The enzyme that catalyses the crucial step in the Calvin–Bassham–Benson cycle of photosynthesis, namely the incorporation of a molecule of carbon dioxide with a molecule of ribulose bisphosphate, forming two molecules of phosphoglycerate. This reaction fixes gaseous carbon dioxide in the form of organic carbon, providing a source of energy and other materials for plants and all their dependent organisms. However, rubisco has a dual activity, being able to catalyse the reaction between oxygen and ribulose bisphosphate, splitting the latter into one molecule of phosphoglycerate and one molecule of phosphoglycolate. This is the wasteful process of photorespiration, which competes with photosynthesis. Reflecting its key role in life, rubisco is the most abundant of all natural proteins.