1. Function introduction to G proteins (more details in another lecture)
Small proteins, bind GDP & GTP
GDP bound at rest
GTP becomes bound when G protein is activated by receptor
G proteins consist of 3 subunits, , , 
 -GTP is the active subunit
different families of G proteins exist

2. At rest at rest, G protein has GDP bound inactive receptor
membrane
receptor
Effector eg adenylate cyclase
G protein

3. Agonist binding
agonist binds to receptor causing a conformational change
G protein becomes activated (ie binds GTP) R AC G
Agonist
GTP

4. Activation of 2nd messenger
G protein diffuses in membrane and binds to enzymes present in membrane (eg adenylate cyclase, phospholipase C) and/or ion channels
this activates or inactivates these enzymes or channels R AC
GTP G
active

5. Summary G protein then breaks down GTP to GDP and is inactivated R AC
Agonist
GTP R AC
GTP G
active R AC
GDP G

6. Function II
G proteins can interact with different receptors and effectors
overall effect depends on receptor and cell
eg adrenaline on beta-adrenoreceptor in smooth muscle
activates adenylate cyclase
increases cAMP
activates a protein kinase
muscle relaxation
G proteins produce amplification
a single agonist/receptor complex can activate several G proteins in turn and each can activate enzymes etc