TARGET BIOLOGY TARUN SIR M- 7679598996

Class - XI NEET Crash Course 9153668079

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Chapter 9 : Biomolecules
INTRODUCTION

➢ Biomolecules include both ----

 micromolecules, e.g. sugar ,amino acids, fatty acids, nitrogenous bases, , etc. and
 macromolecules, such as carbohydrates, proteins, lipids and nucleic acids.

➢ Four elements: carbon, hydrogen, oxygen and nitrogen constitutes 97–99% of the body of living
organisms.
➢ The inorganic constituents can be estimated by analysing the ash formed after burning a tissue
completely.
• Protein- Polymer of amino acids
• Carbohydrates (Polysaccharides)- Polymer of simple sugars, e.g. glucose, fructose
• Fats- Fatty acids and glycerol
• Nucleic acids- Nitrogenous bases, sugar and phosphate
➢ Water constitutes 70-90% of cells. Proteins are 10-15%, nucleic acids are 5-7%, carbohydrates are 3%,
lipids are 2% and the rest 1% are ions.

PRIMARY AND SECONDARY METABOLITES

 Amino acids, fatty acids, sugars, etc. are called primary metabolites, which take part in
the physiological processes.
• Other than these, there are varied compounds present in the cells of plants, microbes and fungi,
which are called secondary metabolites. Secondary metabolites can be grouped under various
categories ----
• Pigments (anthocyanins, carotene, etc.), drugs (curcumin, vinblastine, etc.), gum, rubber, toxins
(ricin, abrin), alkaloids (codeine, morphine), essential oils, terpenoids, lectins (concanavalin A), etc.

ELEMENTS

On the basis of presence and requirement in plants and animals, these are grouped into major (Ca, P, Na,
Mg, S, K, N) and minor (Fe, Cu, Co, Mn, Mo, Zn, I) bio elements.

On the basis of functions, these may be of following types :–

• Framework elements : Carbon, oxygen and hydrogen.

• Protoplasmic elements : Proteins, nucleic acids, lipids, chlorophyll, enzymes, etc.
• Balancing elements : Ca, Mg and K.
 PROTEINS
 Proteins are polypeptides. These are linear chains of amino acids linked by peptide bonds.
 Essential elements in protein are C , H , O, N. Some contain sulphur(S) and phosphorus (P) also.
The structural unit of protein is amino acid.
 There are approximately 300 amino acids known to exist but only 20 types of amino acids are used
in formation of proteins.
 Each amino acid is amphoteric compound because it contains one
weak acidic group – COOH and a weak alkaline group – NH2.

 In protoplasm; free amino acid occurs as ions ( at iso - electric point).

Iso - electric point is that point of pH at which amino acids do not move in
electric field.

 Amongst proteins, 'collagen' is the most abundant protein of animal

world while 'RubisCo' (Ribulose Bisphosphate Carboxylase-Oxygenase)
is the most abundant protein of biosphere.

CLASSIFICATION OF AMINO ACIDS

Amino acids can be classified on the basis of their R (alkyl) group –

➢ Positively charged polar R-group : These contain 2-amino and 1-carboxyl group.
E.g., Lysine, arginine, histidine (Basic Amino acid)
➢ Negatively charged polar R-group : These contain 1-amino and 2-carboxyl groups.
Eg. Aspartic acid, glutamic acid (Acidic Amino acid)

ON THE BASIS OF REQUIREMENTS

On the basis of the synthesis of amino acids in the body and their requirement, these are categorized as :
➢ Essential amino acids : These are not synthesized in the body hence need to be provided in the diet,
e.g., valine, leucine, isoleucine, threonine, lysine, tryptophan, phenylalanine, methionine etc.

➢ Semi-essential amino acids : Synthesized partially in the body but not at the rate to meet the
requirement of individual, e.g., arginine and histidine.

➢ Non-essential amino acids : These amino acids are derived from carbon skeleton of lipids and
carbohydrate metabolism. In humans, there are 12 non-essential amino acids, e.g., alanine, aspartic
acid, cysteine, glutamic acid etc.

CONFIGURATION OF PROTEIN MOLECULE

(1) PRIMARY STRUCTURE

A straight chain of amino acids linked by peptide bonds form primary structure of proteins. The first (or left)
amino acid is called N—terminal (–NH2 group.) amino acid, and the last (or right) amino acid is called C-
terminal (–COOH group) amino acid. Such proteins are non functional proteins. This structure of
proteins is most unstable. Newly formed proteins on ribosomes have primary structure.

(2) SECONDARY STRUCTURE

Protein molecules of secondary structure are spirally coiled. The spiral is stabilized by straight hydrogen
bonds between imide group (– NH –) of one amino acid and carbonyl group (– CO) of fourth amino acid
residue. In this way, all the imide and carbonyl groups become hydrogen bonded.

 α-helix : Right handed rotation of spirally coiled chain with approximately 3½ amino acids in each
turn. This structure has intramolecular hydrogen bonding i.e. between two amino acids of the same
chain e.g., keratin, myosin, tropomyosin.

 β-helix or pleated sheath structure : Protein molecule has zig - zag structure. Two or more
protein molecules are held together by intermolecular hydrogen bonding. The polypeptide chain
may be parallel or antiparallel, e.g. Keratin protein in birds (β– sheets parallel) and silk protein
(fibroin) with antiparallel β–sheets.

(3) TERTIARY CONFIGURATION OR STRUCTURE

Proteins of tertiary structure are highly folded to give a globular appearance. These are soluble in water
(colloid solution) giving us a 3–dimensional view of a protein. Tertiary structure is absolutely necessary
for the many biological activities of proteins. Tertiary structure is stabilised by five types of bonds:

 Peptide bonds : strongest bond in proteins.

 Hydrogen bonds : These occur between hydrogen and oxygen atoms of various groups.
 Disulphide bond : These bonds form between - SH group of amino acids (e.g., methionine,
cysteine). These bonds are second strongest bond and stabilise the tertiary structure of protein.
 Hydrophobic bonds : Present between amino acids which have hydrophobic side chains, e.g.
aromatic amino acids.
 Ionic bonds : Formation of ionic bond occurs between two opposite ends of a protein molecule due
to electrostatic attraction. Majority of proteins and enzymes in protoplasm exhibit tertiary structure.

(4) QUATERNARY STRUCTURE

Two or more polypeptide chains of tertiary structure united by different types of bonds to form quaternary
structure of protein. Different polypeptide chains may be similar (e.g., lactic-dehydrogenase) or dissimilar
types (e.g., haemoglobin, insulin). Quaternary structure is most stable structure of protein.

TYPES OF PROTEINS

a) SIMPLE PROTEINS : Proteins which are composed of only amino acids.

Fibrous Proteins : E.g., Collagen, elastin, keratin etc
Globular Proteins : E.g., Albumin, histones, globin, protamines etc
b) CONJUGATED PROTEINS : Formed by the binding of a simple protein with a non-protein part
(prosthetic group).
Nucleoproteins - Proteins + nucleic acids, e.g., chromatin, ribosomes etc.
Chromoprotein - Proteins + pigment or coloured, e.g., haemoglobin, haemocyanin etc.
Lipoprotein - Proteins + lipids, e.g., cell membrane, lipovitelline of yolk.
Phosphoproteins - Proteins + phosphorus – e.g., casienogen, pepsin, ovovitelline, phosvitin.
Glycoproteins – Proteins + carbohydrates, e.g., hormones like FSH, LH, TSH and HCG etc.
Metallo-protein : Proteing + metal ions –e.g., Cu-tyrosinase, Zn-carbonic anhydrase etc

c) DERIVED PROTEINS : These form by denaturation or hydrolysis of protein.

Primary derived proteins are denaturation products of normal proteins, e.g., fibrin, myosin.
Secondary derived proteins are digestion products of proteins, e.g., proteoses, peptones.

FUNCTIONS OF PROTEINS

 Formation of cells and tissues for growth.

 Repairing of tissues.
 Formation of hormones.
 For muscle contraction (e.g., actin, myosin).
 Formation of enzymes.
 Help in blood clotting.
 For transport (e.g., haemoglobin, transferrin).
 For defence against infections (e.g., antibodies).
 Form hereditary material – nucleoproteins.
 For storage (e.g., myoglobin and ferritin).
 For support (e.g., collagen and elastin).

CARBOHYDRATES
Carbohydrates are polymers of monosaccharides or simple sugars. They are the main energy source of
plants and animal cells. They have a general formula of [Cx(H2O)y]n.
COMPOSITION
It consists of carbon, hydrogen and oxygen in the ratio CnH2nOn. It is also called saccharide with sugars
being the basic components of saccharides.

Maltose

Reducing sugars- All monosaccharides are reducing sugar. Some disaccharides are reducing, e.g.
lactose, maltose, etc. They act as a reducing agent and reduce Tollen’s, Fehling or Benedict’s reagents.
They have free aldehyde or ketone group (in a cyclic form hemiacetal or hemiketal group)
Non-reducing sugars- Sucrose (a disaccharide) is non-reducing as both the carbonyl groups are
involved in the glycosidic bond formation. All the polysaccharides (cellulose, starch) are non-reducing.
 CLASSIFICATION OF CARBOHYDRATES
(A) Monosaccharides : Monosaccharides are colourless, sweet tasting solids that show oxidation,
esterification and fermentation. Due to asymmetric carbon, these exist in different isomeric forms.
These can rotate polarized light hence are dextrorotatory and laevorotatory.

 These are single sugar units : --- trioses-3C, (Glyceraldehyde, dihydroxyace-tone etc.),
tetroses-4C, pentoses-5C, hexoses-6C etc.

 Important Hexoses Glucose : (C6H12O6) are fructose, galactose (important constituent of

glycolipids and glycoproteins).

 Fructose is called fruit sugar (sweetest among natural sugars) and glucose is called
“sugar of body” (blood sugar).

(B) Oligosaccharides : It is formed due to condensation of 2–10 monosaccharide units, the oxygen bridge
is known as “glycoside linkage” and water molecule is eliminated. The bond may be α and β.

 Maltose : Also called “malt sugar” stored in germinating seeds of barley, oats, etc. It is
formed by enzymatic (enzyme amylase) action on starch. It is a reducing sugar.

 Sucrose : “Cane sugar” or “table-sugar”. It is obtained from sugarcane and beetroot and
on hydrolysis splits into glucose and fructose.

 Lactose : “Milk sugar’’, present in mammalian milk. On hydrolysis if yields glucose and
galactose. Streptococcus lacti converts lactose into lactic acid and causes souring of milk.

(c) Polysaccharides : several molecules (300–30000) of monosaccharides.

TYPES OF POLYSACCHARIDES

➢ On the basis of structure

1) Homopolysaccharides : These are made by polymerisation of single kind of
monosaccharides. e.g., starch, cellulose, glycogen, etc.
2) Heteropolysaccharides : These are made by condensation of two or more kinds of
monosaccharides, e.g., chitin, pectin, etc.

➢ On the basis of functions

1) Food storage polysaccharides : These serve as reserve food, e.g., starch and glycogen.
2) Structural polysaccharides : These take part in structural framework of cell wall, e.g.,
chitin and cellulose.
 Cellulose is a homopolymer of 𝛽-D-glucose molecules linked by 𝛽-1,4 glycosidic bonds. Cellulose is
present in the cell wall of plants. made up of unbranched chain of 6000 glucose.
 Starch is a major food reserve of plants. It is a polymer of glucose. It contains amylose and
amylopectin.
 Glycogen is a food reserve for animals. It is a branched polymer of glucose and contains 30,000
glucose units. It is similar to glycogen in structure, but it is more branched.
 Inulin- polymer of fructose, present in many plants. It is found in roots and rhizomes.
 Complex carbohydrates- They are polymer of substituted sugars such as amino sugars or sugar
acids, e.g. N-acetylglucosamine, N-acetylmuramic acid.
 Chitin is a polymer of N-acetyl glucosamine. It is present in the exoskeleton of insects, cell wall of
fungi, scales of fish, etc.
 Peptidoglycan (Murein) is a polymer of sugar and amino acids. Sugars present in peptidoglycan
are the alternating unit of N-acetylglucosamine(NAG) and N-acetylmuramic acid(NAM). The
bacterial cell wall is made up of peptidoglycan.
 Agar-Agar : It is a galactan used to prepare bacterial cultures. It is also used as luxative and
obtained from cell wall of red algae e.g., Gracilaria, Gelidium etc.
 Pectin : It is a cell wall material in collenchyma tissue that may also be found in fruit pulps, rind of
citrus fruits etc.
 LIPIDS
➢ Fat and its derivatives are together known as lipid. The term 'Lipid' was given by German biochemist,
Wilhem Bloor for fat and fat like substances.
➢ Essential constituents are C, H, O but the ratio of hydrogen and oxygen is not 2 : 1. The amount of
oxygen is considerably very less.
➢ Lipids are insoluble in water and soluble in organic solvents like acetones, chloroform, benzene, hot
alcohol, ether etc.
➢ Lipids do not form polymer.
➢ Lipids provide more than double energy as compared to carbohydrates.
➢ Animals store maximum amount of food in the form of lipid.
➢ Lipids are not strictly macromolecules.
TYPES OF lipid

(a) SIMPLE LIPIDS OR NEUTRAL FATS

 These are esters of long chain fatty acids with various alcohols. In the majority of simple lipids, the
alcohol is a trihydroxy sugar alcohol i.e. glycerol.
 Three molecules of fatty acid linked with one molecule of glycerol. The linkage is called “ester bond”,
such type of lipids are called triglycerides. Three molecules of water are released during formation of
triglycerides (dehydration synthesis).

 SATURATED FATTY ACIDS are those in which all the carbon atoms of hydrocarbon chain are
saturated with hydrogen atoms. e.g. Palmitic acid ,Stearic acid .
 Simple lipids with saturated fatty acids remain solid at normal room temperature e.g., fats.
 Saturated fatty acids are less reactive so they tend to store in the body and cause obesity.

 UNSATURATED FATTY ACIDS are those in which some carbon atoms are not fully occupied by
hydrogen atoms.e.g. Oleic acid ,Linoleic acid , Linolenic acid .
 Unsaturated fatty acids also called as essential fatty acids because animals are not able to
synthesize them.
 Simple lipids with unsaturated fatty acids remain liquid at room temperature e.g., oils.
 Oils with polyunsaturates are recommended by physicians for persons who suffer from
high blood cholesterol or cardio-vascular diseases. This is because increasing the
proportion of polyunsaturated fatty acids to saturated fatty acids, without raising the fats in
the diet tend to lower the cholesterol level in blood.

(b) CONJUGATED OR COMPOUND LIPIDS

 PHOSPHOLIPIDS OR PHOSPHATIDE : It is made of 2 molecules of fatty acid + glycerol + H3PO4 +

nitrogenous compounds. Phospholipids have both hydrophilic polar end (H3PO4 and nitrogenous
compound) and hydrophobic non-polar end (fatty acids). Such molecules are called amphipathic.
Due to this property, phospholipids form bimolecular layer in cell membrane.
  Some biologically important phospholipids are as follows : Lecithin or phosphatidylcholine :
Nitrogenous compound in lecithin is choline. Lecithin occurs in egg yolk, oil seeds and blood.
 Sphingolipids or sphingomylins : occur in myelin - sheath of nerves.

(c) DERIVED LIPIDS

 These are derived from simple or conjugated lipids and are complex in structure.
Steroids : Steroids exhibit tetracyclic structure.

FUNCTIONS OF LIPIDS

➢ Oxidation of lipids yield comparatively more energy in the cell than proteins and carbohydrates.
➢ The oil seeds such as groundnut, mustard, coconut store fats to provide nourishment to the embryo
during germination.
➢ These function as structural constituent i.e., all the membrane system of the cell are made up of
lipoproteins.
➢ It works as a heat insulator and is used in the synthesis of hormones.
➢ Fats provide solubility to vitamins A, D, E and K.

NUCLEIC ACIDS

 Nucleic acids are the polymers of nucleotide made up of carbon, hydrogen, oxygen, nitrogen and
phosphorus which control the basic functions of the cell.
 Miescher discovered nucleic acids in the nucleus of pus cell and called it nuclein. The name nucleic
acid was proposed by Altman.
 Composed of N2 base,pentose sugar and phosphate.

 On the basis of structure, nitrogen bases are broadly of two types :

 Pyrimidines : These consist of one pyrimidine ring. Skeleton of ring is composed of two
nitrogen and four carbon atoms. E.g., cytosine, thymine (in DNA) and uracil (in RNA).
 Purines : These consist of two rings i.e. one pyrimidine ring (2N + 4C) and one imidazole ring
(2N + 3C). E.g., adenine and guanine.

 PENTOSE SUGAR
 Nitrogen base forms bond with
first carbon of pentose sugar to
form a nucleoside, nitrogen of
third place (N3) forms bond with
sugar in case of pyrimidines
while in purines nitrogen of
ninth place (N9) forms bond
with sugar.
 Phosphate : Phosphate forms
ester bond (covalent bond) with
fifth carbon of sugar to form a
complete nucleotide.

 When nitrogen bases are found attached to a pentose sugar, then they are called nucleosides, e.g.,
adenosine, guanosine, thymidine, uridine and cytidine.

 Nucleotides are phosphorylated nucleosides. These are formed by condensation of a pentose sugar, a
nitrogen base and at least one phosphoric acid residue, e.g., adenylic acid, thymidylic acid, guanylic
acid, uridylic acid and cytidylic acid.
 Nucleic acids like DNA and RNA consist of nucleotides only. DNA and RNA function as genetic
material.
 DNA (DEOXYRIBONUCLEIC ACID)

 In DNA, pentose sugar is deoxyribose sugar and four types of nitrogen bases are Adenine (A),
Thymine (T), Guanine (G) and Cytosine (C).
 Wilkins and Franklin studied DNA molecule with the help of X-ray crystallography.
 With the help of this study, Watson and Crick (1953) proposed a double helix model for DNA. For this
model, Watson, Crick and Wilkins were awarded noble prize in 1962.

 According to this model, DNA is composed of two polynucleotide chains. Both polynucleotide chains are
complementary and anti-parallel to each other.

 Both strands of DNA are held together by hydrogen bonds. These hydrogen bonds are present between
nitrogen bases of both strands.
 Adenine binds to Thymine by two hydrogen bonds and Cytosine binds to Guanine by three hydrogen
bonds.
 Chargaff’s equivalence rule : In a double stranded DNA, amount of purine nucleotides is equal to
amount of pyrimidine nucleotides.
Purine = Pyrimidine ::::: [A] + [G] = [T] + [C]

Base ratio = = constant for a given species.

CONFIGURATION OF DNA MOLECULE

 In both strands of DNA, direction of phosphodiester bond is opposite, i.e., if direction of phosphodiester
bond in one strand is 3'-5' then it is 5'-3' in another strand.
 Two strands of DNA are helically coiled like a revolving ladder. Backbone of this ladder (Reiling) is
composed of phosphates and sugars while steps (bars) are composed of pairs of nitrogen bases. The
strand turns 36°.
 Distance between two successive steps is 3.4
Å, In one complete turn of DNA molecule,
there are 10 such steps

 Diameter of DNA molecule i.e., distance

between phosphates of two strands is 20Å.

 DNA in chromosomes is linear while in

prokaryotes, mitochondria and chloroplast, it is
circular.
 RNA (RIBONUCLEIC ACID)

Structure of RNA is fundamentally the same as DNA, but there are some differences. The differences are :

 In place of deoxyribose sugar of DNA, ribose sugar is present in RNA.

 In place of nitrogen base thymine present in DNA, nitrogen base uracil is present in RNA.
 RNA is made up of only one polynucleotide chain i.e., RNA is single stranded.
Exception : RNA found in reo-virus is double stranded, i.e., it has two polynucleotide chains.

TYPES OF RNA

 GENETIC RNA OR GENOMIC RNA :- In the absence of DNA, sometimes RNA working as genetic
material and genomic RNA transfer information from one generation to the next generation.
E.g., Reovirus and Tobacco Mosaic Virus (TMV).

 NON-GENETIC RNA :- 3 types : (a) r - RNA (b) t - RNA (c) m - RNA

(1) Ribosomal RNA (r - RNA)

 This RNA is 80% of the cells total RNA.

 It is found in ribosomes and is produced in the nucleolus.
 At the time of protein synthesis, r-RNA provides attachment sites to t-RNA and m-RNA and
attaches them on the ribosome.

(2) Transfer - RNA (t-RNA) / soluble RNA (sRNA) / adapter RNA

 It is 10-15% of total RNA.
 It is synthesized in the nucleus by DNA.

(3) Messenger RNA (m -RNA)

 The m- RNA is 1-5% of the cell’s total RNA.
 It is produced by genetic DNA in the nucleus. This process is known as transcription.

ENZYMES

 Enzymes are biocatalysts made up of proteins (except ribozyme) which increases the rate of
biochemical reactions by lowering down the activation energy, but does not affect the nature of final
product.
 The term enzyme (meaning in yeast) was used by Willy Kuhne (1878) while working on fermentation.
 Zymase (from yeast) was the first discovered enzyme by Buchner.
 The first purified and crystalized enzyme was urease
(by J.B. Sumner) from Canavalia/Jack Bean (Lobia plant) and suggested that enzymes are proteins.

CHARACTERISTICS OF ENZYMES

➢ All enzymes are proteins, but all proteins are not enzymes. Enzymatic proteins consist of 20 amino
acids.
➢ All enzymes are tertiary and globular proteins (isoenzymes quaternary protein). Their tertiary structure
is very specific and important for their biological activity.
➢ Enzymes accelerate the rate of reaction without undergoing any change in themselves. Enzymes lower
the activation energy of substrate or reactions.
➢ Enzymes are macromolecules of amino acids which are synthesized on ribosomes under the control of
genes.
➢ Molecular weight of enzymes are high and these are colloidal substances.
➢ Enzymes are very sensitive to pH and temperature. Optimum temperature for enzymes is 20-35°C.
Most of the enzymes are active at neutral pH, hydrolytic enzymes of lysosomes are active at acidic pH
(5).
➢ Enzymes are required in very minute amounts for biochemical reactions.
➢ Their catalytic power is represented by Michaelis Menten constant or Km constant and turn over
number. ‘‘The number of substrate molecules converted into products per unit time by one molecule of
the enzyme in favourable conditions is called turnover number.’’
➢ Enzymes are very specific to their substrate or reactions.

STRUCTURE OF ENZYMES

1. Simple enzymes : These are exclusively made up of protein i.e., simple proteins.
E.g., pepsin, trypsin, papain.

2. Conjugated enzymes : Enzymes are composed of one or several polypeptide chains. However, there
are a number of cases in which non-protein constituents called co-factors are bound to the enzyme to
make the enzyme catalytically active. The protein part of the enzyme is called apoenzyme. There are 3
kinds of cofactors which are given as follows :

3. Co-enzymes : These are non-protein organic groups, which are loosely attached to apoenzymes.
These are generally made up of vitamins, e.g., coenzyme nicotinamide adenine dinucleotide (NAD),
nicotinamide adenine dinucleotide phosphate (NADP) contains the vitamin niacin, coenzyme A contains
pantothenic acid, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) contains riboflavin
(Vitamin B2), and thiamine pyrophosphate (TPP) contains thiamine (Vitamin B1).

4. Prosthetic group : When non-protein part is tightly or firmly attached to apoenzyme. These are organic
compounds. E.g., in peroxidase and catalase, which catalyze the breakdown of H2O2 to H2O and O.

5. Metal ions : Metal ions play an essential role in regulating the activity of enzymes by forming
coordination bonds with side chains at the active site and at the same time form one or more
coordination bonds with the substrate. E.g., Mn, Fe, Co, Zn, Ca, Mg, Cu.

6. Active site : Specific part of enzyme at which specific substrate is to bind and catalyse the reaction is
known as an active site. Active site of enzyme is made up of very specific sequence of amino acids
which is, determined by genetic codes.

7. Allosteric site : Besides the active site, some enzymes possess additional sites, at which chemicals
other than substrate (allosteric modulators) bind. These sites are known as allosteric sites and enzymes
with allosteric sites are called as allosteric enzymes, e.g., hexokinase, phosphofructokinase.

TERMS RELATED TO ENZYMES

a) Endoenzymes : Enzymes which are functional only inside the cells.

b) Exoenzymes : Enzymes catalysed the reactions outside the cell. E.g., enzymes of digestion, some
enzymes of insectivorous plants, zymase complex of fermentation.
c) Proenzyme/Zymogen : These are precursors of enzymes or inactive forms of enzymes.
E.g., Pepsinogen, Trypsinogen, etc.
d) Iso-enzymes : Enzymes having similar action, but little difference in their molecular configuration are
called isoenzymes. 16 forms of α-amylase of wheat and 5 forms of LDH (Lactate dehydrogenase) are
known. These all forms are synthesised by different genes.
e) Inducible enzymes : When formation of enzyme is induced by substrate availability. E.g., Lactase,
Nitrogenase, β-galactosidase.
f) Biodetergents : Enzymes used in washing powders are known as bio-detergents, e.g., amylase,
lipase, proteolytic enzymes.
g) Housekeeping / constitutive enzymes : These enzymes are always present in constant amount and
are also essential to cell.
 There are six major classes of enzymes:

1. Dehydrogenases or oxidoreductases- One of the substrate gets oxidised and another reduced, e.g.
oxidases, reductases, dehydrogenases
2. Transferases- Catalyses the transfer of a group from one substrate to another, e.g. transaminase,
transketolase, transaldolase
3. Hydrolases- Hydrolysis of various bonds such as glycosidic, ester, peptide, etc, e.g. amylases, lipases,
proteases, nucleases
4. Lyases- Removal of a group by other than hydrolysis. A double bond is formed, e.g. aldolases,
decarboxylases, fumarase, citrate synthase
5. Isomerases- Formation of isomers (positional, geometrical or optical), e.g. isomerase, epimerase,
mutase
6. Ligases- joining of two compounds, i.e. formation of C-O, C-S, C-N, P-O bonds, e.g. synthetases,
carboxylases

MECHANISM OF ENZYME ACTION

 Energy is required to convert the inert molecules into the activated state. The amount of energy
required to raise the energy of molecules at which chemical reaction can occur is called activation
energy.

 Enzymes act by decreasing the activation energy so that the

number of activated molecules is increased at lower energy
levels. If the activation energy required for the formation of the
enzyme-substrate complex is low, many more molecules can
participate in the reaction than would be the case if the
enzymes were absent.

MODE OF ACTION OF ENZYME

(1) LOCK & KEY THEORY OR TEMPLATE THEORY

✓ The theory was given by Emil Fischer.

✓ According to this theory, active sites of enzymes serve as a
lock into which the reactant substrate fits like a key. Enzymes
have specific sites where a particular substrate can only be
attached. This model accounts for enzyme specificity.

(2) ENZYME - SUBSTRATE COMPLEX THEORY

In 1913, Michaelis and Menten proposed that for a catalytic reaction to occur it is necessary that the
enzyme and substrate bind together to form an enzyme substrate complex.

It is amazing that the enzyme-substrate complex breaks up into chemical products different from that which
participated in its formation (i.e., substrates). On the surface of each enzyme, there are many specific sites
for binding substrate molecules called active sites or catalytic sites.

(3) INDUCED FIT THEORY

✓ This hypothesis was proposed by Koshland (1959).

✓ According to this theory, active site is not static but it undergoes a
conformational change which is induced by specific substrate.
 FACTORS / AFFECTING ENZYME ACTION

The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary
structure of the protein. These include temperature, pH, change in substrate concentration or binding of
specific chemicals that regulate its activity.

1. pH : Enzymes are very sensitive to pH. Each enzyme shows its highest activity at optimum pH. Activity
declines both below and above the optimum value.

2. Temperature : Low temperature preserves the enzyme in a temporarily inactive state whereas high
temperature destroys enzymatic activity because proteins are denatured by heat. Generally all
enzymes perform better at body temperature of an organism.

3. Enzyme concentration : The rate of reaction is directly proportional to enzyme concentration. An

increase in enzyme concentration will cause a rise in the rate of reaction upto a point and after which
the rate of reaction becomes constant. Increasing the enzyme concentration, increases the number of
available active sites.

4. Substrate concentration : Increase in substrate concentration increases the activity of enzymes until
all the active sites of enzymes are saturated by the substrate molecules. Therefore, the substrate
molecules occupy the active sites vacated by the products and cannot increase the rate of reaction
further.

Km CONSTANT (MICHAELIS & MENTEN CONSTANT)

 Km constant of an enzyme is the concentration of substrate at which rate of reaction of that enzyme
attains half of its maximum velocity. It is given by Michaelis & Menten. The value of Km should be lower
for an enzyme.

 Km exhibits catalytic activity of an enzyme.

 Km value differs from substrate to substrate because

different enzymes differ in their affinity towards different
substrates. A high Km indicates low affinity while a low Km
shows strong affinity.

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