ribulose bisphosphate carboxylase

(RUBP carboxylase, carboxydismutase) An enzyme that catalyses the carboxylation of ribulose bisphosphate to two molecules of phosphoglyceric acid. This is the first step in the *Calvin cycle. RUBP carboxylase is the fraction 1 protein of chloroplasts. It is very abundant in chloroplasts and may account for up to 15% of the total protein. This enzyme can also act as an oxygenase, forming a molecule of phosphoglyceric acid and a molecule of phosphoglycollic acid from ribulose bisphosphate and oxygen. This reaction forms the basis of photorespiration.