So let's begin by taking a look at the active side of carbonic and hydrase II. So, in our discussion on the mechanisms of enzymes, we said that one mechanism that enzymes use is known as metal ion catalysis. And in metal ion catalysis, the active side of the enzyme uses some type of metal ion to basically form a strong nucleophile. 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.