Rubisco is the acronym of d-ribulose 1,5-bisphosphate carboxylase-oxygenase. This enzyme catalyzes the carboxylation of ribulose 1,5-bisphosphate (RuBP) as a first step in the Calvin-Benson-Bassham cycle yielding two molecules of 3-phosphoglycerate. It is a fundamental activity for many autotrophic systems. In the presence of molecular oxygen, the enzyme oxygenates RuBP, yielding a molecule of 3-phosphoglycerate and one of phosphoglycolate (representing a loss of carbon). Rubisco is an old enzyme with a wide phylogenetic distribution among bacteria and archaea. It has been also involved in other, non-autotrophic metabolic functions like a more efficient use of carbohydrates to synthesize lipids in developing plant embryos. Its enigmatic presence also in non-autotrophic species opens the range of its metabolic functions.