btt-301: carbohydrates

BTT-301: Carbohydrates

Program B.Sc

Subject Biotechnology

Semester 3

University Bangalore University

Session 19

Carbohydrates

Carbohydrates

Introduction and structural aspects of monosaccharides

Recap

• Coenzymes

• Cofactors

Learning Objectives

• To study the role of carbohydrates and its classification in addition to introduction

• To understand stereoisomerism, optical activity, configuration of sugars

• To learn the Fischer projection, Pyranose and Furanose structure of Glucose & Fructose

• To study the chemical reactions like Tautomerization, Oxidation, Reduction, Dehydration, Osazone formation and formation of esters

• To understand the role of physiologically important glycosides & derivatives of monosaccharides To learn about reducing and non– reducing disaccharides

• To learn about the plant & animal Homopolysaccharides

• To understand the various Heteropolysaccharides present in the

animal kingdom

Session Outcomes

Importance of carbohydrates and classification based on sugar

units will be known to students and structures, optical

activity of sugars will also be known

Introduction to carbohydrates

• Provides energy through their oxidation

• Supplies carbon for the synthesis of cell components

• Serves as a stored form of chemical energy

• Forms a part of the structural elements of some cells and tissues

Carbohydrates are polyhydroxy aldehydes or ketones, or

substances that yield such compounds upon hydrolysis.

Example:

(Source: dlc.dcccd.edu)

Classification of carbohydrates

Monosaccharide – a single polyhydroxy aldehyde or ketone

unit

Disaccharide – composed of two monosaccharide units

Polysaccharide – very long chains of linked monosaccharide

units

Monosaccharides

Name Formula Aldoses

(Aldo sugars)

Ketoses

(Keto sugars)

Trioses C3H6O3 Glycerose Dihydroxyacetone

Tetroses C4H8O4 Erythrose Erythrulose

Pentoses C5H1005 Ribose Ribulose

Hexoses C6H12O6 Glucose Fructose

Heptoses C7H14O7 Glucoheptose Sedoheptulose

Structure of Monosaccharides

(Source: bioinfo.org.cn)

Oligosaccharides

Disaccharides: Sucrose, Lactose, Maltose, Cellobiose, Trehalose,

Gentibiose, Melibiose

Trisaccharides: Rhamninose, Gentianose, Raffinose, Rabinose,

Melezitose

Tetrasaccharides: Stachyose, Scorodose

Pentasaccharides: Verbascose

Polysaccharides

Homopolysaccharides: Starch, Glycogen, Cellulose, Inulin, Chitin

Heteropolysaccharides: Hyaluronic acid, Heparin, Chondroitin

sulfate, Dermatan sulfate, Keratan sulfate

Optical isomerism

(Source: jackwestin.com)

Summary

• Introduction to carbohydrates

• Classification of carbohydrates

• Structural aspects of carbohydrates

• Optical isomerism

MCQs

1. Carbohydrates are

a) Polyhydroxy aldehydes and phenols

b) Polyhydroxy aldehydes and ketones

c) Polyhydroxy ketones and phenols

d) Polyhydroxy phenols and alcohols

Ans. a. Polyhydroxy aldehydes and ketones

MCQs

2. Class of carbohydrate which cannot be hydrolyzed further, is

known as?

a) Disaccharides

b) Polysaccharides

c) Proteoglycan

d) Monosaccharides

Ans. d. Monosaccharides

MCQs

3. Minimum number of carbon required for a monosaccharide

a) 1

b) 2

c) 3

d) 4

Ans. c. 3

MCQs

4. An aldohexose will have ----- stereoisomerisms

a) 8

b) 10

c) 14

d) 16

Ans.d. 16

References 1. Lehninger, A.L.(2017). Lehninger Principles of Biochemistry (7th ed).

New York, N, Y: Freeman, W. H. & Company.

2. Stryer, L.(2019). Biochemistry (9th ed). London: Palgrave Macmillan.

3. Voet, D., Voet, J.G., & Pratt, C. W.(2016). Fundamentals of

Biochemistry: Life at the Molecular Level (5th ed). Hoboken, New Jersey: Wiley.

4.Bettelheim, F.A.(2011). Introduction to General, Organic and

Biochemistry (11th ed). Boston, Massachusetts: Cengage Learning.

5. Satyanarayana, U., & Chakrapani, U.(2017). Biochemistry (5th ed). India: Elsevier.

6. Gajera, H.P., & Patel, S.V., & Golakiya, B.A.(2008). Fundamentals of

Biochemistry. India: IBDC.

7. http:// www.Microbe Notes.com/Carbohydrates

8. http:// www.easybiologyclass.com/Carbohydrates

9. http:// www. biologynotes.site/Carbohydrates


Program B.Sc


Semester 3


Session 20

Carbohydrates

Carbohydrates

Structure of Glucose & Fructose and chemical properties of

monosaccharides

Recap

• Introduction to carbohydrates

• Classification of carbohydrates

• Structural aspects of carbohydrates

• Optical isomerism

Session Outcomes

• Fischer projection, Pyranose and Furanose structure of

Glucose & Fructose will be learnt.

• Chemical reactions like Tautomerization, Oxidation,

Reduction, Dehydration, Osazone formation and formation

of esters will be known.

Structure of glucose

(Source: alamy.com)

Structure of fructose

(Source: toppr.com)

Chemical properties of monosaccharides

1. Tautomerization

(Source: jaypeedigital.com)

2. Oxidation

(Source: tutorhelpdesk.com)

3. Reduction

(Source: embibe.com)

4. Dehydration

(Source: slideshare.net)

5. Osazone formation

(Source: toppr.com)

6. Formation of esters

(Source: quizlet.com)

Summary

• Structural aspects of Glucose & Fructose

• Chemical properties of monosaccharides

MCQs

1. What is the molecular formula of glucose?

a) CH3OH

b) C12H22O11

c) C6H12O6

d) C6H12O5

Ans. c. C6H12O6

MCQs

2. Choose a ketohexose

a) Glyceraldehyde

b) Dihydroxy acetone

c) Erythrose

d) Fructose

Ans. d. Fructose

MCQs

3. Which out of the following does not form osazone crystals?

a) Galactose

b) Maltose

c) Lactose

d) Sucrose

Ans. d. Sucrose

MCQs

4. Glucose on reduction with sodium amalgam forms

a) Dulcitol

b) Sorbitol

c) Mannitol

d) Mannitol and sorbitol

Ans. b. Sorbitol

MCQs

5. Glucose on oxidation does not give

Glycoside

Glucosaccharic acid

Gluconic acid

Glucuronic acid

Ans. a. Glycoside


Program B.Sc


Semester 3


Session 21

Carbohydrates

Carbohydrates

Glycosides & Derivatives of Monosaccharides

Recap

• Structural aspects of Glucose & Fructose

• Chemical properties of monosaccharides

Session Outcomes

To understand the role of physiologically important glycosides

& derivatives of monosaccharides

Glycosides

Glycosides are formed when hemiacetal or hemiketal hydroxyl

group (of anomeric carbon) of a carbohydrate reacts with

hydroxyl group of another carbohydrate or non-carbohydrate.

The non-carbohydrate moiety is referred to as aglycone.

Methanol, Phenols, Sterols and Glycerols serves as aglycones.

Classification of glycosides

1. O-glycosides

2. S-glycosides

3. N-glycosides

4. C-glycosides

Physiologically important glycosides

Glucovanillin

Cardiac glycosides

Streptomycin

Ouabain

Saponins

Derivatives of monosaccharides

Sugar acids

Sugar alcohols

Deoxy sugars

Sugar phosphates

Amino sugars

Alditols

Summary

• Glycosides

• Physiologically important glycosides

• Derivatives of monosaccharides

MCQs

1. A sugar alcohol is

a) Mannitol

b) Trehalose

c) Xylulose

d) Arabinose

Ans. a. Mannitol

MCQs

2. The constituents of heteropolysaccharides are

a) Sugar acids

b) Sugar alcohols

c) Amino sugars

d) Deoxy sugars

Ans. c. Amino sugars

MCQs

3. A sugar acid is

a) Sorbitol

b) Mannitol

c) Inositol

d) Gluconic acid

Ans. d. Gluconic acid

MCQs

4. A monosaccharide derivative present in the DNA is

a) Ribose

b) Ribulose

c) Deoxyribose

d) Glucose

Ans. c. Deoxy ribose

MCQs

5. An example of cardiac glycoside is

a) Streptomycin

b) Glucovanillin

c) Digoxin

d) Ouabain

Ans. c. Digoxin


Program B.Sc


Semester 3


Session 22

Carbohydrates

Carbohydrates

Introduction and structural aspects of disaccharides

Recap

• Glycosides

• Physiologically important glycosides

• Derivatives of monosaccharides

Session Outcomes

Structure & importance of Maltose, Sucrose & Lactose will be

learnt

Structure of maltose

(Source: toppr.com)

Importance of maltose

Maltose, also known as malt sugar, is formed from two glucose

molecules (α 1-4 glycosidic bond) and is a reducing sugar.

Malt is formed when grains soften and grow in water, and it is a

component of beer, starchy foods like cereal, pasta, and

potatoes, and many sweetened processed foods.

In plants, maltose is formed when starch is broken down for

food.

It is used by germinating seeds in order to grow.

Structure of lactose

(Source: blogs.creighton.com)

Importance of lactose

Lactose, or milk sugar, is made up of galactose and glucose (β 1-4

glycosidic bond) and is a reducing sugar.

The milk of mammals is high in lactose and provides nutrients for

infants.

Most mammals can only digest lactose as infants, and lose this

ability as they mature.

Structure of sucrose

(Source: researchgate.net)

Importance of sucrose

Sucrose, commonly known as cane or table sugar in its refined

form, is a disaccharide found in many plants.

It is made up of the monosaccharides glucose (C1 of α-glucose)

and fructose (C2 of β-fructose).

In the form of sugar, sucrose is a very important component of

the human diet as a sweetener.

It is called as invert sugar and is a non reducing sugar

Summary

• Introduction to disaccharides

• Structural aspects of disaccharides

• Reducing & Non-reducing sugars

MCQs

1. Which of the following glycosidic linkage found in maltose?

a) Glucose (α-1 – 2β) Fructose

b) Glucose (α1 – 4) Glucose

c) Galactose (β1 – 4) Glucose

d) Glucose (β1 – 4) Glucose

Ans. b. Glucose (α1 – 4) Glucose

MCQs

2. Which of the following is also known as invert sugar?

a) Sucrose

b) Fructose

c) Dextrose

d) Glucose

Ans. a. Sucrose

MCQs

3. Which of the following is not a disaccharide?

a) Hyaluronic acid

b) Maltose

c) Lactose

d) Sucrose

Ans. a. Hyaluronic acid

MCQs

4. What is the molecular formula of sucrose?

a) C12H22O11

b) C10H20O10

c) C6H12O6

d) C12H20O11

Ans. a. C12H22O11

MCQs

5. Sucrose is composed of which two sugars?

a) Glucose and Glucose

b) Glucose and Fructose

c) Glucose and Galactose

d) Fructose and Galactose

Ans. B. Glucose and Fructose


Program B.Sc


Semester 3


Session 23

Carbohydrates

Carbohydrates

Introduction and classification of polysaccharides

Recap

• Introduction to disaccharides

• Structural aspects of disaccharides

• Reducing & Non-reducing sugars

Session Outcomes

• Plant & Animal homopolysaccharides are studied.

• Various heteropolysaccharides present in the animal

kingdom will be understood.

Polysccharides

Polysaccharides are long chain polymeric carbohydrates

composed of monosaccharide units bound together by

glycosidic linkages.

Commonly found monomer units in polysaccharides are glucose,

fructose, mannose and galactose which are simple sugars.

Types of Polysaccharides

• Homo-polysaccharides: are made up of one type of

monosaccharide units. ex: Cellulose, Starch, Glycogen, Inulin,

and Chitin .

• Hetero-polysaccharides: are made up of two or more types of

monosaccharide units or their derivatives. ex. Hyaluronic acid,

Heparin, Chondroitin sulfate, Dermatan sulfate and Keratan

sulfate.

Functions of Polysaccharides

Storage polysaccharides: Starch, Glycogen

Structural polysaccharides: Cellulose, Chitin

Homopolysaccharides

Starch:

Found in plant cells as reserve food and is a polymer of D- glucose.

Exists in two forms:

1. Amylose (15-20%), helical form (α 1-4 linkages)

2.Amylopectin (80-85%), branched, similar to glycogen

(α 1-6 linkages on one in 30 monomers )

Glycogen:

Found in animal cells (muscles and liver) as reserve food and is a

polymer of D- glucose.

Composed of α 1-4 glycosidic bonds with branched α1-6 bonds

present at about every 10 th monomer.

Homopolysaccharides

Cellulose:

Is a structural polysaccharide found in the cell wall of plants.

Cellulose is said to be the most abundant organic molecule on earth.

Cellulose is composed of β-D-Glucose units linked by β (1-4) glycosidic

bonds.

Chitin:

It is composed of N-acetyl D-glucosamine units held together by

β (1-4) glycosidic bonds.

It is a structural polysaccharide found in the exoskeleton of some

Invertebrates e.g. insects, crustaceans.

Inulin: It is a polymer of fructose i.e., fructosan. It occurs in dahlia bulbs

garlic and onion etc.

Heteropolysaccharides

Hyaluronic Acid: Acts as a lubricant in the synovial fluid of joints and

vitreous humor of eyes.

Chondroitin Sulfate: It contributes to tensile strength and elasticity

of cartilages, tendons, ligaments, and walls of the aorta.

Heparin: Is present as an anticoagulant in the blood.

Heteropolysaccharides

Dermatan sulfate: It is found mainly in the skin, and also is in

vessels, heart, lungs. It may be related to coagulation and

vascular diseases and other conditions.

Keratan sulfate: Present in the cornea, cartilage bone and a

variety of other structures as nails and hair.

Summary

• Polysaccharides, Types and Functions

• Homopolysaccharides

• Heteropolysaccharides

MCQs

1. Name the major storage form of carbohydrates in animals?

a) Cellulose

b) Chitin

c) Glycogen

d) Starch

Ans. c. Glycogen

MCQs

2. Which class of carbohydrates is considered as non-sugar?

a) Monosaccharides

b) Disaccharides

c) Polysaccharides

d) Oligosaccharides

Ans. c. Polysaccharides

MCQs

3. Structural polysaccharide includes

a) Cellulose, hemicellulose and chitin

b) Cellulose, starch and chitin

c) Cellulose, starch and glycogen

d) Cellulose, glycogen and chitin

Ans. a. Cellulose, hemicellulose and chitin

MCQs

4. Which of the following is not a homopolysaccharide?

a) Starch

b) Heparin

c) Glycogen

d) Cellulose

Ans. b. Heparin

MCQs

5. The most abundant carbohydrate found in nature is

a) Starch

b) Glycogen

c) Cellulose

d) Chitin

Ans. c. Cellulose

Sem 3: Biochemistry and Biophysics Module-4 Carbohydrates

Digital Learning-DCE Bangalore University Page 1

Session 19

Introduction and structural aspects of monosaccharides

Introduction

The carbohydrates are most abundant naturally occurring organic molecules in the

biosphere and also referred to as “saccharides”

They are primarily produced by plants through photosynthesis from carbon dioxide and

water in the presence of sunlight; 80% of the dry weight of the plant is carbohydrate.

Essential elements present in the carbohydrate are–C. H. O

Carbohydrates, literally means ‘hydrates of carbon’; with an empirical formula (CH2O)n,

where n ≤ 3

Several non-carbohydrate compounds such as acetic acid (C2H4O2), lactic acid (C3H6O3)

etc appear to be carbon hydrates

Further, rhamnohexose (C6H12O5), deoxyribose (C5H10O6) and other genuine

carbohydrates do not satisfy the general formula (CH2O)n

Carbohydrates can be defined as polyhydroxy aldehydes or ketones or compounds

which produce them on hydrolysis.

The carbohydrates which are soluble in water and sweet in taste are called as “sugars”

Monosaccharides are the building blocks of carbohydrates

The carbohydrates can be structurally represented in any of the three forms:

Open chain structure

Hemi-acetal structure

Haworth structure

Functions of carbohydrates

In living organisms, the energy derived upon oxidation of carbohydrates used as fuel for

various metabolic activities.

Carbohydrates provide about 50-70% of total energy and are the most abundant dietary

source of energy (4kcal/g) for all living beings.

Carbohydrates serve as the principal storage form of energy (as glycogen in animals and

starch in plants) instead of other biomolecules such as proteins, lipids and nucleic acids to

meet the immediate energy demands of the body.

In many animals, being the chief energy source, carbohydrates also provide instant

sources of energy in the form of glucose. ATPs are generated upon oxidation of glucose

via glycolysis/Krebs cycle.

https://microbenotes.com/glycolysis-steps-atp-generation-and-significance/

https://microbenotes.com/tca-cycle-citric-acid-cycle-or-krebs-cycle/



They form structural and protective components in many organisms. These include

the cell wall of plants (cellulose), microorganisms (peptidoglycan or murein) and

exoskeleton of some insects (chitin).

Carbohydrates serve as intermediates during the biosynthesis of fats and proteins.

Carbohydrates aid in the regulation of nerve tissue and are the energy source for the

brain.

Carbohydrates combine with lipids and proteins to form glycolipids and glycoproteins

respectively. These conjugate carbohydrates participate as surface antigens, receptor

molecules, vitamins, and antibiotics in living system.

Glycolipids and glycoproteins play significant role in cell-cell communication and in

interactions between cells and other elements of the biological system. They are an

important constituent of connective tissues.

They form structural framework of nucleic acids (ribonucleic acid in RNA and

deoxyribonucleic acid in DNA).

As glycoproteins they help in the modulation of the immune system.

Classification of carbohydrates

Carbohydrates are also called as saccharides (Greek: sakcharon-sugar) and are broadly classified

into following three major groups

1. Monosaccharides

2. Oligosaccharides and

3. Polysaccharides

This categorization is based on the basis of their behavior on hydrolysis. Mono and

oligosaccharides are sweet crystalline substances that are soluble in water, hence, they are

commonly known as sugars.

Fig. 1: Classification of carbohydrates

(Source: www.askiitians)

https://microbenotes.com/cell-wall-plant-fungal-bacterial-structure-and-functions/



1. Monosaccharides

The simplest group of carbohydrates is the monosaccharide that cannot be further hydrolyzed.

'Mono' means 'one' and 'saccharide' means 'sugar'. They are referred to as simple sugars.

Monosaccharides are polyhydroxy aldehydes or ketones with general formula Cn(H20)n. On the

basis of the nature of carbonyl group these are further classified into two groups.

a. Polyhydroxy aldehydes are called aldoses. Example: Glyceraldehyde, Glucose

b. Polyhydroxy ketones are called ketoses. Example: Dihydroxyacetone, Fructose

Fructose Glucose

(Ketose) (Aldose)

Fig. 2: Monosaccharides

(Source: en.wikipedia.org)

Based on the number of carbon atoms, the monosaccharides are regarded as trioses (3C),

tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C).

Name

Formula

Aldoses

(Aldo sugars)

Ketoses

(Keto sugars)

Trioses

C3H6O3

Glycerose

Dihydroxyacetone

Tetroses

C4H8O4

Erythrose

Erythrulose

Pentoses

C5H1005

Ribose

Ribulose

Hexoses

C6H12O6

Glucose

Fructose

Heptoses

C7H14O7

Glucoheptose

Sedoheptulose

Table. 1: Classification of monosaccharides



2. Oligosaccharides

On hydrolysis oligosaccharides (Greek: oligo-few) liberates 2-1O monosaccharide molecules.

They can be further classified based on the number of monosaccharide residues released upon

hydrolysis.

I. Disaccharides

Upon hydrolysis disaccharides liberates two monosaccharide units, which may be the same or

different. For example, hydrolysis of sucrose yields one molecule glucose and fructose, whereas

maltose liberates two units of glucose.

Examples of disaccharides: Sucrose, Lactose, Maltose, Cellobiose, Trehalose, Gentibiose,

Melibiose

C12H22O11 + H2O C6H12O6 + C6H12O6

Sucrose Glucose Fructose

C12H22O11 + H2O 2C6H12O6

Maltose Glucose

II. Trisaccharides: On hydrolysis these carbohydrates yield three molecules of monosaccharides

units.

Examples of trisaccharides: Rhamninose, Gentianose, Raffinose, Rabinose, Melezitose

C18H32O16 + H2O C6H12O6 + C6H12O6 + C6H12O6

Raffinose Glucose Fructose Galactose

Examples of tetrasaccharides: Stachyose, Scorodose

Examples of pentasaccharides: Verbascose

3. Polysaccharides

Polysaccharides (Greek: poly-many) are polymers of a hundreds or thousands of

monosaccharide units which may be either straight or branched chains. The monosaccharide

units are joined together by glycosidic linkages and they usually have high molecular weight.

They are tasteless, so called as non-sugars, and form colloids with water. The general formula

for polysaccharides is (C6H10O5)n. The common and widely distributed polysaccharides appear

to satisfy this formula. Common examples of polysaccharides are starch, cellulose, glycogen,

etc.

The polysaccharides are further classified into-homopolysaccharides and

heteropolysaccharides.



(C6H10O5)n + n H2O n C6H12O6

Starch Glucose

Monosaccharides-structural aspects

Stereoisomerism is an important character of monosaccharides. The compounds with same

structural formulae but differ in their spatial configuration are said to be stereoisomers.

When carbon is attached to four different atoms or groups then, it is said to be asymmetric.

The possible isomers of a given compound are determined by the number of asymmetric

carbon atoms (n), which is equal to 2n. Glucose contains 4 asymmetric carbons, and thus, 16

isomers.

The D and L Notations

The configurations of both carbohydrates and amino acids are designated by D and L notations.

Glyceraldehyde (triose) is the simplest monosaccharide hence, has been chosen as arbitrary

standard for the D and L notation in sugar chemistry. It has one asymmetric carbon atom so,

two possible stereoisomers (enantiomers). Due to this reason, to represent the structure of all

other carbohydrates, glyceraldehyde has been kept as reference carbohydrate.

Fig.3: D- and L-forms of glyceraldehydes

(Source: alchetron. com)

In a Fischer projection, the carbonyl group is always placed on the top position for

monosaccharide. From its structure, if the –OH group attached to the bottom-most asymmetric

center (the carbon that is second from the bottom) is on the right, then, the compound is a

D-sugar. If the –OH group is oriented to left, then; the compound is L-sugar. Almost all sugars

found in nature are D-sugars. The D and L isomers are mirror images of each other. The

structures of D- and L-glucose are based on the reference monosaccharide, D and

L-glyceraldehyde.



Fig. 4: D- and L-forms of glucose

(Source: pediaa.com)

Optical activity of sugars

Compounds with asymmetric carbon atom will exhibit optical activity. These compounds in

solution can rotate a beam of polarized light either to the right or left. Based on the rotation of

the plane of polarized light to the right or to the left the compounds can be term

dextrorotatory (+) and levorotatory (-) respectively. The designation D(+), D(-), L(+) and L(-) of

an optical isomer can be done based on its structural relation with glyceraldehyde. lt may be

noted that the D- and L-configurations of sugars are primarily based on the structure of

glyceraldehyde, the optical activities however, may be different.

A special type of stereoisomers is enantiomers that are mirror images of each other. The two

members are designated as D- and L-sugars. The stereoisomers that are not mirror images of

one another are referred to as diastereomers.

Racemic mixture

lf D- and L-isomers are present in equal concentration, it is known as racemic mixture or DL

mixture. No optical activity is shown by racemic mixture, since, the dextro- and levorotatory

activities cancel each other.

Configuration of D-aldoses

Configuration of D-aldoses is a representation of Killiani-Fischer synthesis. Thus, starting with

an aldotriose (3C), aldotetrose (4C), aldopentoses (5C) and aldohexoses (6C) are formed by

increasing the chain length of an aldose, by one carbon at a time. The most familiar

carbohydrates of the 8 aldohexoses are glucose, mannose and galactose. D-glucose is the only

aldose monosaccharide that predominantly occurs in nature among all the other aldoses.

Configuration of D-ketoses

There are five physiologically important keto-sugars are reported starting from a triose,

dihydroxyacetone.



Epimers

If the configuration of two monosaccharides differs from each other around a single specific

carbon (other than anomeric) atom, they are referred to as epimers. Glucose and galactose are

C4-epimers. That is, both differ in the arrangement of –OH group at C4. The glucose and

mannose are epimers with regard to carbon 2 (C2-epimers).

Epimerization is the process of interconversion of epimers. For instance, interconversion of

glucose to galactose and vice versa is known as epimerization and this process is catalyzed by a

group of enzymes namely-epimerases.

Fig. 5: D-aldoses shown in Fischer projection

(Source: Biochemistry text book by U. Satyanarayana & U.Chakrapani)



Fig. 6: D-ketoses shown in Fischer projection

(Source: Biochemistry text book by U. Satyanarayana & U.Chakrapani)

References

Lehninger, A.L. (2017). Lehninger Principles of Biochemistry (7th

ed). New York, N, Y: Freeman,

W. H. & Company.

2. Stryer, L. (2019). Biochemistry (9th

ed). London: Palgrave Macmillan.

3. Voet, D., Voet, J.G., & Pratt, C. W. (2016). Fundamentals of Biochemistry: Life at the

Molecular Level (5th

ed). Hoboken, New Jersey: Wiley.

4. Bettelheim, F.A. (2011). Introduction to General, Organic and Biochemistry (11th

ed). Boston,

Massachusetts: Cengage Learning.

5. Satyanarayana, U., & Chakrapani, U. (2017). Biochemistry (5th

ed). India: Elsevier.

6. Gajera, H.P., & Patel, S.V., & Golakiya, B.A. (2008). Fundamentals of Biochemistry. India:

IBDC.






Session 20

Structure of Glucose & Fructose and chemical properties of monosaccharides

Structure of glucose

Fischer or Haworth projection formulae are the convenient way to represent configuration of

monosaccharides.

Fischer projection formulae of glucose

Fig.1: Open chain structure of glucose

(Source: nextgurukul.in)

Cyclic structure or Haworth projection formulae of glucose

Six-membered ring pyranose (based on pyran) or a five-membered ring furanose (based on

furan) is the cyclic structures by which Haworth projection formulae are depicted.

Fig.2: Cyclization of aldehyde and ketone

(Source: chem.libretexts.org)

Hemiacetal and hemiketal are formed when the hydroxyl group of monosaccharides reacts with

its own aldehyde or keto functional group. Thus, the two types of cyclic hemiacetals namely

α and β are formed when the aldehyde group of glucose at C1 reacts with alcohol group at C5.

The α and β cyclic forms of D-glucose are known as anomers. They differ from each other in the



configuration only around C1 known as anomeric carbon (hemiacetal carbon). In case of

α-anomer, the -OH group held by anomeric carbon is on the opposite side of the group -CH2OH

of sugar ring. The reverse is true for β-anomer. These anomers differ in their physical and

chemical properties.

Fig.3: Cyclization of D-Glucose

(Source: chem.libretexts.org)

Fig.4: Haworth structures of glucose


Cyclic structure of fructose

Fructose, a ketohexose consists of six carbon atoms and a keto functional group at position 2 of

the carbon chain.

Fig.5: Open chain structure of fructose




The cyclic structure is a five-membered ring. Haworth projection formulae of fructose are

depicted by five-membered ring structure called furanose. Hence, two possible isomeric forms.

These isomers are called anomers. The two cyclic forms differ in the configuration of the -OH

group held by anomeric carbon.

Fig.6: Haworth structures of fructose


Mutarotation

Mutarotation of glucose

Mutarotation is a general property exhibited cyclic sugars bearing a hemiacetal and hemiketal.

It is also a consequence of ring-chain tautomerism.

The α form of crystalline glucose melts at 146°C and has a specific rotation of +112°, while the

β form melts at 150°C and has a specific rotation of +18.7°. When the glucose is dissolved in

water, however, a solution formed will have anomers as well as the straight-chain form. The

change in the conformation of glucose reflects a change in the specific rotation. A freshly

prepared glucose (α anomer) solution whose specific rotation +112.2o will gradually changes

and attains a dynamic equilibrium with a constant value of +52.7o. Mutarotation is defined as

“the change in the specific optical rotation representing the interconversion of α and β forms of

D-glucose to an equilibrium mixture”. The equilibrium mixture contains 63% β-anomer and 36%

α-anomer of glucose with 1% open chain form. ln aqueous solution, the β form is more

predominant due to its stable conformation. The α and β forms of glucose are interconvertible

which occurs through a linear form.

α-D-glucose Equilibrium mixture β- D-glucose

+112° +52.7o +18.7°

Mutarotation of fructose

Fructose also exhibits mutarotation. In case of fructose, the pyranose ring (six-membered) is

converted to furanose (five-membered) in ring, till equilibrium is attained. At equilibrium

fructose has a specific optical rotation of -92 o

.



Fig.7: Mutarotation of D-glucose (a) and D-fructose (b)

(Source: 4 chem.libretexts.org)

Inversion of sucrose

The hydrolysis of sucrose by the enzyme invertase or boiling with a mineral acid gives an

equimolar mixture of D-glucose and D-fructose.

H+

C12H22O11 + H2 C6H12O6 + C6H12O6

Sucrose or invertase D-glucose D-fructose

The optical rotation of sucrose solution changes from dextrorotatory (+) to laevorotatory (-)

upon hydrolysis. The specific rotation of sucrose is + 66.5o, D-glucose and D-fructose have +52

o

& -92o respectively. Therefore, the net specific rotation of an equimolar mixture of D-glucose

and D-fructose in hydrolyzed sucrose solution is -20 o.

+52o

-92o

---------------- = -20

o

2

The specific rotation changes from +66.5o to -20

o upon hydrolysis of sucrose solution. During

the process of hydrolysis the sign of the specific rotation changes from (+) to ( -), or is said to

‘Invert’. Therefore, the hydrolysis of sucrose to D-glucose and D-fructose is called ‘Inversion’ and the hydrolysis mixture is called ‘Invert-sugar’. The enzyme that brings about inversion of

sucrose is named as invertase.

Reactions of monosaccharides

1. Tautomerization or enolization



Tautomerization is the process of producing enediols by shifting a hydrogen atom from one

carbon atom to another. Tautomerization in alkaline solutions is shown the sugars possessing

anomeric carbon atom. Glucose undergoes isomerization to form D-fructose and D-mannose

when it is kept in alkaline solution for several hours. This reaction known as the “Lobry de

Bruyn-von Ekenstein transformation”. This results in the formation of a common intermediate

termed enediol.

Fig. 8: Tautomerization of glucose

(Source: jaypeedigital.com)

Reducing properties of monosaccharides

The sugars are classified as reducing or nonreducing. The free aldehyde or keto group of

anomeric carbon may involve in the reduction. Any carbohydrate capable of reducing either

ferric or cupric ions is said to be a reducing sugar. All monosaccharides are reducing sugars.

ln the laboratory, many tests are employed as simple and rapid diagnostic tests for the

presence of glucose in blood or urine. These include Barfoed's, Benedict's test, Fehling's test,

test etc. In alkaline medium than in the acid medium the reduction is much more efficient.

Examples of reducing sugars: Maltose, Lactose, Melibiose, Cellobiose, Gentiobiose, Fructose,

Mannose, Galactose, Glucose

Examples of Non-reducing sugars include: Sucrose, Trehalose, Raffinose, Stachyose,

Verbascose

Reducing sugars

1. Reducing sugar is any carbohydrate which is capable of being oxidized and causes the reduction

of other substances without having to be hydrolyzed first.

2. Reducing sugars are carbohydrates that can act as reducing agents due to the presence of free

aldehyde groups or free ketone groups.

3. Reducing sugars have a sweet taste.

4. Most of the reducing sugars are Monosaccharides.

5. Reducing sugars give a dark red color (brick color) when they react with Benedict solution.



6. Reducing sugar has a free aldehyde (-CHO) or ketonic (-CO) group.

7. Reducing sugars give a positive reaction towards the Fehling’s test.

8. Reducing sugars have the capacity to reduce cupric ions of Benedict’s or Fehling solution to

cuprous ions.

9. Presence or absence of reducing sugars can be identified by carrying out different tests.

10. Generally, the molecular weight of reducing sugars is relatively low.

Nonreducing sugars

1. Non-reducing sugars are any type of carbohydrate which are unable to be oxidized and do not

reduce other substances.

2. Non-reducing sugars are carbohydrates that cannot act as reducing agents due to the absence

of free aldehyde groups or free ketone groups.

3. Non-reducing sugars have a less sweet taste.

4. Most of non-reducing sugars are polysaccharides while others are disaccharides.

5. Non-reducing sugars do not give a red color when they react with Benedict’s solution, instead remains as green in color.

6. Non-reducing does not have a free aldehyde or ketonic group.

7. Non-reducing sugars give a negative reaction towards the Fehling’s test.

8. Non-reducing sugar fail to reduce the cupric ions of Benedict’s solution to coprous ions.

9. The presence or absence of non-reducing sugars cannot be identified by different tests.

10. Generally, the molecular weight of reducing sugars is relatively high when compared to that of

reducing sugars.

2. Oxidation

The oxidation of terminal aldehyde (or keto) or the terminal alcohol or both the groups may

depend on the type of oxidizing agent. For instance, consider glucose:

Oxidation of aldehyde group (CHO ------>COOH) results in the formation of gluconic acid.

Oxidation of terminal alcohol group (CH2OH ------>C OOH) leads to the production of glucuronic

acid.

Fig. 9: Oxidation of aldehyde group of glucose




Fig.10: Conversion of glucose to glucuronic acid

(Source: osp.mans.edu.eg)

Fig. 11: Oxidation of aldehyde and alcoholic group of glucose

(Source: chtf.Stuba.sk)

3. Reduction

The aldehyde or keto group of monosaccharide is reduced to corresponding alcohol when

treated with reducing agents such as sodium amalgam.

The important monosaccharides and their corresponding alcohols are given below.

D-Glucose D-Sorbitol

D-Galactose D-Dulcitol

D-Mannose D-Mannitol

D-Fructose D-Mannitol + D-Sorbitol

D-Ribose D-Ribitol

Fig. 12: Reduction of glucose




4. Dehydration

When treated with concentrated sulfuric acid, monosaccharides undergo dehydration with an

elimination of 3 water molecules. Thus, on dehydration pentoses give furfural while hexoses

give hydroxymethyl furfural. The coloured products are formed when these furfurals condense

with phenolic compounds (a-naphthol). Dehydration is the basis of Molisch test.

Fig. 13: Dehydration of monosaccharides

(Source: Biochemistry text book by U. Satyanarayana & U. Chakrapani)

5. Osazone formation

Osazones are formed when phenylhydrazine in acetic acid is boiled with reducing sugars. The

the first two carbons (C1 and C2) of sugars are involved in osazone formation. Since the

difference on these two carbons is masked by binding with phenylhydrazine, the same type of

osazones is given by sugars that differ in their configuration for first two carbons. Thus, the

same type (needle-shaped) osazones are formed from glucose, fructose and mannose.

Reducing disaccharides also give osazones-maltose sunflower-shaped, and lactose powder-puff

shaped.

Fig. 14: Osazone formation

(Source: Biochemistry text book by U. Satyanarayana & U. Chakrapani)



6. Formation of esters

By non-enzymatic or enzymatic reactions the alcoholic groups of monosaccharides may be

esterified. Esterification of alcoholic group a carbohydrate with phosphoric acid is a common

reaction in metabolism. Glucose 6-phosphate and glucose 1-phosphate are good examples. ATP

donates the phosphate moiety in ester formation.

References

Lehninger, A.L. (2017). Lehninger Principles of Biochemistry (7th

ed). New York, N, Y: Freeman,

W. H. & Company.

2. Stryer, L. (2019). Biochemistry (9th

ed). London: Palgrave Macmillan.

3. Voet, D., Voet, J.G., & Pratt, C. W. (2016). Fundamentals of Biochemistry: Life at the

Molecular Level (5th

ed). Hoboken, New Jersey: Wiley.

4. Bettelheim, F.A. (2011). Introduction to General, Organic and Biochemistry (11th

ed). Boston,

Massachusetts: Cengage Learning.

5. Satyanarayana, U., & Chakrapani, U. (2017). Biochemistry (5th

ed). India: Elsevier.

6. Gajera, H.P., & Patel, S.V., & Golakiya, B.A. (2008). Fundamentals of Biochemistry. India:

IBDC.




BangaloreUniversity

Biotechnology

III Sem -Biochemistry and Biophysics

Module-4 Carbohydrates

Session19: Introduction and structural aspects of monosaccharides

1. Carbohydrates are

a) Polyhydroxy aldehydes and phenols

b) Polyhydroxy aldehydes and ketones

c) Polyhydroxy ketones and phenols

d) Polyhydroxy phenols and alcohols

Ans. a. Polyhydroxy aldehydes and ketones

2. Class of carbohydrate which cannot be hydrolyzed further, is known as?

a) Disaccharides

b) Polysaccharides

c) Proteoglycan

d) Monosaccharides

Ans. d. Monosaccharides

3. Minimum number of carbon required for a monosaccharide

a) 1

b) 2

c) 3

d) 4

Ans. c. 3

4. An aldohexose will have ----- stereoisomerisms

a) 8

b) 10

c) 14

d) 16

Ans.d. 16

Session20: Structure of Glucose & Fructose and chemical properties of

monosaccharides

5. What is the molecular formula of glucose?

a) CH3OH

b) C12H22O11

c) C6H12O6

d) C6H12O5

Ans. c. C6H12O6

6. Choose a ketohexose

a) Glyceraldehyde

b) Dihydroxy acetone

c) Erythrose

d) Fructose

Ans. d. Fructose

7. Which out of the following does not form osazone crystals?

a) Galactose

b) Maltose

c) Lactose

d) Sucrose

Ans. d. Sucrose

8. Glucose on reduction with sodium amalgam forms

a) Dulcitol

b) Sorbitol

c) Mannitol

d) Mannitol and sorbitol

Ans. b. Sorbitol

9. Glucose on oxidation does not give

a) Glycoside

b) Glucosaccharic acid

c) Gluconic acid

d) Glucuronic acid

Ans. a. Glycoside

Session21: Glycosides & Derivatives of Monosaccharides

10. A sugar alcohol is

a) Mannitol

b) Trehalose

c) Xylulose

d) Arabinose

Ans. a. Mannitol

11. The constituents of heteropolysaccharides are

a) Sugar acids

b) Sugar alcohols

c) Amino sugars

d) Deoxy sugars

Ans. c. Amino sugars

12. A sugar acid is

a) Sorbitol

b) Mannitol

c) Inositol

d) Gluconic acid

Ans. d. Gluconic acid

13. A monosaccharide derivative present in the DNA is

a) Ribose

b) Ribulose

c) Deoxyribose

d) Glucose

Ans. c. Deoxy ribose

14. An example of cardiac glycoside is

a) Streptomycin

b) Glucovanillin

c) Digoxin

d) Ouabain

Ans. c. Digoxin

Session22: Introduction and structural aspects of disaccharides

15. Which of the following glycosidic linkage found in maltose?

a) Glucose (α-1 – 2β) Fructose

b) Glucose (α1 – 4) Glucose

c) Galactose (β1 – 4) Glucose

d) Glucose (β1 – 4) Glucose

Ans. b. Glucose (α1 – 4) Glucose

16. Which of the following is also known as invert sugar?

a) Sucrose

b) Fructose

c) Dextrose

d) Glucose

Ans. a. Sucrose

17. Which of the following is not a disaccharide?

a) Hyaluronic acid

b) Maltose

c) Lactose

d) Sucrose

Ans. a. Hyaluronic acid

18. What is the molecular formula of sucrose?

a) C12H22O11

b) C10H20O10

c) C6H12O6

d) C12H20O11

Ans. a. C12H22O11

19. Sucrose is composed of which two sugars?

a) Glucose and Glucose

b) Glucose and Fructose

c) Glucose and Galactose

d) Fructose and Galactose

Ans. b. Glucose and Fructose

Session23: Introduction and classification of polysaccharides

20. Name the major storage form of carbohydrates in animals?

a) Cellulose

b) Chitin

c) Glycogen

d) Starch

Ans. c. Glycogen

21. Which class of carbohydrates is considered as non-sugar?

a) Monosaccharides

b) Disaccharides

c) Polysaccharides

d) Oligosaccharides

Ans. c. Polysaccharides

22. Structural polysaccharide includes

a) Cellulose, hemicellulose and chitin

b) Cellulose, starch and chitin

c) Cellulose, starch and glycogen

d) Cellulose, glycogen and chitin

Ans. a. Cellulose, hemicellulose and chitin

23. Which of the following is not a homopolysaccharide?

a) Starch

b) Heparin

c) Glycogen

d) Cellulose

Ans. b. Heparin

24. The most abundant carbohydrate found in nature is

a) Starch

b) Glycogen

c) Cellulose

d) Chitin

Ans. c. Cellulose

btt-301: carbohydrates

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