| Definition | As shown in Figure 2 (left), RuBisCO is one of many enzymes in the Calvin cycle. Substrates. During carbon fixation, the substrate molecules for RuBisCO are ribulose 1,5-bisphosphate, carbon dioxide (distinct from the "activating" carbon dioxide) and water. RuBisCO can also allow a reaction to occur with molecular oxygen (O2) instead of carbon dioxide (CO2). Products. 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. When molecular oxygen is the substrate, the products of the oxygenase reaction are phosphoglycolate and 3-phosphoglycerate. Phosphoglycolate initiates a sequence of reactions called photorespiration, which involves enzymes and cytochromes located in the mitochondria and peroxisomes. 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 air 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 potential 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 C4 carbon fixation, crassulacean acid metabolism and using pyrenoid.; Ribulose-1,5-bisphosphate carboxylase oxygenase, most commonly known by the shorter name RuBisCO , is an enzyme (EC 4.1.1.39) that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as sucrose. RuBisCO catalyzes either the carboxylation or the oxygenation of ribulose-1,5-bisphosphate (also known as RuBP) with carbon dioxide or oxygen. |
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