NCERT Textbook: Chemistry in Everyday Life (Old NCERT) | Chemistry Class 12 - NEET PDF Download

 Page 1


After studying this Unit you will be
able to
• visualise the importance of
Chemistry in daily life;
• explain the term ‘chemotherapy’;
• describe the basis of classification
of drugs;
• explain drug-target interaction of
enzymes and receptors;
• explain how various types of
drugs function in the body;
• know about artificial sweetening
agents and food preservatives;
• discuss the chemistry of cleansing
agents.
Objectives
From living perception to abstract thought, and from this to practice.
V.I. Lenin.
16
Unit Unit Unit Unit Unit
16
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
By now, you have learnt the basic principles of
chemistry and also realised that it influences every
sphere of human life.  The principles of chemistry have
been used for the benefit of mankind. Think of
cleanliness — the materials like soaps, detergents,
household bleaches, tooth pastes, etc. will come to your
mind. Look towards the beautiful clothes — immediately
chemicals of the synthetic fibres used for making clothes
and chemicals giving colours to them will come to your
mind. Food materials — again a number of chemicals
about which you have learnt in the previous Unit will
appear in your mind. Of course, sickness and diseases
remind us of medicines — again chemicals. Explosives,
fuels, rocket propellents, building and electronic
materials, etc., are all chemicals. Chemistry has
influenced our life so much that we do not even realise
that we come across chemicals at every moment; that
we ourselves are beautiful chemical creations and all
our activities are controlled by chemicals. In this Unit,
we shall learn the application of Chemistry in three
important and interesting areas, namely – medicines,
food materials and cleansing agents.
Drugs are chemicals of low molecular masses (~100 – 500u). These
interact with macromolecular targets and produce a biological response.
When the biological response is therapeutic and useful, these chemicals
are called medicines and are used in diagnosis, prevention and
treatment of diseases. Most of the drugs used as medicines are potential
poisons, if taken in doses higher than those recommended. Use of
chemicals for therapeutic effect is called chemotherapy.
16.1 16.1 16.1 16.1 16.1Drugs and Drugs and Drugs and Drugs and Drugs and
their their their their their
Classification Classification Classification Classification Classification
2022-23
Page 2


After studying this Unit you will be
able to
• visualise the importance of
Chemistry in daily life;
• explain the term ‘chemotherapy’;
• describe the basis of classification
of drugs;
• explain drug-target interaction of
enzymes and receptors;
• explain how various types of
drugs function in the body;
• know about artificial sweetening
agents and food preservatives;
• discuss the chemistry of cleansing
agents.
Objectives
From living perception to abstract thought, and from this to practice.
V.I. Lenin.
16
Unit Unit Unit Unit Unit
16
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
By now, you have learnt the basic principles of
chemistry and also realised that it influences every
sphere of human life.  The principles of chemistry have
been used for the benefit of mankind. Think of
cleanliness — the materials like soaps, detergents,
household bleaches, tooth pastes, etc. will come to your
mind. Look towards the beautiful clothes — immediately
chemicals of the synthetic fibres used for making clothes
and chemicals giving colours to them will come to your
mind. Food materials — again a number of chemicals
about which you have learnt in the previous Unit will
appear in your mind. Of course, sickness and diseases
remind us of medicines — again chemicals. Explosives,
fuels, rocket propellents, building and electronic
materials, etc., are all chemicals. Chemistry has
influenced our life so much that we do not even realise
that we come across chemicals at every moment; that
we ourselves are beautiful chemical creations and all
our activities are controlled by chemicals. In this Unit,
we shall learn the application of Chemistry in three
important and interesting areas, namely – medicines,
food materials and cleansing agents.
Drugs are chemicals of low molecular masses (~100 – 500u). These
interact with macromolecular targets and produce a biological response.
When the biological response is therapeutic and useful, these chemicals
are called medicines and are used in diagnosis, prevention and
treatment of diseases. Most of the drugs used as medicines are potential
poisons, if taken in doses higher than those recommended. Use of
chemicals for therapeutic effect is called chemotherapy.
16.1 16.1 16.1 16.1 16.1Drugs and Drugs and Drugs and Drugs and Drugs and
their their their their their
Classification Classification Classification Classification Classification
2022-23
448 Chemistry
Drugs can be classified mainly on criteria outlined as follows:
(a) On the basis of pharmacological effect
This classification is based on pharmacological effect of the drugs. It
is useful for doctors because it provides them the whole range of
drugs available for the treatment of a particular type of problem. For
example, analgesics have pain killing effect, antiseptics kill or arrest
the growth of microorganisms.
(b) On the basis of drug action
It is based on the action of a drug on a particular biochemical process.
For example, all antihistamines inhibit the action of the compound,
histamine which causes inflammation in the body. There are various
ways in which action of histamines can be blocked. You will  learn
about this in Section 16.3.2.
(c) On the basis of chemical structure
It is based on the chemical structure of the drug. Drugs classified in this
way share common structural features and often have similar
pharmacological activity.  For example, sulphonamides have common
structural feature, given below.
Structural features of sulphonamides
(d) On the basis of molecular targets
Drugs usually interact with biomolecules such as carbohydrates, lipids,
proteins and nucleic acids. These are called target molecules or drug
targets. Drugs possessing some common structural features may have
the same mechanism of action on targets. The classification based on
molecular targets is the most useful classification for medicinal chemists.
Macromolecules of biological origin perform various functions in the
body. For example, proteins which perform the role of biological catalysts
in the body are called enzymes, those which are crucial to
communication system in the body are called receptors. Carrier proteins
carry polar molecules across the cell membrane. Nucleic acids have
coded genetic information for the cell. Lipids and carbohydrates are
structural parts of the cell membrane. We shall explain the drug-target
interaction with the examples of enzymes and receptors.
(a) Catalytic action of enzymes
For understanding the interaction between a drug and an enzyme,
it is important to know how do enzymes catalyse the reaction
(Section 5.2.4). In their catalytic activity, enzymes perform two
major functions:
(i) The first function of an enzyme is to hold the substrate for a chemical
reaction. Active sites of enzymes hold the substrate molecule in a
suitable position, so that it can be attacked by the reagent effectively.
16.1.1
Classification of
Drugs
16.2 16.2 16.2 16.2 16.2 Drug-Target Drug-Target Drug-Target Drug-Target Drug-Target
Interaction Interaction Interaction Interaction Interaction
16.2.1 Enzymes
as Drug
Targets
2022-23
Page 3


After studying this Unit you will be
able to
• visualise the importance of
Chemistry in daily life;
• explain the term ‘chemotherapy’;
• describe the basis of classification
of drugs;
• explain drug-target interaction of
enzymes and receptors;
• explain how various types of
drugs function in the body;
• know about artificial sweetening
agents and food preservatives;
• discuss the chemistry of cleansing
agents.
Objectives
From living perception to abstract thought, and from this to practice.
V.I. Lenin.
16
Unit Unit Unit Unit Unit
16
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
By now, you have learnt the basic principles of
chemistry and also realised that it influences every
sphere of human life.  The principles of chemistry have
been used for the benefit of mankind. Think of
cleanliness — the materials like soaps, detergents,
household bleaches, tooth pastes, etc. will come to your
mind. Look towards the beautiful clothes — immediately
chemicals of the synthetic fibres used for making clothes
and chemicals giving colours to them will come to your
mind. Food materials — again a number of chemicals
about which you have learnt in the previous Unit will
appear in your mind. Of course, sickness and diseases
remind us of medicines — again chemicals. Explosives,
fuels, rocket propellents, building and electronic
materials, etc., are all chemicals. Chemistry has
influenced our life so much that we do not even realise
that we come across chemicals at every moment; that
we ourselves are beautiful chemical creations and all
our activities are controlled by chemicals. In this Unit,
we shall learn the application of Chemistry in three
important and interesting areas, namely – medicines,
food materials and cleansing agents.
Drugs are chemicals of low molecular masses (~100 – 500u). These
interact with macromolecular targets and produce a biological response.
When the biological response is therapeutic and useful, these chemicals
are called medicines and are used in diagnosis, prevention and
treatment of diseases. Most of the drugs used as medicines are potential
poisons, if taken in doses higher than those recommended. Use of
chemicals for therapeutic effect is called chemotherapy.
16.1 16.1 16.1 16.1 16.1Drugs and Drugs and Drugs and Drugs and Drugs and
their their their their their
Classification Classification Classification Classification Classification
2022-23
448 Chemistry
Drugs can be classified mainly on criteria outlined as follows:
(a) On the basis of pharmacological effect
This classification is based on pharmacological effect of the drugs. It
is useful for doctors because it provides them the whole range of
drugs available for the treatment of a particular type of problem. For
example, analgesics have pain killing effect, antiseptics kill or arrest
the growth of microorganisms.
(b) On the basis of drug action
It is based on the action of a drug on a particular biochemical process.
For example, all antihistamines inhibit the action of the compound,
histamine which causes inflammation in the body. There are various
ways in which action of histamines can be blocked. You will  learn
about this in Section 16.3.2.
(c) On the basis of chemical structure
It is based on the chemical structure of the drug. Drugs classified in this
way share common structural features and often have similar
pharmacological activity.  For example, sulphonamides have common
structural feature, given below.
Structural features of sulphonamides
(d) On the basis of molecular targets
Drugs usually interact with biomolecules such as carbohydrates, lipids,
proteins and nucleic acids. These are called target molecules or drug
targets. Drugs possessing some common structural features may have
the same mechanism of action on targets. The classification based on
molecular targets is the most useful classification for medicinal chemists.
Macromolecules of biological origin perform various functions in the
body. For example, proteins which perform the role of biological catalysts
in the body are called enzymes, those which are crucial to
communication system in the body are called receptors. Carrier proteins
carry polar molecules across the cell membrane. Nucleic acids have
coded genetic information for the cell. Lipids and carbohydrates are
structural parts of the cell membrane. We shall explain the drug-target
interaction with the examples of enzymes and receptors.
(a) Catalytic action of enzymes
For understanding the interaction between a drug and an enzyme,
it is important to know how do enzymes catalyse the reaction
(Section 5.2.4). In their catalytic activity, enzymes perform two
major functions:
(i) The first function of an enzyme is to hold the substrate for a chemical
reaction. Active sites of enzymes hold the substrate molecule in a
suitable position, so that it can be attacked by the reagent effectively.
16.1.1
Classification of
Drugs
16.2 16.2 16.2 16.2 16.2 Drug-Target Drug-Target Drug-Target Drug-Target Drug-Target
Interaction Interaction Interaction Interaction Interaction
16.2.1 Enzymes
as Drug
Targets
2022-23
449 Chemistry in Everyday Life
Fig. 16.2
Drug and substrate
competing for active
site
(ii) The second function of an enzyme is to provide functional groups
that will attack the substrate and carry out chemical reaction.
(b) Drug-enzyme interaction
Drugs inhibit any of the above mentioned activities of enzymes. These
can block the binding site of the enzyme and prevent the binding of
substrate, or can inhibit the catalytic activity of the enzyme. Such
drugs are called enzyme inhibitors.
Drugs inhibit the attachment of substrate on active site of enzymes
in two different ways;
(i) Drugs compete with the natural substrate for their attachment
on the active sites of enzymes. Such drugs are called competitive
inhibitors (Fig. 16.2).
Fig. 16.1
(a) Active site of an
enzyme (b) Substrate
(c) Substrate held in
active site of the
enzyme
(ii) Some drugs do not bind to the
enzyme’s active site. These bind
to a different site of enzyme
which is called allosteric site.
This binding of inhibitor at
allosteric site (Fig.16.3) changes
the shape of the active site in
such a way that substrate can-
not recognise it.
If the bond formed between
an enzyme and an inhibitor is
a strong covalent bond and
Substrates bind to the active site of the enzyme through a variety
of interactions such as ionic bonding, hydrogen bonding, van der
Waals interaction or dipole-dipole interaction (Fig. 16.1).
Fig. 16.3:  Non-competitive inhibitor changes the active
site of enzyme after binding at allosteric site.
2022-23
Page 4


After studying this Unit you will be
able to
• visualise the importance of
Chemistry in daily life;
• explain the term ‘chemotherapy’;
• describe the basis of classification
of drugs;
• explain drug-target interaction of
enzymes and receptors;
• explain how various types of
drugs function in the body;
• know about artificial sweetening
agents and food preservatives;
• discuss the chemistry of cleansing
agents.
Objectives
From living perception to abstract thought, and from this to practice.
V.I. Lenin.
16
Unit Unit Unit Unit Unit
16
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
By now, you have learnt the basic principles of
chemistry and also realised that it influences every
sphere of human life.  The principles of chemistry have
been used for the benefit of mankind. Think of
cleanliness — the materials like soaps, detergents,
household bleaches, tooth pastes, etc. will come to your
mind. Look towards the beautiful clothes — immediately
chemicals of the synthetic fibres used for making clothes
and chemicals giving colours to them will come to your
mind. Food materials — again a number of chemicals
about which you have learnt in the previous Unit will
appear in your mind. Of course, sickness and diseases
remind us of medicines — again chemicals. Explosives,
fuels, rocket propellents, building and electronic
materials, etc., are all chemicals. Chemistry has
influenced our life so much that we do not even realise
that we come across chemicals at every moment; that
we ourselves are beautiful chemical creations and all
our activities are controlled by chemicals. In this Unit,
we shall learn the application of Chemistry in three
important and interesting areas, namely – medicines,
food materials and cleansing agents.
Drugs are chemicals of low molecular masses (~100 – 500u). These
interact with macromolecular targets and produce a biological response.
When the biological response is therapeutic and useful, these chemicals
are called medicines and are used in diagnosis, prevention and
treatment of diseases. Most of the drugs used as medicines are potential
poisons, if taken in doses higher than those recommended. Use of
chemicals for therapeutic effect is called chemotherapy.
16.1 16.1 16.1 16.1 16.1Drugs and Drugs and Drugs and Drugs and Drugs and
their their their their their
Classification Classification Classification Classification Classification
2022-23
448 Chemistry
Drugs can be classified mainly on criteria outlined as follows:
(a) On the basis of pharmacological effect
This classification is based on pharmacological effect of the drugs. It
is useful for doctors because it provides them the whole range of
drugs available for the treatment of a particular type of problem. For
example, analgesics have pain killing effect, antiseptics kill or arrest
the growth of microorganisms.
(b) On the basis of drug action
It is based on the action of a drug on a particular biochemical process.
For example, all antihistamines inhibit the action of the compound,
histamine which causes inflammation in the body. There are various
ways in which action of histamines can be blocked. You will  learn
about this in Section 16.3.2.
(c) On the basis of chemical structure
It is based on the chemical structure of the drug. Drugs classified in this
way share common structural features and often have similar
pharmacological activity.  For example, sulphonamides have common
structural feature, given below.
Structural features of sulphonamides
(d) On the basis of molecular targets
Drugs usually interact with biomolecules such as carbohydrates, lipids,
proteins and nucleic acids. These are called target molecules or drug
targets. Drugs possessing some common structural features may have
the same mechanism of action on targets. The classification based on
molecular targets is the most useful classification for medicinal chemists.
Macromolecules of biological origin perform various functions in the
body. For example, proteins which perform the role of biological catalysts
in the body are called enzymes, those which are crucial to
communication system in the body are called receptors. Carrier proteins
carry polar molecules across the cell membrane. Nucleic acids have
coded genetic information for the cell. Lipids and carbohydrates are
structural parts of the cell membrane. We shall explain the drug-target
interaction with the examples of enzymes and receptors.
(a) Catalytic action of enzymes
For understanding the interaction between a drug and an enzyme,
it is important to know how do enzymes catalyse the reaction
(Section 5.2.4). In their catalytic activity, enzymes perform two
major functions:
(i) The first function of an enzyme is to hold the substrate for a chemical
reaction. Active sites of enzymes hold the substrate molecule in a
suitable position, so that it can be attacked by the reagent effectively.
16.1.1
Classification of
Drugs
16.2 16.2 16.2 16.2 16.2 Drug-Target Drug-Target Drug-Target Drug-Target Drug-Target
Interaction Interaction Interaction Interaction Interaction
16.2.1 Enzymes
as Drug
Targets
2022-23
449 Chemistry in Everyday Life
Fig. 16.2
Drug and substrate
competing for active
site
(ii) The second function of an enzyme is to provide functional groups
that will attack the substrate and carry out chemical reaction.
(b) Drug-enzyme interaction
Drugs inhibit any of the above mentioned activities of enzymes. These
can block the binding site of the enzyme and prevent the binding of
substrate, or can inhibit the catalytic activity of the enzyme. Such
drugs are called enzyme inhibitors.
Drugs inhibit the attachment of substrate on active site of enzymes
in two different ways;
(i) Drugs compete with the natural substrate for their attachment
on the active sites of enzymes. Such drugs are called competitive
inhibitors (Fig. 16.2).
Fig. 16.1
(a) Active site of an
enzyme (b) Substrate
(c) Substrate held in
active site of the
enzyme
(ii) Some drugs do not bind to the
enzyme’s active site. These bind
to a different site of enzyme
which is called allosteric site.
This binding of inhibitor at
allosteric site (Fig.16.3) changes
the shape of the active site in
such a way that substrate can-
not recognise it.
If the bond formed between
an enzyme and an inhibitor is
a strong covalent bond and
Substrates bind to the active site of the enzyme through a variety
of interactions such as ionic bonding, hydrogen bonding, van der
Waals interaction or dipole-dipole interaction (Fig. 16.1).
Fig. 16.3:  Non-competitive inhibitor changes the active
site of enzyme after binding at allosteric site.
2022-23
450 Chemistry
cannot be broken easily, then the enzyme is blocked permanently.
The body then degrades the enzyme-inhibitor complex and
synthesises the new enzyme.
Receptors are proteins that are crucial to body’s communication
process. Majority of these are embedded in cell membranes (Fig.
16.4). Receptor proteins are embedded in the cell membrane in such
a way that their small part possessing active site projects out of the
surface of the membrane and opens on the outside region of the cell
membrane (Fig. 16.4).
16.2.2 Receptors
as Drug
Targets
Fig. 16.4
Receptor protein
embedded in the cell
membrane, the
active site of the
receptor opens on
the outside region of
the cell.
Fig. 16.5: (a) Receptor receiving chemical messenger
(b) Shape of the receptor changed after attachment of messenger
(c) Receptor regains structure after removal of chemical messenger.
There are a large number of different receptors in the body that
interact with different chemical messengers. These receptors show
selectivity for one chemical messenger over the other because their binding
sites have different shape, structure and amino acid composition.
In the body, message between two neurons and that between neurons
to muscles is communicated through certain chemicals. These chemicals,
known as chemical messengers are received at the binding sites of receptor
proteins. To accommodate a messenger, shape of the receptor site changes.
This brings about the transfer of message into the cell. Thus, chemical
messenger gives message to the cell without entering the cell (Fig. 16.5).
2022-23
Page 5


After studying this Unit you will be
able to
• visualise the importance of
Chemistry in daily life;
• explain the term ‘chemotherapy’;
• describe the basis of classification
of drugs;
• explain drug-target interaction of
enzymes and receptors;
• explain how various types of
drugs function in the body;
• know about artificial sweetening
agents and food preservatives;
• discuss the chemistry of cleansing
agents.
Objectives
From living perception to abstract thought, and from this to practice.
V.I. Lenin.
16
Unit Unit Unit Unit Unit
16
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
Chemistr Chemistr Chemistr Chemistr Chemistry y y y y     in in in in in
E E E E Ev v v v ver er er er eryday yday yday yday yday     L L L L Life ife ife ife ife
By now, you have learnt the basic principles of
chemistry and also realised that it influences every
sphere of human life.  The principles of chemistry have
been used for the benefit of mankind. Think of
cleanliness — the materials like soaps, detergents,
household bleaches, tooth pastes, etc. will come to your
mind. Look towards the beautiful clothes — immediately
chemicals of the synthetic fibres used for making clothes
and chemicals giving colours to them will come to your
mind. Food materials — again a number of chemicals
about which you have learnt in the previous Unit will
appear in your mind. Of course, sickness and diseases
remind us of medicines — again chemicals. Explosives,
fuels, rocket propellents, building and electronic
materials, etc., are all chemicals. Chemistry has
influenced our life so much that we do not even realise
that we come across chemicals at every moment; that
we ourselves are beautiful chemical creations and all
our activities are controlled by chemicals. In this Unit,
we shall learn the application of Chemistry in three
important and interesting areas, namely – medicines,
food materials and cleansing agents.
Drugs are chemicals of low molecular masses (~100 – 500u). These
interact with macromolecular targets and produce a biological response.
When the biological response is therapeutic and useful, these chemicals
are called medicines and are used in diagnosis, prevention and
treatment of diseases. Most of the drugs used as medicines are potential
poisons, if taken in doses higher than those recommended. Use of
chemicals for therapeutic effect is called chemotherapy.
16.1 16.1 16.1 16.1 16.1Drugs and Drugs and Drugs and Drugs and Drugs and
their their their their their
Classification Classification Classification Classification Classification
2022-23
448 Chemistry
Drugs can be classified mainly on criteria outlined as follows:
(a) On the basis of pharmacological effect
This classification is based on pharmacological effect of the drugs. It
is useful for doctors because it provides them the whole range of
drugs available for the treatment of a particular type of problem. For
example, analgesics have pain killing effect, antiseptics kill or arrest
the growth of microorganisms.
(b) On the basis of drug action
It is based on the action of a drug on a particular biochemical process.
For example, all antihistamines inhibit the action of the compound,
histamine which causes inflammation in the body. There are various
ways in which action of histamines can be blocked. You will  learn
about this in Section 16.3.2.
(c) On the basis of chemical structure
It is based on the chemical structure of the drug. Drugs classified in this
way share common structural features and often have similar
pharmacological activity.  For example, sulphonamides have common
structural feature, given below.
Structural features of sulphonamides
(d) On the basis of molecular targets
Drugs usually interact with biomolecules such as carbohydrates, lipids,
proteins and nucleic acids. These are called target molecules or drug
targets. Drugs possessing some common structural features may have
the same mechanism of action on targets. The classification based on
molecular targets is the most useful classification for medicinal chemists.
Macromolecules of biological origin perform various functions in the
body. For example, proteins which perform the role of biological catalysts
in the body are called enzymes, those which are crucial to
communication system in the body are called receptors. Carrier proteins
carry polar molecules across the cell membrane. Nucleic acids have
coded genetic information for the cell. Lipids and carbohydrates are
structural parts of the cell membrane. We shall explain the drug-target
interaction with the examples of enzymes and receptors.
(a) Catalytic action of enzymes
For understanding the interaction between a drug and an enzyme,
it is important to know how do enzymes catalyse the reaction
(Section 5.2.4). In their catalytic activity, enzymes perform two
major functions:
(i) The first function of an enzyme is to hold the substrate for a chemical
reaction. Active sites of enzymes hold the substrate molecule in a
suitable position, so that it can be attacked by the reagent effectively.
16.1.1
Classification of
Drugs
16.2 16.2 16.2 16.2 16.2 Drug-Target Drug-Target Drug-Target Drug-Target Drug-Target
Interaction Interaction Interaction Interaction Interaction
16.2.1 Enzymes
as Drug
Targets
2022-23
449 Chemistry in Everyday Life
Fig. 16.2
Drug and substrate
competing for active
site
(ii) The second function of an enzyme is to provide functional groups
that will attack the substrate and carry out chemical reaction.
(b) Drug-enzyme interaction
Drugs inhibit any of the above mentioned activities of enzymes. These
can block the binding site of the enzyme and prevent the binding of
substrate, or can inhibit the catalytic activity of the enzyme. Such
drugs are called enzyme inhibitors.
Drugs inhibit the attachment of substrate on active site of enzymes
in two different ways;
(i) Drugs compete with the natural substrate for their attachment
on the active sites of enzymes. Such drugs are called competitive
inhibitors (Fig. 16.2).
Fig. 16.1
(a) Active site of an
enzyme (b) Substrate
(c) Substrate held in
active site of the
enzyme
(ii) Some drugs do not bind to the
enzyme’s active site. These bind
to a different site of enzyme
which is called allosteric site.
This binding of inhibitor at
allosteric site (Fig.16.3) changes
the shape of the active site in
such a way that substrate can-
not recognise it.
If the bond formed between
an enzyme and an inhibitor is
a strong covalent bond and
Substrates bind to the active site of the enzyme through a variety
of interactions such as ionic bonding, hydrogen bonding, van der
Waals interaction or dipole-dipole interaction (Fig. 16.1).
Fig. 16.3:  Non-competitive inhibitor changes the active
site of enzyme after binding at allosteric site.
2022-23
450 Chemistry
cannot be broken easily, then the enzyme is blocked permanently.
The body then degrades the enzyme-inhibitor complex and
synthesises the new enzyme.
Receptors are proteins that are crucial to body’s communication
process. Majority of these are embedded in cell membranes (Fig.
16.4). Receptor proteins are embedded in the cell membrane in such
a way that their small part possessing active site projects out of the
surface of the membrane and opens on the outside region of the cell
membrane (Fig. 16.4).
16.2.2 Receptors
as Drug
Targets
Fig. 16.4
Receptor protein
embedded in the cell
membrane, the
active site of the
receptor opens on
the outside region of
the cell.
Fig. 16.5: (a) Receptor receiving chemical messenger
(b) Shape of the receptor changed after attachment of messenger
(c) Receptor regains structure after removal of chemical messenger.
There are a large number of different receptors in the body that
interact with different chemical messengers. These receptors show
selectivity for one chemical messenger over the other because their binding
sites have different shape, structure and amino acid composition.
In the body, message between two neurons and that between neurons
to muscles is communicated through certain chemicals. These chemicals,
known as chemical messengers are received at the binding sites of receptor
proteins. To accommodate a messenger, shape of the receptor site changes.
This brings about the transfer of message into the cell. Thus, chemical
messenger gives message to the cell without entering the cell (Fig. 16.5).
2022-23
451 Chemistry in Everyday Life
Drugs that bind to the receptor site and inhibit its natural function
are called antagonists. These are useful when blocking of message is
required. There are other types of drugs that mimic the natural
messenger by switching on the receptor, these are called agonists.
These are useful when there is lack of natural chemical messenger.
In this Section, we shall discuss the therapeutic action
of a few important classes of drugs.
Over production of acid in the stomach causes irritation and pain. In
severe cases, ulcers are developed in the stomach. Until 1970, only
treatment for acidity was administration of antacids, such as sodium
hydrogencarbonate or a mixture of aluminium and magnesium
hydroxide. However, excessive hydrogencarbonate can make the stomach
alkaline and trigger the production of even more acid. Metal hydroxides
are better alternatives because of being insoluble, these do not increase
the pH above neutrality. These treatments control only symptoms, and
not the cause. Therefore, with these metal salts, the patients cannot be
treated easily. In advanced stages, ulcers become life threatening and its
only treatment is removal of the affected part of the stomach.
A major breakthrough in the treatment of hyperacidity came through
the discovery according  to which a chemical, histamine, stimulates the
secretion of pepsin and hydrochloric acid in the stomach. The drug
cimetidine (Tegamet), was designed to prevent the interaction of
histamine with the receptors present in the stomach wall. This resulted
in release of lesser amount of acid.  The importance of the drug was
so much that it remained the largest selling drug in the world until
another drug, ranitidine (Zantac), was discovered.
16.3 16.3 16.3 16.3 16.3 Therapeutic Action of Therapeutic Action of Therapeutic Action of Therapeutic Action of Therapeutic Action of
Different Classes of Drugs Different Classes of Drugs Different Classes of Drugs Different Classes of Drugs Different Classes of Drugs
16.3.1 Antacids
Histamine is a potent vasodilator. It has various functions. It contracts
the smooth muscles in the bronchi and gut and relaxes other muscles,
such as those in the walls of fine blood vessels. Histamine is also
responsible for the nasal congestion associated with common cold and
allergic response to pollen.
Synthetic drugs, brompheniramine (Dimetapp) and terfenadine
(Seldane), act as antihistamines. They interfere with the natural action
16.3.2
Antihistamines
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