Without that metal ion the substrate would not be able to bind into the active side because it would not have the proper orientation. And so ultimately, it's the ATP metal ion complex that is the substrate of that active side because only when the metal atom binds with the ATP molecule will the interactions between the active side and the substrate be perfect and very stabilizing interactions. Now, the final thing that I'd like to mention about MP kinases is that they not only use the metal ion catalysis, but they also use catalysis by proximity and orientation. And to see what we mean by that, let's take a look at the structure of our NMP kinase, namely the adenolid kinase. And so this is the domain that we basically spoke of earlier and this is our P loop. So what happens is once the magnesium binds onto the ATP molecule, that creates a correct conformation and then the ATP magnesium complex can move into the active side of this molecule and bind with that P loop structure.