And that's exactly what we do in the active side of carbonic and hydrase, too. So to see what we mean, let's take a look at the following diagram. So, inside the active side of carbonic and hydrase two and all of these carbonic and hydrates, we have a zinc metal atom. And anytime any biological system uses a zinc atom in nature, the zinc atom always contains an oxidation state of positive two. So it has a net charge of positive two. And what that means is we have four different groups which are bound to that zinc atom.