So adenosine triphosphate, but other high energy molecules can also be used. It's just this ATP is the most common. Now, what is an example of our protein kinase? Well, one type of protein kinase is protein kinase A or PKA. So PKA is an enzyme that phosphorylates proteins on specific amino acids along that polypeptide chain. Now, when cyclic adenosine monophosphate allosterically activates PKA at a specific type of site on that PKA enzyme.