And so we have to use that ATP molecule to actually activate that CO2 molecule to make it much more reactive so that we can actually attach that molecule onto Pyruvate. Why? Why do we have to activate the CO2? 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.