This document discusses signal transduction pathways, which allow cells to respond to extracellular signals. There are two main advantages to multi-step signal transduction pathways over single-step pathways: 1) a small number of signal molecules can produce widespread cellular responses, and 2) multi-step pathways provide more opportunities for coordination and regulation. Common events in signal transduction include phosphorylation, where protein kinases transfer phosphate groups to activate proteins, and the use of second messengers like cyclic AMP and calcium ions to amplify the signal and trigger downstream responses.

1. Transduction of a Signal is
Usually a Multi-Step Pathway
In this case, there are two advantages:
1. A small number of extracellular signal molecules
can produce widespread cellular response.
2. Provides more opportunities for coordination &
regulation than in simpler systems
Analogy:
vs
.
On/off switch
is like a single
step pathway Dimmer=
multi-step

2. Cell Communication –Signal
Transduction
Ch. 5
section 5.6

3. Signal Transduction Pathways
•The binding of a specific
signal to its receptor
triggers a chain reaction
that leads to a response.
•Relay molecules are the proteins that “carry the
message” down a reaction chain

4. Ligand doesn’t enter cell
Remember: theRemember: the
signal moleculesignal molecule
itselfitself doesndoesn’t’t
physicallyphysically
get passed downget passed down
the pathway.the pathway.

5. 2 common transduction
events
1. Phosphorylation= the transfer of a phosphate group
from ATP to a protein – thereby activating the protein

6. Protein kinase is the enzyme that
transfers phosphate groups from ATP to
proteins

7. Protein kinases are numerous and help regulate
cell reproduction. Therefore, abnormal kinase
activity can contribute to cancer.

8. In addition to protein kinases, protein
phosphatases also play a role in the
cascade
Protein phosphatases are
enzymes that dedephosphorylate
proteins
phosphatases turn off signal transduction
Phosphatases also make kinases available for re-use

9. QUICK THINKQUICK THINK
Explain how a phosphorylation
cascade gets turned off

10. 2 common transduction
events
2. Second messengers: small, nonprotein, water-soluble
molecules or ions; the signal molecule being the “first
messenger.”

11. 2 common second
messengers
 Cyclic AMP (or cAMP)

12. Example of
cAMP as a Second Messenger
The binding of
epinephrine,
the first messenger,
activates adenylyl
cyclase, which then
catalyzes the synthesis
of cAMP in a liver cell.
The response is
the breakdown of
glycogen
within the cell.
Boosts
your
energy!

13. cAMP Malfunctioning
Vibrio cholerae produces a toxin that
modifies the G protein, which is now
stuck in the on position.
G protein is
unable to
hydrolyze GTP
to GDP
Now high
concentrations
of cAMP
cause
intestinal cells
to secrete
water & salts-
may lead to
death w/in

14. cAMP’s role in cells
1. Caffeine artificially boosts your energy by
blocking the breakdown of cAMP, keeping
you alert even without epinephrine

15. 2 common second messengers
•Ca2+
the ER released stored calcium
•cause cellular responses including muscle
contraction & cell division.