Figure 1:

Schematic overview of photosynthesis and photorespiration.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) catalyzes both CO₂ and O₂ fixation. The product of CO₂ fixation is phosphoglycerate (P-glycerate) that enters the Calvin cycle. During oxygenation, equimolar amounts of P-glycerate and phosphoglycolate (P-glycolate) are formed. P-glycolate is recycled to P-glycerate in the photorespiratory pathway. In this reaction cascade, reducing equivalents (NAD(P)H) and energy equivalents are consumed. Ammonia (NH₃) and CO₂ are released and have to be refixed. Under current atmospheric gas concentrations and moderate environmental conditions, approximately each fourth reaction catalyzed by RUBISCO is an oxygenase reaction.