1. Cell Communication Part II

3. Many receptors are associated with the cell membrane.
When a ligand attaches to a receptor it starts a process that results in a specific response.
Reversibility is important because in most cases the cell needs to stop responding to a signal after the appropriate response has occurred.
Inhibitors (or antagonist) can also bind to a receptor protein, preventing the binding of the normal ligand.

4. Ion Channel Receptors Ligand–gated Channels for ions.
Ex. Muscle Contractions

5. Protein Kinase Receptors
Change shape upon ligand binding.
Exposes or activates a catalytic domain on the cytoplasmic side of the transmembrane protein that has protein kinase activity.
ATP + Protein ADP + Phosphorylated Protein
Protein Kinase

6. G Protein-Linked Receptors
Ligand binding on the extracellular domain of the receptor exposes a site on the cytoplasmic side that can bind to a mobile membrane protein called a G protein.
G protein is partially inserted into the lipid bilayer and partially exposed on the cytoplasmic surface of the membrane.
The activated G protein-linked receptor functions as a guanine nucleotide exchange factor.

7. G Protein-Linked Receptors
Many G proteins have three polypeptide subunits and can bind three different molecules.
The G protein –linked receptor
GDP and GTP (guanine diphosphate & guanine triphosphate)
Nucleotide like ADP and ATP
An Effector Protein
A protein that cause an effect in the cell.

8. Signal Transduction Pathways
Signaling at the cell membrane initiates a cascade of events.
Proteins interact with other proteins until the final response is achieved.
Initial signal can be amplified and distributes to cause several different responses.

9. A cell may respond…
Opening of ion channels change the balance of the ion concentration between both the inside and outside the cell.
This results in a change in the electrical potential across the membrane.
Serious consequence for nerve and muscle cells.

10. A cell may respond… Alterations in gene expression.
Some genes may be switched on while others are switched off.
This alters the proteins encoded by these genes. (enzymes)

11. A cell may respond… Alteration of enzyme activity.
Alteration in enzyme activity is a much more rapid response than one that involves a change in gene expression.

12. A cell may respond…
The same signal can lead to different responses in different types of cells.
Ex. Epinephrine.
Heart muscle results in glucose mobilization for energy and muscle contraction.
Smooth muscle of the digestive tract causes it to relax.

13. Second Messenger can stimulate Signal Transduction
A small molecule that brings about later steps in a signal transduction pathway.
Do NOT have enzymatic activity themselves.
They act to regulate target enzymes by binding to them noncovalently.
Distributes the initial signal.
Amplify the signal.

14. Cyclic AMP (cAMP) Second Messenger
Activates many target proteins by binding to them noncovalently.
This causes a change in the proteins shape
Leads to activating or inhibiting its function.

15. Example of Signaling Cascade
Epinephrine in the Liver
Results in the activation of G protein.
Followed by the production of cAMP which activates protein kinase A.
Prevents glucose from being stored in glycogen.
Results in the liberation of glucose molecules from glycogen.
Each molecule of epinephrine that arrives at the cell membrane ultimately results in 10,000 molecules of blood glucose.

16. Signal Transduction is Highly Regulated
Signal Transduction gets “turned off” when the cell has responded.
Temporary Event
Certain enzymes convert signal transduction molecules back to their inactive state.
The balance between the regulating enzymes and the signaling enzymes is what determines the response to a signal.
Cells can alter their balance by:
Synthesis or breakdown of the enzymes involved.
Activation or inhibition of the enzymes by other molecules.

17. Assignment
Create a color diagram with labels and descriptions for the following:
Signal transduction pathway
Ion Channel receptor in use.
Protein kinase receptor in use
G protein-linked receptor in use.
You may use your book or the computer to find examples.