Now, we actually discussed metal ion catalysis previously when we discussed proteases and carbonic and hydrates. But the major difference between that type of metaline catalysis and the metaline catalysis that takes place within an MP kinases is here that metal ion doesn't actually interact with the active sites enzyme, but it interacts with the substrate molecule itself. So we see that NMP kinases require the presence of a metal atom such as magnesium or a manganese. And so we have to have some type of divalent metal atom. Divalent simply means it has a charge of positive two. So to demonstrate why this is so, let's suppose we have the ATP molecule as our substrate.