Well, because we can't simply attach the CO2 because that would require an input of energy. And so we have to activate it, make it more reactive, and that's where that ATP comes into play. So once again, Pyruvate carboxylase consists of four identical subunits that each have a domain that has a Covalently attached biotin prosthetic group. So this helper group that helps to bind that CO2. So we call this group the biotin binding domain that is used to bind the CO2 molecule and bring it into the active site of the enzyme. So basically the active side that contains that Pyruvate and we'll talk more about that in just a moment.