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A copper protein that catalyses the formation of 2 moles of 3-phosphoglycerate from ribulose 1,5-biphosphate in the presence of carbon dioxide and is responsible for carbon dioxide fixation in photosynthesis.

carbon dioxide is combined with ribulose diphosphate to give two molecules of 3 phosphoglycerate, as part of the calvin Benson cycle. It is the sole carbon dioxide fixing enzyme in c3 plants and collaborates with pEP carboxylase in carbon dioxide fixation in c4 plants.

In the presence of oxygen the products of the reaction are one molecule of phosphoglyceric acid and one molecule of phosphoglycolic acid. The latter is the initial substrate for photorespiration and this oxygenase function occurs in C3 plants where the enzyme is not protected from ambient oxygen, in C4 plants the enzyme acts exclusively as a carboxylase since it is protected from oxygen.