoses :

Monosaccharides containing ketone group e.g. fructose, ribulose and dihydroxyacetone.

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STEREOISOMERS • Compounds having same structural formula, but

differ in spatial configuration are known as stereoisomers.

• All monosaccharides can be considered as molecules derived from glyceraldehyde.

• Depending on the configuration of Hand OH around the reference carbon atom, the two mirror forms are designated as Land D forms.

• All naturally occurring sugars are D sugars.

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of Aldoses Epimerism

• When sugars are different from one another, only in configuration with regard to a single carbon atom (other than the reference carbon atom), they are called epimers.

• For example, glucose and mannose

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Anomerism of Sugars

• This is explained by the fact that D-glucose has two anomers, alpha and beta varieties.

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Monosaccharides of physiologic importance

1-Pentoses: • They are sugars containing 5 carbon

atoms. Ribose is a constituent of RNA while deoxyribose is seen in DNA.

• Ribose is also seen in co-enzymes

such as ATP and NAD.

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Hexoses Galactose, Mannose and Glucose

• They are the common aldohexoses.

• Glucose is the sugar in human blood. It is the major source of energy.

• Mannose is a constituent of many glycoproteins. Mannose was isolated from plant mannans; hence the name.

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b) D-Fructose : ( Fruit sugar = Levulose). It is found in fruits, honey

and obtained from sucrose and inulin by hydrolysis.

c)D-Galactose : It is obtained from hydrolysis of lactose (milk sugar). .

It is a constituent of galactolipids, glycoprotein.

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DlSACCHARIDES

• When two monosaccharides are combined together by glycosidic linkage, a disaccharide is formed. The important disaccharides are sucrose, maltose, isomaltose and lactose.

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. Sucrose1 • It is the sweetening agent known as cane sugar. It is

present in sugarcane and various fruits. Hydrolysis of sucrose will produce one molecule of glucose and one molecule of fructose.

• ii. The enzyme producing hydrolysis of sucrose is called sucrase.

• Sucrose is not

a reducing sugar.

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. Lactose2 • Reducing disaccharide.

• Capable of forming osazone.

• It is only found in milk.

• By acid or lactase enzyme in the intestine,

it yields D-galactose and D-glucose

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. Maltose3 • Maltose contains two glucose residues with

alpha1,4 linkage.

• It is found in germinating cereals and malt.

• Intermediate product of the action of amylases on starch. It is reducing disaccharide

• Hydrolysis: By acid or by maltase enzyme into

two D-glucose units.

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Isomaltose. 4

• It is also a reducing sugar. It contains 2 glucose units combined in alpha -1, 6 linkage.

• Partial hydrolysis of glycogen and starch produces isomaltose.

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POLYSACCHARIDES

• These are polymerized products of many monosaccharide units.

• They may be homoglycans composed of single kind of monosaccharides, e.g. starch. glycogen and cellulose.

• Heteroglycans are composed of two or more different monosaccharides.

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Starch. 1 • It is the reserve carbohydrate of plant as in

potatoes, rice, wheat. 10-20% is soluble part called amylose. The insoluble part is called amylopectin.

• Amylose is made up of glucose units with alpha-1,4 glycosidic linkages to form an unbranched long chain.

• Amylopectin is also made up of glucose units, but is highly branched with molecular weigh more than 1 million. The branching points are made by alpha-1,6 linkage (similar to isomaltose) .

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2. Glycogen • It is the reserve carbohydrate in animals. It is

stored in liver and muscle. About 5% of weight of liver is made up by glycogen.

• Glycogen is composed of glucose units joined by alpha-1 ,4 and alpha-1 ,6 glycosidic linkages.

3. Cellulose • It is the chief carbohydrate in plants.

• The enzyme act on hydrolysis of cellulose is absent in animal and human digestive system, and hence cellulose cannot be digested.

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4. Inulin • It is the reserve carbohydrate present in onion,

garlic, etc.

• 5. Chitin • It is present in exoskeletons of crustacea and

insects

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B- Heteroglycan: These are polysaccharides containing more than one

type of sugar residues.

1.Agar: • It is prepared from sea weeds and contains

galactose, glucose and other sugars.

• It is used as a supporting medium for electrophoresis.

• 2. Mucopolysaccharides: • Mucopolysaccharides or glycosaminoglycans (GAG)

are carbohydrates containing uronic acid and amino sugars.

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3. Hyaluronic acid: • It is present in connective tissues, tendons,

4. Heparin: • It is an anticoagulant

• 5. Keratan sulphate: • It is found in cornea and tendons.

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Glycoproteins and Mucoproteins

• When the carbohydrate chains are attached to a polypeptide chain it is called a proteoglycan.

• If the carbohydrate content is less than 10%, it is generally named as a glycoprotein.

• If the carbohydrate content is more than 10% it is a mucoprotein.

• They are seen in almost all tissues and cell membranes.

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Digestion of carbohydrates

• Cooking helps in breaking of glycosidic linkages in polysaccharides and thus makes the digestion process easier.

• In the diet carbohydrates are available as polysaccharides (starch, glycogen), and to a minor extent, as disaccharides (sucrose and lactose).

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• This process of digestion starts in mouth by the salivary alpha-amylase. However, the time available for digestion in the mouth is limited. The gastric hydrochloric acid will inhibit the action of salivary amylase.

• In the pancreatic juice another alpha-amylase is available which will hydrolyze the alpha-1,4 glycosidic linkages randomly, so as to produce smaller subunits like maltose, isomaltose, dextrin and oligosaccharides.

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• The intestinal juice (succus entericus) and brush border of intestinal cells contain enzymes, which will hydrolyze disaccharides into component monosaccharaides.

• These enzymes are sucrase, maltase, isomaltase and lactase.

• The monosaccharides are then absorbed.

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Lactose intolerance

• This is produced by the deficiency of lactase. This enzyme hydrolyses lactose to glucose and galactose.

• In this condition, lactose accumulate in the gut produce irritant diarrhea.

• If milk is withdrawn temporarily, the diarrhea will be limited. Curd is also an effective treatment, because the lactobacilli present in curd contains the enzyme lactase.

• Lactase activity is high during infancy and it decreases to adult levels by 5-7 years of age.

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Absorption of carbohydrates

• Only monosaccharides are absorbed by the intestine. Minute quantities of disaccharides that may be absorbed, are immediately eliminated through kidneys.

• The duodenum and upper jejunum absorb the bulk of the dietary sugars.

• Insulin is not required for the uptake of glucose by intestinal cells.

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• However, different sugars have different mechanisms of absorption.

1. galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions (sodium-dependent monosaccharide transporter SGluT).

2. Fructose uptake requires a sodium-independent monosaccharide transporter (GLUT-5) for its absorption. 3. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2.

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