And the family of enzymes that regulates this process, this family is known as the protein kinase family. In fact, inside our body we have over 500 homologous protein kinases which are responsible for regulating the rates of this process. So basically these different protein kinases catalyze the phosphorylation of different substrate molecules, different proteins and different enzymes. Now what exactly does the process of phosphorylation actually entail? Well in this process that functional group that we transfer from some molecule onto that target enzyme of protein is the phosphoryl group. Now the question is what is the source of this phosphoryl group?