But when the Cofactor is bound to the APO enzyme, we call that a hollow enzyme. So the hollow enzyme is simply an enzyme bound to its Cofactor. Now, we have many, many different types of Cofactors, as we'll eventually see. But we can categorize Cofactors into two groups, into two categories. We have metal ions and we also have organic molecules known as coenzymes that are usually formed from vitamins. Now, one example of a metal ion that acts as a cofactor for carbonic and hydrates is the zinc atom.