In state A the phosphorptokinase to this structure is active while this one is inactive. And the reason it's active is because this serene residue found on this region here is not actually phosphorylated. Now, if protein kinase A takes an ATP and phosphorylates that serine residue on this domain, we create state B and in state B this becomes inactive while the other one becomes active. Now, we can go back to this state by the action of phosphor protein phosphatase which uses a water molecule to hydrolyze that ester bond and remove that inorganic phosphate reforming state A. So we can basically cycle it between state A and state B. So let's begin by looking at state A.