So in step number one, the enzyme that catalyzed the step is carbon phosphate synthetase. So this carboil phosphate synthetase ultimately utilizes two ATP, two ATPs in this three step process. So we begin with a carbon dioxide, but actually the carbon dioxide is in the form of Bicarbonate. So remember, carbon dioxide in our body exists as Bicarbonate. Now, Bicarbonate is a stable molecule and we have to increase its energy and that's where we use the first ATP. So we phosphorylate the carbon to form carboxy phosphate.