<P> During carbon fixation, the substrate molecules for RuBisCO are ribulose - 1, 5 - bisphosphate and carbon dioxide (distinct from the "activating" carbon dioxide). RuBisCO also catalyses a reaction between ribulose - 1, 5 - bisphosphate and molecular oxygen (O 2) instead of carbon dioxide (CO). </P> <P> When carbon dioxide is the substrate, the product of the carboxylase reaction is a highly unstable six - carbon phosphorylated intermediate known as 3 - keto - 2 - carboxyarabinitol - 1, 5 - bisphosphate, which decays virtually instantaneously into two molecules of glycerate - 3 - phosphate . The extremely unstable molecule created by the initial carboxylation was unknown until 1988, when it was isolated . The 3 - phosphoglycerate can be used to produce larger molecules such as glucose . Also, Rubisco side activities can lead to useless or inhibitory by - products; one such product is xylulose - 1, 5 - bisphosphate, which inhibits Rubisco activity . </P> <P> When molecular oxygen is the substrate, the products of the oxygenase reaction are phosphoglycolate and 3 - phosphoglycerate . Phosphoglycolate is recycled through a sequence of reactions called photorespiration, which involves enzymes and cytochromes located in the mitochondria and peroxisomes (this is a case of metabolite repair). In this process, two molecules of phosphoglycolate are converted to one molecule of carbon dioxide and one molecule of 3 - phosphoglycerate, which can reenter the Calvin cycle . Some of the phosphoglycolate entering this pathway can be retained by plants to produce other molecules such as glycine . At ambient levels of carbon dioxide and oxygen, the ratio of the reactions is about 4 to 1, which results in a net carbon dioxide fixation of only 3.5 . Thus, the inability of the enzyme to prevent the reaction with oxygen greatly reduces the photosynthetic capacity of many plants . Some plants, many algae, and photosynthetic bacteria have overcome this limitation by devising means to increase the concentration of carbon dioxide around the enzyme, including C carbon fixation, crassulacean acid metabolism, and the use of pyrenoid . </P> <P> Some enzymes can carry out thousands of chemical reactions each second . However, RuBisCO is slow, being able to fix only 3 - 10 carbon dioxide molecules each second per molecule of enzyme . The reaction catalyzed by RuBisCO is, thus, the primary rate - limiting factor of the Calvin cycle during the day . Nevertheless, under most conditions, and when light is not otherwise limiting photosynthesis, the speed of RuBisCO responds positively to increasing carbon dioxide concentration . However, our descriptive knowledge will become more usable when we can translate them into quantitative models that can enable us to calculate the outcome of the reaction under a given condition . Since RubisCO reacts with RuBP (ribulose 1, 5 bisphosphate) first to produce enediol and next with CO that after some intermediate changes produces PGA (3 - phosphoglycerate), a biochemical model is developed to represent the effects of these steps quantitatively . Since carboxylation or fixation of CO is possible only after the synthesis of enediol, thus it is suggested that the role of RubisCO is to produce enediol that is carboxylase and oxygenase (EnCO). Accordingly, RubisCO is called enolase - phosphglycerase (EPGase) since it is neither carboxylase nor oxygenase . </P>

Which enzyme catalyzes the fixation of carbon dioxide in the calvin cycle