FIGURE 22.24 • The ribulose bisphosphate carboxylase reaction. Enzymatic abstraction of the C-3 proton of RuBP yields a 2,3-enediol intermediate (I), which is stereospecifically car-boxylated at C-2 to create the six-carbon ^-keto acid intermediate (II) known as 2-carboxy,3-keto-arabinitol. Intermediate II is rapidly hydrated to give the gem-diol form (III). Deprotonation of the C-3 hydroxyl and cleavage yield two 3-phosphoglycerates. Mg2+ at the active site aids in stabilizing the 2,3-enediol transition state for CO2 addition and in facilitating the carbon-carbon bond cleavage that leads to product formation. Note that CO2, not HCO3 (its hydrated form), is the true substrate.
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