1RBA

Ribulose-bisphosphate carboxylase/oxygenase (RuBisCO) is the enzyme responsible for the initial step of carbon fixation during photosynthesis. RuBisCO carboxylates one molecule of ribulose-bisphosphate (RuBP), yielding two molecules of 3-phosphoglycerate(PGA). The latter is subsequently used partly to regenerate RuBP in the Calvin cycle and partly to build up sugar compounds. RuBisCO also acts as an oxygenase with RuBP as substrate. The oxygenation of RuBP initiates photorespiration, a pathway in which energy initially accumulated through the light reactions is lost as heat, with no apparent benefit for the plant. In higher plants and the majority of photosynthetic microorganisms, RuBP-carboxylase is composed of eight large and eight small subunits, forming a hexadecameric structure, L8S8. In contrast, in some photosynthetic bacteria, RuBisCO is a dimer of only large subunits.