How a signal brought to the cell by a 
single message molecule is amplified 
through the second messenger system. 
Step 
Action 
Product 
1 . 
A message molecule binds 
to receptor and activates it. 
2 . 
Each activated receptor 
activates many G proteins 
(two shown here), which 
each activate one molecule 
of the enzyme adenylate 
cyclase. 
3 . 
Using ATP, each molecule 
of adenylate cyclase makes 
many molecules of cyclic AMP. 
Cyclic AMP 
4 . 
Each cyclic AMP molecule 
makes one molecule of an 
enzyme called protein 
kinase. 
Protein kinase 
5 . 
Each protein kinase 
molecule activates many 
molecules of some 
enzyme. 
6 . 
Each copy of the 
activated enzyme 
produces many copies of 
its protein product. 
•As 
#>• 
m 
% 
w 
Enzyme "X" - varies 
depending on the 
protein kinase 
Products of 
enzyme "X" 
54 
