RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is activated by a CO2 molecule that carbamylates a specific lysine residue within its active site, enabling the binding of Mg2+. This Mg2+ binding completes the activation process, rendering the enzyme catalytically competent.
The Activation Process in Detail
Newly synthesized RuBisCO isn't immediately functional. Its active site isn't fully formed and ready to catalyze the carboxylation or oxygenation of ribulose-1,5-bisphosphate (RuBP). Activation involves a specific modification:
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Carbamylation: A CO2 molecule, distinct from the one involved in the actual carboxylation reaction, binds to a specific lysine residue (Lys201 in Spinach RuBisCO) in the RuBisCO active site. This reaction is called carbamylation.
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Mg2+ Binding: The carbamylated lysine now facilitates the binding of a magnesium ion (Mg2+). This Mg2+ ion is crucial for stabilizing the transition state during the RuBisCO reaction.
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Formation of the Active Enzyme: The binding of Mg2+ completes the formation of the functional active site. RuBisCO is now primed to bind RuBP and catalyze the carboxylation or oxygenation reaction.
The Role of RuBisCO Activase
The activation of RuBisCO isn't always spontaneous. In vivo, RuBisCO activase, a chaperone protein, is essential for maintaining RuBisCO in its activated state. RuBisCO activase is particularly important because RuBP, a substrate of RuBisCO, can bind to the enzyme in its inactive state, preventing carbamylation and Mg2+ binding. RuBisCO activase removes these inhibitory RuBP molecules from the active site, allowing for the activation process to proceed. The activase uses ATP hydrolysis to drive the removal of RuBP.
Summary
In summary, RuBisCO activation is a two-step process that requires the carbamylation of a lysine residue by CO2, followed by the binding of a magnesium ion (Mg2+). This process is often facilitated by RuBisCO activase, which removes inhibitory RuBP molecules, ensuring that the enzyme remains in its active and functional state.
