Dr. Riddhi Datta

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis.

Dr. Riddhi Datta

• Many, but not all, carbohydrates have the empirical formula (CH₂O)_n, [n≥3]; some also contain nitrogen, phosphorus, or sulfur.

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• Carbohydrate literally means ‘hydrates of carbon’.

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• Carbohydrates are the most abundant biomolecules on Earth.

Occurrence & Function

• Certain carbohydrates (sugar and starch) are a dietary staple and abundant dietary source of energy (4 cal/g)

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• 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

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Dr. Riddhi Datta

Occurrence & Function

• Complex carbohydrate polymers that are covalently attached to proteins or lipids are called glyco-conjugates.
• act as signals that determine the intracellular location or metabolic fate of these hybrid molecules

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• Carbohydrates are precursors of many organic molecules (fats, amino acids, etc.)

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• They serve as storage form of energy (Ex- Glycogen, Starch)

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Classification

• The word “saccharide” is derived from the Greek ‘sakcharon’, meaning “sugar”

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• Monosaccharides (simple sugars): Consist of a single polyhydroxy aldehyde or ketone unit.

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• Oligosaccharides: Consist of short chains of monosaccharide units, or residues (2-10), joined by characteristic linkages called glycosidic bonds.

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Glucose

Classification

• Disaccharides:
• Consists of two monosaccharide units joined by glycosidic bond
• Ex- Sucrose (Glucose + Fructose)
• 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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Monosaccharides

• Simplest carbohydrates that can not be hydrolyzed to smaller carbohydrates.

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• General chemical formula of unmodified monosaccharide is (C.H₂O)_n where n≥3

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• Consist of a single polyhydroxy aldehyde or ketone unit.

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Monosaccharides

• The most abundant monosaccharide in nature is the six-carbon sugar D-glucose.

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• Monosaccharides of more than four carbons tend to have cyclic structures.

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• Ex- Glyceraldehyde, Glucose, Fructose, etc.

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Classification of monosaccharides

• Classified according to 3 different characteristics:
• Placement of its carbonyl group
• Number of carbon atoms present
• Chiral handedness

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Classification based on placement of 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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Classification based on number of carbon atoms

• Triose (3 C)
• Tetrose (4 C)
• Pentose (5 C)
• Hexose (6 C)
• Heptose (7 C)

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Stereoisomers

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• Stereoisomers: Compounds that have same structural formulae but differ in their spatial configuration.

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• A carbon is said to be asymmetric (chiral) when it is attached to four different atoms or groups.

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• The number of asymmetric carbon atoms (n) determines the possible number of isomers of a given compound which is equal to 2ⁿ.

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• Stereoisomerism is a characteristic feature of all sugars except Dihydroxyacetone.

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• 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.

Classification 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.

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• 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.

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• D-sugars are biologically more common.

Dr. Riddhi Datta

Optical activity of sugars

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• 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.

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• The terms dextrorotatory (+) and levorotarory (-) are used to compounds that respectively rotate the plane of polarized light to the right or to the left.

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Optical activity of sugars

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• It may be noted that the D and L configurations of sugars are primarily based on the structure, optical activities may be different.

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• Racemic mixture: If dextrorotatory and levorotatory 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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Epimers

• If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric or carbonyl 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.

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• 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

Common Monosaccharides Have Cyclic Structures

• 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.

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• 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.

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• These structures contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms.

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Common Monosaccharides Have Cyclic Structures

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Hemiacetal formation

Hemiketal formation

• 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.

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• These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran.

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• The systematic names for the two ring forms of D-glucose are α-D-glucopyranose and β-D-glucopyranose.

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• Only aldoses having five or more carbon atoms can form pyranose rings.

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• Aldohexoses also exist in cyclic forms having five membered rings, which, because they resemble the five membered ring compound furan, are called furanoses.

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• The six-membered aldopyranose ring is much more stable than the aldofuranose ring and predominates in aldohexose solutions.

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• Ketohexoses, like fructose, forms five membered furanose rings.

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Dr. Riddhi Datta

Anomers

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• Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers.

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• The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon.

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• 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.

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• The α- and β-anomers of D-glucose interconvert in aqueous solution by a process called mutarotation.

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• 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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• Ketohexoses also occur in α and β anomeric forms.

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• 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.

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• D-Fructose readily forms the furanose ring, the more common anomer of this sugar in combined forms or in derivatives is D-fructofuranose.

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• The specific optical rotation of fructose is -92° at equilibrium.

Dr. Riddhi Datta

Monosaccharides Are Reducing Agents

• 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.

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Dr. Riddhi Datta

Let’s sum up

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds upon hydrolysis.
• Monosaccharides: Single sugar units (e.g., glucose, fructose).
• Oligosaccharides: Short chains of 2 to 10 monosaccharide units linked by glycosidic bonds (e.g., sucrose, lactose).
• Disaccharides: 2 monosaccharide units linked by a glycosidic bond (e.g., sucrose).
• Polysaccharides: Long polymers containing more than 10—and often hundreds or thousands—of monosaccharide units (e.g., starch, glycogen).
• Monosaccharides are classified as aldoses (containing an aldehyde group) or ketoses (containing a keto group).
• Isomerism: Except for dihydroxyacetone, all sugars contain asymmetric (chiral) carbon atoms, allowing for the existence of stereoisomers (D and L forms). They are also categorized by their optical activity as dextrorotatory (+) or levorotatory (-).

Dr. Riddhi Datta

Let’s sum up

• Cyclic Structures: In aqueous solution, monosaccharides with five or more carbons predominantly form cyclic structures known as pyranoses (six-membered rings) or furanoses (five-membered rings).
• Reducing Sugars: Sugars with a free aldehyde or ketone group can reduce cupric or ferric ions; these are termed reducing sugars (e.g., glucose, maltose). Others can not reduce cupric or ferric ions and are called non-reducing sugars (e.g., sucrose).
• Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers (α and β forms).
• If they differ from each other in their configuration around a single specific carbon (other than anomeric), they are referred to as epimers to each other.

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Disaccharides

• 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.

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• Example: maltose, lactose, and sucrose

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• 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.

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• N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

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• General formula: C_n(H₂O)_n-1

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• 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).

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• 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.

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• 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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• 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.

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• 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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Nomenclature of disaccharides (or oligosaccharides)

• 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.

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Short name:

Glc(α1 4)Glc

Sucrose

• 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.

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• 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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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.

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• Hydrolysis of sucrose by sucrase or dilute acid yields one molecule of glucose and one molecule of fructose.

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• 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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Lactose

• 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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Maltose

• 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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Cellobiose

• It is identical to maltose, except that in it the linkage is β(1 4) glycosidic bond.
• It is formed during hydrolysis of cellulose.

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Examples of other oligosaccharides

• Raffinose (trisaccharides): Fructose+Galactose+Glucose
• Stachyose (Tetrasaccharide): Galactose+Galactose+Glucose+Fructose
• Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose

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Trehalose

• 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.

Polysaccharides

• Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their derivatives linked by glycosidic linkages are called polysaccharides.

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• They are primarily concerned with 2 important functions:
• Structural role
• Storage of energy

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• 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.

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• Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and form colloids with water.

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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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Starch

• 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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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).

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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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Cellulose

• 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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Glycogen

• 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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Chitin

• 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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Peptidoglycan

• The rigid component of bacterial cell walls is a heteropolymer of alternating (β1 4)-linked N-acetylglucosamine and N-acetylmuramic acid residues.

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• The enzyme lysozyme kills bacteria by hydrolyzing the (β1 4) glycosidic bond between N-acetylglucosamine and Nacetylmuramic acid.

Dr. Riddhi Datta

Sugar derivatives

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.

Dr. Riddhi Datta

• 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.

Sugar derivatives

• 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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Dr. Riddhi Datta

Sugar derivatives

• 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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Sugar derivatives

• 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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Sugar derivatives

• Sugar alcohols: Sugar alcohols (polyols) are produced by reduction of aldoses or ketoses.
• Examples:
• Sorbitol from glucose
• Mannitol from mannose

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• 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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Let’s sum up

• Disaccharides: Formed by two monosaccharides joined by an O-glycosidic bond.
• Sucrose: Glucose + Fructose; it is a non-reducing sugar.
• Lactose: Galactose + Glucose; found in milk.
• Maltose: Two glucose units; produced during starch digestion.
• Polysaccharides: Classified into two types:
• Homopolysaccharides: Yield only one type of monosaccharide upon hydrolysis (e.g., starch, glycogen, cellulose, and chitin).
• Heteropolysaccharides: Yield a mixture of different monosaccharides or derivatives (e.g., peptidoglycan in bacterial cell walls).

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Dr. Riddhi Datta

Let’s sum up

The sugar derivatives can be classified into four categories:

• Sugar Acids (Oxidation): Formed when the aldehyde or alcohol groups are oxidized to carboxyl groups, giving the molecule a negative charge (e.g., Gluconic acid, Glucuronic acid).
• Amino Sugars (Substitution): An –OH group is replaced by an amino (–NH2) group, which is often acetylated. These are vital for structural polymers like chitin and bacterial cell walls (e.g., Glucosamine, N-acetylglucosamine).
• Deoxy Sugars (Substitution): An –OH group is replaced by a hydrogen atom. The most notable example is 2-Deoxy-D-ribose, the sugar component of DNA.
• Sugar Alcohols (Reduction): The carbonyl group is reduced to a hydroxyl group, creating polyols used as sweeteners or metabolic precursors (e.g., Sorbitol, Mannitol, Glycerol, and Xylitol).

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Dr. Riddhi Datta

Thank you

Dr. Riddhi Datta