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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