Exam 4 - Ligands & Receptors

What is cell signaling?

It’s how cells send, receive, and respond to messages from their environment, basically how cells talk to each other.

Why is cell signaling important in pharmacology?

Because most drugs work by changing or mimicking the signals that cells normally use to communicate.

What do many drugs act as?

They act as ligands molecules that bind to receptors on cells to trigger or block normal signals.

Pharmacology is built on physiology because…

drugs usually target the same systems the body already uses to function normally.

What are endogenous ligands?

They’re the body’s natural signaling molecules, like hormones, neurotransmitters, or peptides, that bind to receptors to cause a response.

What does specificity of actions mean?

It means a ligand only binds to its specific receptor, kind of like a key fitting into one specific lock. This ensures the right signal goes to the right place.

What are receptors?

Proteins on or inside cells that recognize and bind to ligands, which then trigger a change or response in the cell.

What is modulation of ligand-receptor interactions?

It’s when something changes how well a ligand binds or how strong the response is for example, drugs can increase or block the signal.

Who are the three main players in cell signaling?

The signaling cell, the ligand, and the recipient cell.

What does the signaling cell do?

It makes and releases a ligand, usually just one specific kind to send a message.

What is a ligand?

It’s the chemical messenger that carries information from one cell to another.

What does the recipient cell do

It receives the ligand through receptors on its surface (or inside the cell) and responds to the message. Can receive many ligands

What does the ligand from Cell A do?

It acts as the messenger, released by Cell A to send information to another cell.

What is the receptor on Cell B?

It’s a protein that recognizes and binds the ligand, like a lock matching its key.

What happens after the ligand binds to the receptor on Cell B?

The binding triggers an intracellular change inside Cell B, this is the cell’s response to the signal.

How are substrate-enzyme interactions and drug-receptor interactions similar?

Both involve binding, a substrate or drug attaches to a specific site on an enzyme or receptor.

What is substrate-enzyme interactions

the substrate is chemically changed into a new product.

What is a drug-receptor interactions?

the drug causes an effect but is not changed chemically, it usually just detaches afterward.

What happens in a substrate-enzyme interaction?

The enzyme binds a substrate, performs a chemical reaction, and makes a new product.

What happens in a drug-receptor interaction?

The drug binds to a receptor, activates or blocks it to cause an effect, and then leaves the receptor unchanged.

What is an endogenous ligand?

Chemical that is synthesized by a cell within the body, typically stored, and then released by one cell to interact with a receptor.

What does classification of ligands mean?

it’s how we group ligands based on their chemical structure, where they act, or how they send signals inside the cell.

What’s one way ligands are classified?

By their chemistry and structure

Why does charge at physiological pH matter?

Because it affects whether a ligand can cross cell membranes. Charged molecules have trouble entering the cell. Non-charged molecules can move through the membrane more easily.

How else can ligands be classified?

By the receptor they interact with

Ex: Insulin binds to insulin receptors

How are ligands classified by anatomy and physiology?

Based on where and how far they act

Neurotransmitters act…

locally between nearby cells

Hormones act by…

travel through the bloodstream to distant targets

Steroids act…

inside the cells (intracellular)

What does signal transduction mechanism refer to?

It’s how the receptor passes the message into the cell, the steps that turn an outside signal into an inside response.

If a ligand is charged can it go through the lipid bilayer?

No, charged molecules cannot go through the lipid bilayer

Is there a single, universally accepted system for classifying ligands?

No, classification system for ligands does not exist.

How can ligands be classified based on source?

By the cell or tissue that produces the ligand.

How can ligands be classified by chemical properties?

Examples include small organic molecules, peptides/proteins, and whether they are charged or uncharged at physiological pH.

How can ligands be classified by receptor associations?

By the receptor they bind to, Ex: norepinephrine binds adrenergic receptors, insulin binds the insulin receptor.

How can ligands be classified by anatomy/physiology?

By the tissue or organ system where the ligand acts.

How can ligands be classified by signal transduction?

By the intracellular signaling pathway triggered after the ligand binds its receptor.

What are glands?

Organs that produce and release secretions.

What are exocrine glands?

Glands that secrete substances into ducts (not the bloodstream) to produce local effects (sweat glands, salivary glands.)

Do exocrine glands work via receptors?

Typically, no. Receptors are usually not involved in exocrine gland function.

What are endocrine glands?

Ductless glands that secrete hormones directly into the bloodstream to produce effects at distant receptor sites.

Why are hormones important?

They regulate growth, development, and homeostasis.

An example of an endocrine cell is…

Beta cell in pancreas

An example of a hormone molecule

Insulin

Insulin receptors on muscle cells is an example of

Target cell on receptor

How are hormones commonly classified?

By chemistry (chemical structure) and by receptor location.

What type of hormones use intracellular receptors?

Hydrophobic (lipophilic) hormones that can cross the cell and nuclear membranes.

Examples of hormones that use intracellular receptors

Sex hormone, thyroid hormone, adrenal hormone

What is a key effect of hormones that bind intracellular receptors?

They modulate gene expression in target cells.

What type of hormones use extracellular receptors?

Most charged hormones that cannot cross the cell membrane.

How do extracellular hormones signal?

They bind to cell-surface receptors and use second messengers to transmit their signal.

How is endocrine hormone signaling terminated?

Mainly by elimination from the blood, usually after metabolism in the liver or other sites.

What are the main modes of intercellular signaling?

Endocrine signaling and local signaling.

What is endocrine signaling?

Hormones are released into the bloodstream to act on distant target cells.

What is local signaling?

signaling that only affects nearby cells

What are three key features of local signaling?

bloodstream transport is necessary, effects occur near the source of signal, effects are relatively short term.

What are the three types of local signaling?

Paracrine, autocrine, and juxtacrine.

What is the most important type of local signaling?

Paracrine signaling

What happens in paracrine signaling?

Cell A releases a ligand that moves through the extracellular fluid to act on neighboring Cell B.

What are common examples of paracrine signaling?

Growth factors and autacoids.

How is neurotransmission related to paracrine signaling?

It is a special form where the ligand (neurotransmitter) crosses the synapse.

What happens in autocrine signaling?

Cell A releases a ligand that acts on itself (Cell A)

What happens in juxtacrine signaling?

Cell A and B have direct contact. (Ex:Gap junctions)

Which glands are not part of intercellular signaling?

Exocrine gland 

What is another term for endocrine signaling?

Blood-borne pathogen

What types of signaling are considered paracrine (local)?

Paracrine signaling, autocrine signaling, and juxtacrine signaling.

Which types of local signaling are not typically major players?

Autocrine and juxtacrine signaling

How does a receptor interact with ligands?

The ligand must fit the receptor’s binding site stereochemically, and a cell can have many receptors for different ligands.

What does it mean that a receptor’s binding site is “highly stereoselective”?

It means the receptor can only interact with ligands that have a matching 3D shape or orientation.

What must a ligand exhibit to bind to a receptor?

Stereo complementarity, its shape must complement the receptor’s binding site.

What determines whether a ligand-receptor interaction occurs?

The structural and stereochemical compatibility between the ligand and receptor binding site.

How many types of receptors does a cell have?

A cell has numerous different receptors that can respond to various ligands or combinations of ligands.

What is the first step in ligand–receptor interaction?

Recognition, the ligand binds to the receptor, usually through a reversible process.

What happens during Signal Transduction?

The ligand–receptor complex changes conformation, producing an effect and leading to release of the ligand.

What important step is not displayed in the equations?

The conformational change in the ligand–receptor complex

What are the two main types of receptors?

Noncatalytic receptors and Catalytic receptors.

What are receptor subtypes?

Variations of a receptor that respond differently to ligands but often mediate similar overall functions.

What does Signal Transduction refer to overall?

The process by which a ligand–receptor interaction triggers a biological response inside the cell.

What happens after substrate–enzyme binding?

A product is formed, the substrate is chemically changed.

What happens after drug–receptor binding?

An effect occurs, but the drug remains chemically unchanged and eventually leaves the receptor.

What is the key takeaway comparing the two interactions?

Binding is similar, but the result after binding differs, enzymes transform substrates, while receptors simply trigger effects.

What is an example of receptor subtypes using norepinephrine (NE)?

NE binds to alpha₁, alpha₂, beta₁, beta₂, and beta₃ noradrenergic receptors.

What is the main point of receptor subtypes?

They allow scientists to design drugs that bind to only one specific subtype of a receptor.

what happens if a drug binds to many receptor subtypes?

It may have multiple therapeutic uses, but also a higher chance of side effects.

What is the first messenger in signal transduction?

The original ligand that binds to the receptor.

What are signal transduction molecules?

Molecules inside the cell that help transmit the signal from the receptor to the appropriate intracellular targets.

What is a second messenger?

Short-lived compounds within the cell that carry the signal to specific sites in the cytoplasm or nucleus.

What is the main role of second messengers?

To amplify and transmit the signal from the receptor to intracellular effectors.

Why do many ligands bind extracellular cell-surface receptors?

They are polar (lipophobic, hydrophilic) and cannot cross the cell membrane.

Name the main types of cell-surface receptors.

Ion Channel Receptors, G-Protein Coupled Receptors (GPCRs), Enzymatic Cell-Surface Receptors. 

Why do some ligands bind intracellular receptors?

They are nonpolar (lipophilic) and can cross the cell membrane.

What family do ion channel receptors belong to?

Cell-surface receptors.

Which ions commonly pass through ion channels?

Na⁺, K⁺, Ca²⁺, Cl⁻

How are ionic gradients maintained?

By active, ion-specific pumps.

Why can ions not pass through lipid membranes easily?

Ions are charged, so they require specific ion channels to cross the membrane.

What types of ion channels exist?

Ligand-gated (opened by binding of a molecule) and voltage-gated (opened by changes in membrane potential).

What type of receptor is the GABAₐ receptor?

A ligand-gated ion channel receptor

Which ion is primarily associated with the GABAₐ receptor?

Chloride (Cl⁻) ions.

Where are chloride ions maintained before the receptor is activated?

Outside the cell, due to the ionic gradient.