Now, the enzyme that catalyzes step three is phosphoglucom mutates. Remember that a mutase basically transfers a group from one location to a different location on that same molecule. So what phosphorluca mutase actually does is it transfers that phosphoryl group from carbon number one to carbon number six. Now we begin with glucose one phosphate and we end with glucose six phosphate. But what we actually don't show in this diagram is that we actually have an intermediate molecule involved and that intermediate molecule is glucose 116 bisphosphate. So if we examine the active side of this enzyme, phosphogluca mutase, we're going to see a modified serine residue.