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Antagonists - Introduction, Drug Target Interactions, Receptor as Drug Target, Antagonist Types, Practice Problems & FAQs

Antagonists - Introduction, Drug Target Interactions, Receptor as Drug Target, Antagonist Types, Practice Problems & FAQs

Every child loves playing games whether it may be indoor or outdoor. A very few of you who don’t enjoy playing.

Mahi, a ten-year-old boy, was very lazy. His mother always keeps shouting at him to wake up, to get ready for school but when she comes to check on him, she finds him sleeping. After this repeated behaviour of Mahi, her mother decided to hide some of his games and comics which he loves doing in his free time.

When Mahi got to know about these things, he started crying and promised her mother that he would wake up on time and complete his homework. Wait! Why am I telling you about Mahi’s story?

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In this concept page, we will study an important concept of Antagonist. Now I can relate this to Mahi’s story to the work of the Antagonist. Here Mahi’s mother acts as an antagonist as she inhibits Mahi’s natural tendency of sleeping and hides his favourite belongings so that he can go back to his work.

An antagonist is somehow similar to Mahi’s mother, they bind to the drug sites and inhibit their main functions.

Let’s study more about this important term and how it interacts with drugs in a bit more detail!

TABLE OF CONTENT

  • What is an Antagonist?
  • What is an agonist?
  • Drug target interactions
  • Receptors as drug targets
  • Types of Antagonist
  • Practice Problems
  • Frequently asked questions-FAQs

What is an Antagonist?

  • An example of a receptor ligand is a receptor antagonist. Drugs that bind to receptors and block them rather than stimulating them as an agonist would.

  • This stops or dampens a biological response. Antibodies prevent receptor proteins from functioning normally. They are referred to as blockers at times; examples include calcium channel blockers, beta-blockers, and alpha-blockers.
  • In pharmacology, antagonists bind to their corresponding receptors with affinity but lack effectiveness, disrupting the connection and inhibiting the activity of an agonist or inverse agonist at the receptor.

  • In order to exert their effects, antagonists either attach to the receptor's active site or allosteric site, or they may connect at special binding sites that are not typically involved in the biological control of the receptor's activity.
  • The lifespan of a common antagonist-receptor molecule, which in turn is based on the type of antagonist-receptor binding, determines whether antagonist action is reversible or irreversible. The majority of pharmacological antagonists work to block receptors at their physically specified binding sites by out-competing internal ligands or substrates.

What is an agonist?

An agonist replicates the action of a general natural ligand and has a similar biological effect when it binds to a receptor. In the absence of the natural ligand, it attaches to the similar binding site and causes either a complete or partial response. It is referred to as a partial agonist in the latter scenario. The actions of the ligand, agonist and partial agonist are depicted in the figure below.

Another agonist subtype goes by the odd moniker of inverse agonist. Only when referring to a receptor that is constitutively active if there isn't one binding ligand does this word make sense. The receptor site is bound by either a natural ligand or an agonist, increasing the basal activity. But if an antagonistic agonist binds, the activity is reduced.

Drug target interactions:

Before discussing interactions, let's discuss how cells operate.

  • Each cell performs a certain role. The interplay of specific signals transmitted by chemical messengers such as neurotransmitters and hormones through specific protein molecules located outside, on the cell membrane, inside the cell membrane, or in the cytoplasm of the cell determines how well each performs.

  • 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.

  • The biological cell's free proteins (enzymes) or bound proteins are where drugs also work via binding.
  • Some proteins, known as carriers or transporter proteins, may help carry the drug molecule into the cell's cytoplasm. The receptor protein is the protein on or within the mammalian cell that binds the medicines to start the biological effect.
  • 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:

  • The small region of receptor proteins that contains the active site flexes from either the inner or outer surface of the membrane according to the way they are lodged in the cell membrane.
  • Chemical messengers drawn to them include hormones, neurotransmitters, mediators, and medicines. Receptors are intricate proteins with very selective binding molecules that they may recognise and adhere to.
  • Additionally, it possesses several binding sites that can connect to various regions of the same molecule. On the conditions, they display reversible binding expenditure.
  • Due to these characteristics, the receptor is a possible target for the creation of medicines that regulate biological functions.

Types of Antagonist:

There are two types of Antagonist, let us discuss these two antagonists.

  • Competitive antagonists
  • Non-competitive antagonists

Competitive antagonists:

  • Competitive antagonists attach to receptors at the same binding location (active site) as the naturally occurring ligand or agonist, but do not cause the receptor to become activated. Both agonists and antagonists "compete" for the same receptor binding site. Once bound, an antagonist will prevent the binding of an agonist.
  • A reduced frequency of receptor activation will occur from the displacement of the agonist from the binding sites by sufficient quantities of an antagonist. The relative affinity of each molecule for the site and their proportional concentrations will define the receptor's degree of activity.
  • Larger doses of the antagonist are needed to achieve the same level of binding site occupancy as higher concentrations of a competitive agonist will increase the fraction of receptors that the agonist occupies.
  • Competitive antagonists are employed to stop drug action as well as to undo drug-related effects after consumption. Narcan, commonly known as naloxone, is used to counteract opioid overdoses brought on by heroin or other opiates.
  • Depending on how they interact with the receptor protein targets, competitive antagonists can be categorized as reversible (surmountable) or irreversible (insurmountable) antagonists.
  • Reversible antagonists, which bind to the receptor by non-covalent intermolecular interactions, gradually disassociate from the receptor, releasing it to attach to other molecules once more.
  • Covalent intermolecular forces are used to bind irreversible antagonists. The link is virtually "permanent," meaning the receptor-antagonist complex will never dissociate because there isn't enough free energy in the immediate environment to break covalent bonds.

Non-competitive antagonists:

  • Noncompetitive antagonists are substances that decrease the biological activity of a receptor by binding to a different location on the receptor than the agonist, natural ligand, or partial agonist (or, perhaps more broadly, mixed antagonists).
  • The noncompetitive antagonist, similar to the non-competitive and mixed enzyme restraint, may alter the appearance Kd for the ligand, common agonist, or partial agonist (the ligand concentration necessary to produce half-maximal biological effects), but will alter the strongest reaction to the ligand (much like mixed inhibitors alter the apparent Vmax ).
  • Allosteric site activity is where the second type of "non-competitive antagonists" act. These antagonists exert their activity on that receptor through the other binding site, which they bind to at a position that is clearly different from the agonist. They do not struggle to bind to the active site in competition with agonists.

Practice Problems:

Q1. When a natural chemical messenger is absent, drugs known as _____ are employed.

  1. Antagonist
  2. Agonist
  3. Analgesics
  4. Narcotics

Answer: B

Solution: Agonists are drugs that resemble the characteristics and behavior of the chemical messenger and activate the receptor. When the real messenger is not present in the body, these are utilized.

Q2. A specific substance, compound X, filled the enzyme's active site's exact opposite location. The active site's form changed as a direct result of this right away. X is known as ____________.

  1. competitive inhibitor
  2. non-competitive inhibitor
  3. competitive messenger
  4. receptor

Answer: B

Solution: Non-competitive inhibitors are medicines that interfere with an enzyme's ability to catalyze reactions by altering the structure of the active site by attaching to the enzyme at a different location. By doing this, the enzyme cannot bond to the substrate.

3. Antagonists are broadly categorized into ___________ types.

  1. Three
  2. Two
  3. Four
  4. Eight

Answer: B

Solution: Antagonist is broadly categorized into two types. These are mentioned below.The first one is the competitive antagonist and the other one is the non-competitive antagonist. Competitive antagonists attach to receptors at the same binding location whereas non-competitive antagonists bind to a different position on the receptor than the general natural ligand.

Q4. Chemicals are used to transmit messages between two neurons and between neurons and muscles. These substances are referred to as__________.

  1. Chemical messengers
  2. Antihistamine
  3. Tranquilizers
  4. Antagonist

Answer: A

Solution: In the body, various substances are used to transmit messages between two neurons and between neurons and muscles. These substances, often referred to as chemical messengers, are taken up by receptor proteins at their binding sites.

Frequently Asked Questions-FAQs:

1. What transpires once an antagonist is introduced?
Answer:
The opposite of what an agonist does is an antagonist. It attaches to receptors and prevents the receptor from eliciting the desired effect. Reverting to the example, it would be like plugging the coin slot of the machine, preventing it from operating until the obstruction is cleared.

2. What distinguishes a substance as an agonist or an antagonist?
Answer:
Drugs that bind to the target receptor with affinity are considered agonists, and they also have intrinsic effectiveness (they change receptor activity to produce a response). Antagonists bind to the target receptor but do not cause a reaction because they have affinity but lack intrinsic effectiveness.

3. Does caffeine act as an antagonist or not?
Answer:
We can consider caffeine as antagonist because adenosine's agonist effects are reversed by caffeine's antagonistic action, which raises dopamine levels in the brain.

4. What exactly is a direct antagonist?
Answer:
The neurotransmitters themselves are prevented from binding to the receptors by a direct acting antagonist, which binds to and inhibits the receptors. The medication atropine is one illustration of this.

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