Receptors and Signal Transduction

Signal Transduction

Converts one form of signal into a different form. In a cell refers to the transmission of a signal

from 1 side of the cell membrane to the other

What determines a cells response to a message?

Ligand (thing that binds to)

Receptor (what the thing binds to)

First Messenger

Ligand. A signal, extracellular molecule binds to and activates a membrane receptor.

• ex: Insulin, epidermal growth factor, adrenaline, cytokines, neurotransmitters

Ligand

The molecule that binds to a proetin. Depend on its ability to enter the cell.

• Lipophilic: can cross the cell membrane.

• Lipophobic: cannot cross the membrane, so it uses a membrane receptor.

Receptor

A cellular protein that binds to a ligand. Detects the signal and converts binding into a biochemical change.

• ex: Receptor tyrosine kinases, GPCRs, cytokine receptors, ion channels, nuclear receptors

Transducer

Relays the signal inside the cell through enzymatic activity or protein-protein interactions.

• ex: G proteins, kinases, adaptor proteins, scaffold proteins

Second Messenger

Brings the message to the cytoplasm (activating enzymes) or the nucleus (activating genes). Small intracellular molecule that rapidly amplifies or distributes the signal. Facilitate transduction of a lipophobic message to the interior of a cell. These messengers may

• open or close membrane channels

• modulate metabolic enzymes

• activate or deactivate transport proteins

• ex: cAMP, cGMP, Ca2+, IP3, DAG, PIP3

Signal Transduction Pathway: Cascade reactions

Starts when a stimulus converts inactive molecule (the receptor) to an active form. Ex: a(inactive)→a(activate)→b(inactive)→b(active)→ and so on, until the final step a substrate is converted into a product. Like a domino falling effect. Blood clotting is an important example of extracellular cascade.

Signal Transduction Pathway: Amplification

Turns one signal molecule into multiple second messenger intracellular molecules. Begins when the first messenger ligand combines with its receptor. The receptor-ligand complex turns on an amplifer enzyme, this activates several molecules which in turn each activate several more molecules as the cascade proceeds.

Tyrosine Kinase

A catalytic enzyme receptor (the ligand binds to it) and is a kinase (a phosphorylating enzyme). It transfers a phosaphate group from ATP to a tyrosine (an amino acid) of a protein.

G protein-coupled receptors (GPCRs)

Can link receptors to ion channels. Can link receptors to amplifier enzymes

What are the four steps of signal transduction?

1. signal molecule binds to receptor that

2. activates a protein that

3. creates a second messenger that

4. creates a response

What happens during amplification?

It turns one signal molecule into multiple second messenger molecules.

Why do steroid hormones not require signal transduction and second messengers to exert their action?

Steriods are lipophilic, so they can enter cells and bind to intracellular receptors.

Name four categories of membrane receptors

Receptors are either:

• ligand gated ion channels

• receptor enzymes

• G protein-coupled receptors

• or integrins

What do receptors, enzymes, and transporters have in common

They are all porteins

Insulin increases the number of glucose transporters on a skeletal muscle cell but not the membrane of a liver cell. List two possible mechanisms that could explain how this on hormone can have these two different effects.

It could be one receptor with different second messenger system or two different receptor isoforms

How can one chemical signal have opposite effects in two different tissues?

A signal can have opposite effects by using different signal pathways.

Signal transduction

Allow cells to meet their goals: reaching only the correct recipients, imparting correct information, timeliness, causing the desired effect, effective termination.