So in this step, an enzyme called phosphoglycerate mutase catalyzes the movement of a phosphoryl group from the third carbon on this three phosphaglycerate onto the second carbon of that same molecule. And so this is what we form. Now, this is the net overall reaction. But it's not actually this simple because we also have an important molecule involved that is present in catalytic amounts in a very small amount. And this molecule is two or three bisphosphoglycerate or two three BPG. Now, this is the same exact molecule that we discussed when we mentioned hemoglobin's ability to bind oxygen.