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 systemsAnalogy:

vs.

On/off switchis like a singlestep pathway Dimmer=

multi-step

Cell Communication –Signal Transduction

Ch. 5 section

5.6

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

Ligand doesn’t enter cell

Remember: the Remember: the signal molecule signal molecule itselfitself doesn doesn’’t t physically physically get passed down get passed down the pathway.the pathway.

2 common transduction events

1. Phosphorylation= the transfer of a phosphate group from ATP to a protein – thereby activating the protein

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

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

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

QUICK THINKQUICK THINKExplain how a phosphorylation cascade gets turned off

2 common transduction events

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

2 common second messengers Cyclic AMP (or cAMP)

Example ofcAMP 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 isthe breakdown of glycogen within the cell.

Boosts your energy!

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 hours

cAMP’s role in cells1. Caffeine artificially boosts your

energy by blocking the breakdown of cAMP, keeping you alert even without epinephrine

2 common second messengers•Ca2+ the ER released stored calcium •cause cellular responses including muscle

contraction & cell division.