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Receptors as drug targets - Drug target interactions, Hormones as chemical messengers, Agonists and antagonists, Practice problems, FAQs

You must have seen your family member or somebody falling sick and taking medicines.  Do you know that for a fever there are multiple drugs and you have to be careful in choosing them? 

Do you know which disease causes the maximum mortality across the world? More people die not because of war, accidents or natural calamity every year but because of Cancer. There are many types of cancer and so are the drugs for them.

Science has advanced to the extent of developing drugs for each cancer for example. These drugs attack only specific cancer cells and not other normal cells and improve recovery. These are called target therapy.

Come let us understand some of the basics of these drugs and their targets. 

Why do we study this concept?
Falling sick and taking medicines are common affairs today for all ages. But, most of the drugs do not differentiate between normal and sick cells. This is the reason for slow recovery and the worst side effect of drugs. More specific the drug attacking the sick cells or cells that are responsible for the disease (or target) fast will be the recovery and less will be the side effect. To develop a drug that can stop or destroy the target cell, we need to know what are drugs and natural immune chemicals produced in our body and their function.

Come on, let us understand the concept of target therapy.

Table of contents:

What are drugs?

Drugs or medicines are chemicals that produce changes in the functional and or mental nature of animals. It is used to diagnose or prevent, cure diseases and to alleviate pain. Drugs are classified in terms of their chemical structure mode of action, physiological effect and mechanism of action.

Drug target interactions:

  • Before going into interaction let us understand the functioning of cells.
  • Each cell has a unique function. The performance of each is dictated by the interaction of certain signals carried by the chemical messengers like neurotransmitters, and hormones through certain protein molecules present outside, on the cell membrane, inside the cell membrane or in the cytoplasm in the cell.
  • Drugs also act by binding to these free proteins(enzymes) or bound proteins of the biological cell.
  • Some of the proteins may assist in taking the drug molecule into the cytoplasm of the cell and are called the carrier or transporter proteins. The protein on or within the mammalian cell that binds the drugs on them to initiate biological action is called receptor protein. These proteins are essential for the good and bad activity of the cell.
  • The hormones are all selective in reaching a particular cell having a particular receptor. But, the drug does not differentiate the cells and hence the ability to induce the right result will be less. Instead, if the drug is directed only to a particular cell of concern, then the effect of the drug will be more and we can avoid side effects. This is where the receptor proteins come into play. A drug can be prepared in such a way that it attacks only the cell we want and the receptor of the cell becomes the target protein.

Receptors as drug targets:

  • Receptor proteins are embedded in the cell membrane in such a way that their active site-containing small portion protrudes from the membrane's inner or outer surface. 
  • They attract chemical messengers like hormones, neurotransmitters, mediators and drugs. Receptors are complex proteins having high selectivity of recognition and attachment of the binding molecule. 
  • It also has multiple sites that can bind to different parts of the same molecule. They exhibit reversible binding spending on the conditions.
  • These properties make the receptor a potential target for developing drugs to control biological effects.

Functioning of receptor proteins:

Chemical messengers are substances that are sent to receptor proteins' binding sites. The shape of the receptor site alters to accommodate a message.

As a result, a chemical messenger sends a message to the cell without really entering it.


The steps involved in this process are as follows:

1. Receptors bind to the specific chemical messenger.

2. The receptor's shape changes when the chemical messenger is attached.

3. The receptor regains its structure once the chemical messenger is removed.

Hormones as Chemical messengers

  • Do you know that our instinct actions of time-specific eating, sleeping, waking up, digestion of sweets, and running away out of fear,  are all because of the chemical messengers called Hormones.
  • Hormones are chemical messengers that flow throughout the body and help to coordinate complicated activities including growth, development, metabolism, and reproduction. Endocrine cells, which are found in glands, produce these chemical compounds. The hormones subsequently reach the bloodstream, where they circulate and trigger target cells

Agonists and Antagonists:

  • An agonist is a messenger chemical that binds to the receptor and produces a reaction that is expected to do. 
  • An antagonist, on the other hand, is a medication that binds to the receptor either at the primary location or at a different place, preventing the receptor from creating its usual response.


Inverse agonist and mixed agonist & antagonist

  • Inverse agonist is the drug that produces a biological reaction opposite to the agonist response.
  • A mixed agonist-antagonist is a drug that produces an active effect at one target receptor and an opposite effect on another receptor.

Practice problems: 

Q1. Give an example to differentiate between agonist and antagonist?
Answer: The fundamental difference between these two medications is that one simulates the desired reaction, while the other attaches to the receptor and inhibits/slows it. Agonists fundamentally imitate the actions of regular neurotransmitters like acetylcholine and the responses they elicit from the receptors they attach to. The binding of atropine on the other hand prevents the reaction given by acetylcholine.

Q2. Neurotransmitters are examples of 

A. Medicine
B. Drug acceptor
C. Chemical messenger
D. Drug receptor


Answer: (C)
A neurotransmitter is a chemical that allows cells to communicate with one another and acts as chemical messengers.

Q3. What are the binding sites in a protein?
Answer: A binding site is a location on a protein that binds to a ligand, which is a smaller molecule than the protein. Binding sites are tiny spaces on the tertiary structure of proteins where ligands bind with modest forces (non-covalent bonding)

Q4. Morphine is an example of 

A. Receptor
B. Agonist
C. Analgesic
D. Histamine

Answer: C
A medication that binds to a receptor and elicits a functional response is known as an agonist. Morphine is an example of an analgesics, also called painkillers, which are medications that relieve different types of pain.

Frequently asked questions-FAQs

Q1. What is the difference between drugs and medicine?

Answer: A drug is any substance that, when consumed or otherwise introduced into the body, has a physiological impact. Medicine refers to a substance that is used in treating a disease or relieving pain, and that is usually in the form of a pill or a liquid. Medicine has a favourable effect on our health.

Q2. The average molecular mass of a drug is

A. 50-60 g mol-1
B. 40-70 g mol-1
C.150-200 g mol-1
D. 160-480 g mol-1

Answer: (D)
Drugs are heavy molecule with huge molecular masses in between 160-480 g mol-1

Q3. Write the uses of Medicines.

Answer: Medicines are used to treat, prevent, and diagnose diseases.
Most pharmaceuticals used as medicines are potential poisons, thus we should not take them in higher doses than those recommended.

Q4. Receptors are located mostly in the

A. Inner part of the cell membrane
B. Middle part of the cell membrane
C. Outer part of the cell membrane
D. Left corner of the cell membrane

Answer: (C)

Solution: The body's communication system relies heavily on receptors. Receptors are placed on the cell membrane's outer surface so that their little active site protrudes from the membrane's surfaces and opens onto the cell membrane's outside region.

Q5 What type of binding force exists between the messenger and the receptor?

Answer: The messenger may be attached to the target receptor by ionic, covalent, van der Waals or hydrogen bonding forces. 

Related topics:

Chemicals in medicines Chemicals in food
Enzymes as drug target Pesticides
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