Basic biochemistry of Carbohydrates suitable for undergraduate students. This presentation has been started from the basics to enable easy understanding.

1. Dr. Riddhi Datta
Semester I
Botany Core Course II (Group A)

2. • Carbohydrates are polyhydroxy aldehydes or ketones, or
substances that yield such compounds on hydrolysis.
• Many, but not all, carbohydrates have the empirical formula
(CH2O)n, [n≥3]; some also contain nitrogen, phosphorus, or sulfur.
• Carbohydrate literally means „hydrates of carbon‟.
• Carbohydrates are the most abundant biomolecules on Earth.
Dr. Riddhi Datta

3. • Certain carbohydrates (sugar and starch) are a dietary staple. Most abundant dietary source of
energy (4 cal/g)
• Insoluble carbohydrate polymers serve as structural and protective elements:
• in the cell walls of bacteria and plants
• in the connective tissues of animals
• lubricate skeletal joints
• participate in recognition and adhesion between cells
• Complex carbohydrate polymers that are covalently attached to proteins or lipids are called
glycoconjugates.
• act as signals that determine the intracellular location or metabolic fate of these hybrid
molecules
• Carbohydrates are precursors of many organic molecules (fats, amino acids, etc.)
• They serve as storage form of energy (Ex- Glycogen, Starch) Dr. Riddhi Datta

4. • The word “saccharide” is derived from the Greek „sakcharon’, meaning “sugar”
• Monosaccharides (simple sugars):
• Consist of a single polyhydroxy aldehyde or ketone unit.
• Oligosaccharides:
• consist of short chains of monosaccharide units, or residues (2-10), joined by characteristic
linkages called glycosidic bonds.
• Disaccharides:
• Consists of two monosaccharide units joined by glycosidic bond
• Ex- Sucrose (Glucose + Sucrose)
• Polysaccharides:
• sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds
or thousands of units
• Ex- Cellulose, Glycogen
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5. • Simplest carbohydrates that can not be hydrolyzed to smaller carbohydrates.
• General chemical formula of unmodified monosaccharide is (C.H2O)n where n≥3
• Consist of a single polyhydroxy aldehyde or ketone unit.
• The most abundant monosaccharide in nature is the six-carbon sugar D-glucose.
• Monosaccharides of more than four carbons tend to have cyclic structures.
• Ex- Glyceraldehyde, Glucose, fructose, etc.
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6. • Classified according to 3 different characteristics:
• Placement of its carbonyl group
• Number of carbon atoms present
• Chiral handedness
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7. • Classes based on placement of its carbonyl group:
• ALDOSE: Functional group is an aldehyde group (-CHO)
• Ex- Glyceraldehyde, Glucose, etc
• KETOSE: Functional group is a keto group (>C=O)
• Ex- Dihydroxyacetone, Fructose, etc.
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8. • Classes based on number of carbon atoms present:
• Triose (3 C)
• Tetrose (4 C)
• Pentose (5 C)
• Hexose (6 C)
• Heptose (7 C)
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9. Dr. Riddhi Datta

10. Dr. Riddhi Datta

11. o Stereoisomers: Compounds that have same structural formulae but differ in their spatial
configuration.
o A carbon is said to be asymmetric (chiral) when it is attached to four different atoms or groups.
o The number of asymmetric carbon atoms (n) determines the possible isomers of a given
compound which is equal to 2n.
o Stereoisomerism is a characteristic feature of all sugars except Dihydroxyacetone.
o Example-
Glucose has 4 asymmetric carbon atoms. No. of isomers = 24 = 16
Glyceraldehyde has 1 asymmetric carbon atom. No. of isomers = 21 = 2
Dihydroxyacetone has no asymmetric carbon atoms. Hence, no isomer is possible.
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12. • Classes based on chiral handedness:
• D and L isomers: Assignment of D or L isomer is made
according to the orientation of the asymmetric carbon
atom furthest from the carbonyl group.
• In a standard Fischer projection if the hydroxyl group is on
the right, the molecule is D sugar, and if the hydroxyl
group is on the left, the molecule is L sugar.
• D-sugars are biologically more common.
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13. • Optical activity of sugars:
• It is the characteristic feature of compounds with asymmetric carbon atoms. When a beam of
polarized light is passed through a solution of an optical isomer, it will be rotated to either
the right or left.
• The terms dextrorotatory (+) and levorotarory (-) are used to compounds that respectively
rotate the plane of polarized light to the right or to the left.
• It may be noted that the D and L configurations of sugars are primarily based on the
structure, optical activities may be different.
• Racemic mixture: If D and L isomers are present in equal concentration, it is known as
racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity, since the
dextro- and levorotatory activities cancel each other.
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14. • Epimers
• If two monosaccharides differ from each other in their configuration around a single specific
carbon (other than anomeric carbon), they are referred to as epimers to each other.
• D-glucose and D-mannose differ only in the stereochemistry at C-2, are epimers.
• D-glucose and D-galactose which differ at C-4, are epimers.
Inter-conversions of epimers (eg.- glucose
to galactose and vice versa) is known as
epimerization and is catalyzed by a
group of enzymes called epimerases
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15. • In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in
the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has
formed a covalent bond with the oxygen of a hydroxyl group along the chain.
• The formation of these ring structures is the result of a general reaction between alcohols and
aldehydes or ketones to form derivatives called hemiacetals or hemiketals.
• These structures contain an additional asymmetric carbon atom and thus can exist in two
stereoisomeric forms.
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16. • D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group
at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and
producing two stereoisomers, designated as α and β.
• These six-membered ring compounds are called pyranoses because they resemble the six
membered ring compound pyran.
• The systematic names for the two ring forms of D-glucose are α-D-glucopyranose and β-D-
glucopyranose.
• Only aldoses having five or more carbon atoms can form pyranose rings.
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17. • Aldohexoses also exist in cyclic forms having five membered rings, which,
because they resemble the five membered ring compound furan, are called
furanoses.
• The six-membered aldopyranose ring is much more stable than the aldofuranose
ring and predominates in aldohexose solutions.
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18. Anomers
• Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or
hemiketal carbon atom are called anomers.
• The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon.
• In case of α-anomer, the –OH group held by anomeric carbon is on the opposite side of the
–CH2OH group of the sugar ring. The opposite is true for β-anomers.
• The α- and β-anomers of D-glucose interconvert in aqueous solution by a process called
mutarotation.
• Thus, a solution of α-D-glucose and a solution of β-D-glucose eventually form identical
equilibrium mixtures having identical optical properties. This mixture consists of about one-third
α-D-glucose (36%), two-thirds β-D-glucose (63%), and very small amounts of the linear and
five-membered ring (glucofuranose) forms (1%).
α-D-glucose Equilibrium mixture β-D-glucose
+112.2° +52.7° +18.7°
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19. • Ketohexoses also occur in α and β anomeric forms.
• In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto
group at C-2, forming a furanose (or pyranose) ring containing a hemiketal
linkage.
• D-Fructose readily forms the furanose ring, the more common anomer of this
sugar in combined forms or in derivatives is D-fructofuranose.
• The specific optical rotation of fructose is -92° at equilibrium.
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20. • Monosaccharides can be oxidized by relatively mild oxidizing
agents such as ferric (Fe3+) or cupric (Cu2+).
• The carbonyl carbon is oxidized to a carboxyl group.
• Sugars capable of reducing ferric or cupric ion are called
reducing sugars. They have free aldehyde or ketone group
present in their structure.
• Ex- Glucose
• Sugars not capable of reducing ferric or cupric ion are called
non-reducing sugars. They do not have free aldehyde or ketone
group present in their structure.
• Ex- Sucrose
• This property is the basis of Fehling‟s reaction, a qualitative test
for the presence of reducing sugar.
Dr. Riddhi Datta

21. There are a number of sugar
derivatives in which a hydroxyl
group in the parent compound is
replaced with another substituent,
or a carbon atom is oxidized to a
carboxyl group.
• In amino sugars, an –NH2 group
replaces one of the -OH groups
in the parent
• hexose.
• Substitution of –H for –OH
produces a deoxy sugar.
• The acidic sugars contain a
carboxylate group, which confers
a negative charge at neutral pH.
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22. • Sugar acids: Oxidation of aldehyde or primary alcohol groups in the
monosaccharide results in sugar acids.
• The acidic sugars contain a carboxylate group, which confers a negative charge at
neutral pH.
• Examples:
• Gluconic acid is produced from glucose by oxidation of aldehyde group.
• Glucuronic acid is formed from glucose by oxidation of primary alcohol group (C6).
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23. • Amino sugars: When one or more hydroxyl groups of the monosaccharide are
replaced by amino groups, the products formed are called amino sugars.
• They are present as constituents of heteropolysaccharides.
• Examples:
• D-glucosamine
• D-galactosamine
• They are sometimes acetylated.
• Examples:
• N-acetyl-D-glucosamine
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24. • Deoxysugars: They contain one oxygen less than that of their parent molecule.
• The groups –CHOH and –CH2OH become –CH2 and –CH3 due to absence of one
oxygen atom.
• Examples:
• D-2-Deoxyribose
• L-Rhamnose
• L-Fucose
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25. • Sugar alcohols: Sugar alcohols (polyols) are produced by reduction of aldoses or
ketoses.
• Examples:
• Sorbitol from glucose
• Mannitol from mannose
• Alditols: The monosaccharides on reduction yield polyhydroxy alcohols known as
alditols.
• Examples:
• Ribitol (constituent of flavin coenzymes)
• Glycerol (Component of lipid)
• Xylitol (Sweetener used in sugarless gums and candies)
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26. • Disaccharides consist of two monosaccharides joined
covalently by an O-glycosidic bond, which is formed
when a hydroxyl group of one sugar reacts with the
anomeric carbon of the other.
• Example: maltose, lactose, and sucrose
• Glycosidic bonds are readily hydrolyzed by acid but
resist cleavage by base. Thus disaccharides can be
hydrolyzed to yield their free monosaccharide
components by boiling with dilute acid.
• N-glycosyl bonds join the anomeric carbon of a sugar
to a nitrogen atom in glycoproteins and nucleotides.
• General formula: Cn(H2O)n-1
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27. • The oxidation of a sugar‟s anomeric carbon by cupric or ferric ion (the reaction that
defines a reducing sugar) occurs only with the linear form, which exists in equilibrium
with the cyclic form(s).
• When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot
take the linear form and therefore becomes a non-reducing sugar.
• The end of a chain with a free anomeric carbon (one not involved in a glycosidic
bond) is commonly called the reducing end.
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28. • The disaccharide maltose contains
two D-glucose residues joined by a
glycosidic linkage between C-1 (the
anomeric carbon) of one glucose
residue and C-4 of the other.
• Because the disaccharide retains a
free anomeric carbon (C-1 of the
glucose residue on the right), maltose
is a reducing sugar.
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29. • By convention, the name describes the compound with its nonreducing end to the left.
• Give the configuration (α or β) at the anomeric carbon joining the first monosaccharide unit (on
the left) to the second.
• Name the nonreducing residue; to distinguish five- and six-membered ring structures, insert
“furano” or “pyrano” into the name.
• Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow
connecting the two numbers; for example, (1 4) shows that C-1 of the first-named sugar residue
is joined to C-4 of the second.
• Name the second residue.
• If there is a third residue, describe the second glycosidic bond by the same conventions.
Short name:
Glc(α1 4)Glc
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30. • Sucrose (cane sugar) is made up of α-D-glucose and β-D-fructose linked by a glycosidic
bond (α1 β2). The reducing groups of glucose and fructose are involved in glycosidic
bond formation. Hence, sucrose is non-reducing sugar and it cannot form osazones.
• The systematic name of sucrose is α-D-glucopyranosyl-(1 2)- β-D-fructofuranoside. This
indicates:
• It is composed of two monosaccharides: glucose and fructose
• Ring type: Glucose is pyranose and fructose is furanose
• Linkage: oxygen on C1 of α-D-glucose is linked to C2 of β-D-fructose
• Suffix –oside and indicates that the anomeric carbon of both the monosaccharides
participate in glycosidic bond formation
• Sucrose is a major carbohydrate produced in
photosynthesis. It has the advantage as storage
and transport as its functional groups are held
together and are protected from oxidative attacks.
• Intestinal enzyme, sucrase hydrolyze sucrose to
glucose and fructose.
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31. Inversion of sucrose:
• Sucrose is dextrorotatory (+66.5°). But when hydrolyzed, it becomes levorotatory (-
28.2°). The process of change in optical rotation from dextrorotatory(+) to
levorotatory (-) is referred to as inversion. The hydrolyzed mixture of sucrose,
containing glucose and fructose, is known as invert sugar.
• Hydrolysis of sucrose by sucrase or dilute acid yeilds one molecule of glucose and
one molecule of fructose.
• Sucrose first splits into α-D-glucopyranose (+) and β-D-fructofuranose (+). But β-D-
fructofuranose is less stable and gets converted into β-D-fructopyranose (-). The
overall effect in the mixture becomes levorotatory (-).
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32. • Lactose (milk sugar) is composed of β-D-galactose and β-D-glucose held together by
β-(1 4) glycosidic bond.
• The anomeric carbon of C1 of glucose is free. Hence lactose exhibits reducing
properties and forms osazones (powder-puff or hedgehog shape).
• The systematic name is β-D-galactopyranosyl-(1 4) β-D-glucopyranose.
• It is hydrolyzed by intestinal enzyme lactase into glucose and galactose.
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33. • Maltose (malt sugar) is produced during digestion of starch by enzyme amylase.
• Maltose is composed of two α-D-glucose units held together by α(1 4)
glycosidic bond. A free aldehyde group is present on C1 of the second glucose
unit and hence maltose exhibits reducing properties and forms osazones
(sunflower shaped).
• It can be hydrolyzed by dilute acid or enzyme maltase.
• In isomaltose, the glucose units are held together by α(1 6) glycosidic bond.
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34. • It is identical to maltose, except that in it the linkage is β(1 4) glycosidic bond.
• It is formed during hydrolysis of cellulose.
• Raffinose (trisaccharides): Fructose+Galactose+Glucose
• Stachyose (Tetrasaccharide): Galactose+Galactose+Glucose+Fructose
• Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose
• It is identical to maltose, except that in it the linkage is (α1 α1) glycosidic bond.
• It is formed during hydrolysis of cellulose.
• It is a non-reducing sugar.
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35. • Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their
derivatives linked by glycosidic linkages are called polysaccharides.
• They are primarily concerned with 2 important functions:
• Structural role
• Storage of energy
• Polysaccharides can be linear or branched. The occurrence of branched polysaccharides is
due to the fact that glycosidic linkages can be formed at any one of the –OH groups of a
monosaccharide.
• Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and
form colloids with water.
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36. Polysaccharides are of two types:
• Homopolysaccharides (Homoglycans): They,
on hydrolysis, yield only one type of
monosaccharide. They are named based on the
nature of the monosaccharide unit.
Example: Glucan (polymer of glucose),
Fructosan (polymer of fructose)
• Heteropolysaccharides (heteroglycans): They,
on hydrolysis, yield a mixture of a few types of
monosaccharide units or their derivatives.
Example: Peptidoglycan (polymer of N-
acetylglucosamine and N-acetylmuramic
acid residues)
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37. • Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals.
• Starch is a homopolysaccharide composed of D-glucose units held by glycosidic bonds.
• It is known as glucosan or glucan
• Starch consists of two polysaccharide components:
Water soluble amylose (15-20%)
Water insoluble amylopectin (80-85%)
• Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by (α1 4) glycosidic
linkage.
• Amylopectin is a branched chain with (α1 6) glycosidic bonds at the branching points and (α1 4)
glycosidic bonds everywhere else.
• Starches are hydrolyzed by amylases (pancreatic or salivary) to liberate dextrins and finally maltose and
glucose units. Amylase acts specifically on the (α1 4) glycosidic bonds.
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38. Dextrins
• These are the breakdown products of starch by the enzyme amylase or dilute acids.
• Starch is hydrolyzed through different dextrins and finally to maltose and glucose.
• The various intermediates (identified by iodine coloration) are soluble starch (blue),
amylodextrin (violet), erythrodextrin (red) and achrodextrin (no colour).
Inulin
• Inulin is a polymer of fructose.
• It occurs in dahlia bulbs, garlic, onion, etc.
• It is a low molecular weight (~ 5000) polysaccharide easily soluble in water.
• Inulin is not utilized by the body.
• It is used for assessing kidney function through measurement of glomerular filtration rate
(CFR).
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39. • Cellulose occurs extensively in plants and is totally absent in animals.
• Cellulose is composed of β-D-glucose units linked by β(1 4) glycosidic bonds.
• Cellulose can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic
bonds. Hydrolysis of cellulose yields a disaccharide, cellobiose, which is further broken down
to β-D-glucose units.
• It is a major constituent of fibers, the non-digestible carbohydrate.
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40. • Glycogen is the main storage polysaccharide of animal cells.
• Like amylopectin, glycogen is a polymer of (α1 4)-linked subunits of glucose, with
(α1 6)-linked branches, but glycogen is more extensively branched (on average, every 8 to
12 residues) and more compact than starch.
• Glycogen is especially abundant in the liver, it is also present in skeletal muscle.
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41. • Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.
• The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2
with an acetylated amino group.
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42. • The rigid component of bacterial cell walls is a heteropolymer of alternating (β1 4)-linked
N-acetylglucosamine and N-acetylmuramic acid residues.
• The enzyme lysozyme kills bacteria by hydrolyzing the (β1 4) glycosidic bond between N-
acetylglucosamine and Nacetylmuramic acid.
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43. Dr. Riddhi Datta