The enzyme ribulose bisphosphate carboxylase/oxygenase, or rubisco, fixes carbon dioxide. The carbon dioxide is incorporated into a five‑carbon compound, which is then hydrolyzed to yield two three‑carbon compounds. Arrange the steps of the rubisco carboxylation reaction in order, beginning with the substrate.

1. Rubisco removes a proton from C3 of ribulose-1,5-bisphosphate
2. Substrate
3. Water attacks C3 of the six-carbon beta-keto acid, hydrating it
4. An enediolate intermediate nucleophilically attacks CO2, incorporating it at C2
5. Product
6. The six-carbon molecule splits, yielding two 3-phosphoglycerate molecules