What is that molecule that exists inside our cells that we can actually detach the phosphoryl group and attach it onto that protein? Well, inside our body, inside our cells we have a very high abundance of high energy ATP molecules adenosine triphosphates which are produced by the mitochondria of the cell. And so because we have this abundancy of high energy ATP molecules that contain the phosphoryl groups, these are the molecules that are the source of this phosphoryl group. So basically, in this process, the protein kinase catalyze the transfer of a terminal phosphoryl group, shown in red from that ATP molecule, onto a hydroxyl containing group, hydroxyl containing side chain found on that protein, the target protein. And because serine 13 and Tyrosine all contain hydroxyl groups on the side chains of these amino acids, these are the three residues that are capable of actually using the hydroxyl group to accept that transfer, that phosphoryl group. So in this process, on the reactant side, we have an ATP molecule and the target enzyme of protein.