Receptors I

Who developed receptor theory

• Paul Ehrlich

What is Ehrlich's core principle of receptor theory?

Substances do not act unless bound." Drugs must bind to receptors to produce effects

What are receptors?

specialized proteins that bind to specific drugs or ligands and initiate biological responses

Where can receptors be located?

Cell membrane, cytoplasm, or intracellular organelles such as the nucleus

Why is receptor theory clinically important?

Understanding receptor binding helps explain drug selectivity, therapeutic effects, and guides rational drug design and personalized medicine.

What is the "selectivity rule" of receptor theory?

Molecular structure determines binding specificity and therapeutic effects

What does the lock-and-key model describe?

The fit between a drug (key) and receptor (lock) determines binding and drug effect.

How does the lock-and-key model explain drug selectivity?

Only drugs with the correct molecular shape can effectively bind specific receptors and produce a response

What is a structure-activity relationship (SAR)?

The relationship between a drug's chemical structure and its pharmacologic activity. Small structural changes can greatly alter function.

How can small structural changes affect a drug?

They can dramatically alter receptor binding and pharmacological activity.

What beta-blocker example demonstrates SAR?

Para substitution produces the highest β1-selectivity, meta substitution produces intermediate selectivity, and ortho substitution produces the least β1-selectivity.

What determines whether a drug successfully binds to a receptor?

Structural compatibility between the drug and receptor binding site

What is the law of mass action in drug-receptor binding?

Drug and receptor reversibly bind to form a drug-receptor complex that produces a stimulus and response.

What happens to receptor occupancy as drug concentration increases?

Higher drug concentration leads to greater receptor occupancy.

What determines the magnitude of a drug response?

The degree of receptor occupancy.

What is the dissociation constant (Kd)?

The ligand concentration at which half of the receptor binding sites are occupied.

What does Kd measure?

Binding affinity between a drug and receptor.

What is affinity?

The strength of drug-receptor binding.

What is the relationship between affinity and Kd?

Affinity = 1/Kd, so affinity and Kd are inversely related.

What does a low Kd indicate?

High affinity because less drug is needed to occupy 50% of receptors.

What does a high Kd indicate?

Low affinity because more drug is required to occupy 50% of receptors

Why are enzymes common drug targets?

Enzymes possess specific active sites that can be blocked or modified by drugs.

What is competitive inhibition?

Reversible competition between the drug and substrate for the enzyme's active site.

What is noncompetitive inhibition?

Drug binding to an allosteric site changes enzyme shape and function

What is reversible inhibition?

Temporary enzyme inactivation through non-covalent interactions

What is irreversible inhibition?

Permanent enzyme inactivation through covalent modification

How do enzyme inhibitors affect the body?

They alter the production or breakdown of specific molecules

Why does competitive inhibition fit receptor theory?

The inhibitor structurally resembles the natural substrate and competes for the same binding site.

What clinical example illustrates competitive enzyme inhibition?

Statins compete with HMG-CoA for HMG-CoA reductase.

Match the drug with the enzyme target:

• Captopril

• Allopurinol

• Warfarin

• Aspirin

• Aciclovir

• Neostigmine

• Captopril → Angiotensin-converting enzyme (ACE)

• Allopurinol → Xanthine oxidase

• Warfarin → Vitamin K epoxide reductase

• Aspirin → Cyclo-oxygenase (COX)

• Aciclovir → Herpes virus DNA polymerase

• Neostigmine → Acetylcholinesterase

What are ion channels?

Transmembrane proteins that control ion flow and cellular excitability

What is a ligand-gated ion channel?

A channel that opens or closes when a ligand binds to it.

How do drugs affect ion channels?

They can block, open, or modulate channel activity

What occurs when acetylcholine binds nicotinic receptors?

A Na⁺/K⁺ channel opens.

What occurs when GABA binds GABA-A receptors?

A Cl⁻ channel opens

What occurs when glutamate and glycine bind NMDA receptors?

A Ca²⁺ channel opens

Why are ion channels considered receptor theory applications?

They contain specific ligand-binding sites where drugs interact to produce biological effects.

What is state-dependent binding?

Drug binding that depends on the channel state, allowing selective targeting of ion channels.

What major receptor class includes ion channels?

Membrane receptors

What are the 5 take-home messages of receptor theory?

• Drugs must bind to act.

• Molecular structure determines selectivity.

• Mass action governs receptor occupancy.

• Same principles apply to enzymes, ion channels, and receptors.

• Receptor theory drives rational drug design and personalized medicine.

What are the four enzyme inhibition types you must know?

Competitive, noncompetitive, reversible, and irreversible inhibition

What three ligand-gated ion channel receptors are emphasized in this lecture?

Nicotinic receptors, GABA-A receptors, and NMDA receptors.