Cell Communication

Page
5

The molecules that relay a signal from receptor to response are mostly proteins

Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated

At each step, the signal is transduced into a different form, usually a shape change in a protein

Slide 35

Protein Phosphorylation and Dephosphorylation

In many pathways, the signal is transmitted by a cascade of protein phosphorylations

Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation

Slide 36

Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation

This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off

Slide 37

Fig. 11-9

Signaling molecule

Receptor

Activated relay

molecule

Inactive

protein kinase

1

Active

protein

kinase

1

Inactive

protein kinase

2

ATP

ADP

Active

protein

kinase

2

P

P

PP

Inactive

protein kinase

3

ATP

ADP

Active

protein

kinase

3

P

P

PP

i

ATP

ADP

P

Active

protein

PP

P

i

Inactive

protein

Cellular

response

Phosphorylation cascade

i

Slide 38

Small Molecules and Ions as Second Messengers

The extracellular signal molecule that binds to the receptor is a pathway’s “first messenger”

Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion

Second messengers participate in pathways initiated by G protein-coupled receptors and receptor tyrosine kinases

Cyclic AMP and calcium ions are common second messengers

Slide 39

Cyclic AMP

Cyclic AMP (cAMP) is one of the most widely used second messengers

Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal

Slide 40

Adenylyl cyclase

Fig. 11-10

Pyrophosphate

P

P

i

ATP

cAMP

Phosphodiesterase

AMP

Slide 41

Many signal molecules trigger formation of cAMP

Other components of cAMP pathways are G proteins, G protein-coupled receptors, and protein kinases

cAMP usually activates protein kinase A, which phosphorylates various other proteins