Details Of Enzymatic Receptors 

What are Enzymatic Receptors? 

Enzymatic receptors have 2 ends: Extracellular end: drug binds here; Intracellular end: enzyme is present here; So drug binds outside and enzyme is activated inside and action is produced. Also known as Tyrosine Kinase Receptors because mostly the associated enzyme is tyrosine kinase.

Types of Enzymatic Receptors

1. Intrinsic Tyrosine Kinase Activity

Whenever a drug binds outside.The tyrosine kinase enzyme gets activated inside E.g. Insulin receptor

2. No Intrinsic Tyrosine Kinase Activity

The receptor will work through tyrosine kinase, but itself does not possess this activity. Some proteins are present on enzymes that recruit tyrosine kinase from the cytoplasm & activate the tyrosine activity. The receptor itself does not possess enzyme activity 

E.g.

• JAK-STAT (JAK recruits STAT, that will result in enzymatic activity)
• Prolactin, Growth hormone
• Cytokines

3. Guanylate Cyclase

Whenever a drug binds outside, guanylate cyclase gets activated inside and generates cGMP. Substances which act through Guanylate cyclase are ANP, BNP, and CNP (natriuretic peptides)

Also read: Frequently Asked Question in General Pharmacology

What are Spare Receptors?

At a particular number of receptor stimulations, the response becomes maximum, & those receptors that are present in the body beyond that are known as spare receptors.

What is Receptor Regulation?

When a receptor in the body is constantly being stimulated, it gets saturated. Whereas when a receptor is not getting stimulated over a long time, its level will increase. So constant presence of agonist will decrease the number of receptors and constant presence of antagonist increases the number of receptors

1. Constant stimulation (Agonist +++)

The mechanisms involved are:

i. – The receptors present Masking on surface of membrane quickly go below and the number of

receptors decrease. So, action decreases.

ii. Down – regulation – It is the actual decrease in the number of receptors. It occurs via: Less synthesis of new receptors & More degradation of existing receptors

iii. Uncoupling – There is uncoupling of signal transduction response For e.g.: When β-receptors are stimulated constantly, it activates an enzyme called β-adrenergic receptor kinase (BARK). Kinase attacks the phosphate group. This targets the G-protein coupled receptor. This leads to binding of a protein called Arrestin to the phosphate group. So GPCR cannot produce its action

Also read: Important One-liners in Pharmacology

2. Constant Non-stimulation

For example: Clonidine inhibits release of Nor-adrenaline. So nor-adrenaline is decreased and there is decreased stimulation of α and β receptors. As these receptors are not stimulated for a long period, their number is decreased. Now when we stop giving clonidine from outside, NA

gets released. This NA gets plenty of α and β receptors which are lying free. This leads to substantial increase in response. This results in Rebound HTN. The mechanisms involved in Constant Non-stimulation are:

i. Un-masking: All receptors come to the surface and get stimulated

ii. Up-regulation: Actual increase in number of receptors. It occurs via: Increase in synthesis of receptors & Decrease in metabolism or degradation of receptors

iii. Increase in coupling

Important Points to Remember 

• Whenever a drug binds outside.The Tyrosine Kinase enzyme gets activated inside.
• Un-masking: All receptors come to the surface and get stimulated
• Up-regulation: actual increase in number of receptors. It occurs via an increase in the synthesis of receptors & a decrease in metabolism or degradation of receptors.
• Receptor itself does not possess enzyme activity. JAK-STAT (JAK recruits STAT, that will result in enzymatic activity), Prolactin, Growth hormone, and cytokines

Also read: Pharmacology Important Questions for NEET PG/FMGE 2025

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