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RuBisCO, shorthand for ribulose 1,5-bisphosphate carboxylase/oxygenase [EC], is an enzyme of plant chloroplast and photosynthetic microorganisms most directly responsible for the net assimilation of CO2 into metabolically useful compounds by these organisms. This process is referred to as CO2 fixation. The figure below shows the carboxylation reaction catalyzed by RuBisCO, the key step in the conversion of CO2 into carbohydrates. Carbon dioxide adds to a five-carbon compound, ribulose 1,5-bisphosphate (RuBP), to form an intermediate that is hydrolyzed to two molecules of the three-carbon compound 3-phosphoglycerate (3PG).

The reaction catalyzed by RuBisCO, with structural formulas   3PG is an intermediate of both glycolysis and gluconeogenesis, and is subsequently converted by gluconeogenic reactions to glyceraldehyde 3-phosphate (GAP), which in turn can be built up into six carbon sugars such as glucose and fructose, or used to regenerate the Ru5P substrate for RuBisCO.
The system of reactions, including the RuBisCO carboxylase reaction, that convert CO2 into five- and six-carbon sugars is called the Calvin cycle, or sometimes referred to as the "dark" reactions of photosynthesis. If three CO2 molecules are fixed by RuBisCO and other Calvin cycle enzymes, six 3-carbon GAP molecules are produced. Five of these are used to regenerate three RuBP molecules, and the the sixth represents a net production of a 3-carbon carbohydrate from the input of three CO2 to the cycle.

RuBisCO - the carboxylation reaction

The figure below illustrates the mechanism of RuBisCO's carboxylation reaction. The formation of the enediolate is the rate-limiting step of the mechanism, and is also the first step in the mechanism of the competing oxygenase reaction (see below). The carboxylation of RuBP forms an unstable β-keto acid intermediate that becomes hydrated. The latter then undergoes a cleavage reaction that leads directly to the formation of a 3-phosphoglycerate molecule, and the carbanion form of another.

Mechanism of RuBisCO carboxylation reaction
The mechanism of RuBisCO depends critically upon the presence of a magnesium ion in the active site. The carboxylation of a lysine residue by addition of carbon dioxide to the ε-amino group of Lys provides a negatively-charged carbamide ligand to Mg2+. Thus, carbon dioxide is not only a substrate, but also participates as an activating component that is required for enzyme function. The bound magnesium is essential in facilitating the conversion of the RuBP substrate to an enediolate form that acts as a nucleophile toward a substrate CO2 molecule

RuBisCO - the oxygenase reaction

When oxygen replaces carbon dioxide in the active site of RuBisCO, it can catalyze the oxygenase reaction. A mechanistic portrayal of this reaction is diagrammed below.

Diagram of mechanism for oxygenase reaction catalyzed by RuBisCO
Note that the oxygenase reaction depends on the formation of the same enediolate form of RuBP as in the carboxylase reaction. The products, on the other hand, are the 2-carbon molecule phosphoglycolate and the 3-carbon 3-phosphoglycerate (3PG), instead of the two 3PG molecules that result from the carboxylase reaction. The oxygenase reaction is certainly nonproductive, in terms of carbon fixation. Moreover, it is held to be wasteful due to the energy lost as the carbon atoms from two molecules of 2-phosphoglycolate are reconfigured into serine plus CO2. Since oxygen is input and carbon dioxide given off, this process is called photorespiration (related entry: glyoxylate).


  1. RuBisCO - Molecule of the Month @ Protein Data Bank
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© 2014 Jeff D Cronk, Gonzaga University