And the rate at which it actually the rate at which it takes place and the rate at which the protein kinases catalyze the reactions basically depends on the conditions found inside our cells. If we have to undergo this process, the protein kinases can basically allow the process to take place very quickly. But if we don't want to carry out the process, these protein kinases can easily adjust the kinetics and slow down the speed at which this reaction actually takes place. Now, number four is actually what we spoke about previously. It's the fact that inside our cells we have these high energy adenosine triphosphate molecules that can act as the source of that phosphoryl group. So we see that phosphorylation requires the transfer of this phosphoryl group from some molecule onto that residue of that enzyme.