Reaction intermediate partitioning by ribulose-bisphosphate carboxylases with differing substrate specificities.

Area of Science:

• Biochemistry
• Enzymology
• Photosynthesis research

Background:

• Ribulose-1,5-bisphosphate carboxylase (RuBisCO) catalyzes a key step in carbon fixation.
• Understanding RuBisCO's reaction mechanism and intermediate handling is crucial for improving photosynthetic efficiency.
• The enzyme exhibits both carboxylase and oxygenase activities, impacting plant productivity.

Purpose of the Study:

• To characterize the 6-carbon intermediate (3-keto-2-carboxyarabinitol 1,5-bisphosphate) in the RuBisCO reaction.
• To investigate the enzyme's partitioning of this intermediate between decarboxylation and hydrolysis pathways.
• To explore how enzyme source and activating metal affect substrate specificity.

Main Methods:

• Isolation of the 3-keto-2-carboxyarabinitol 1,5-bisphosphate intermediate via acid denaturation.
• Carbon-13 Nuclear Magnetic Resonance (NMR) spectroscopy for structural analysis in solution.
• Enzymatic assays using RuBisCO from different sources with varying activating metal ions (Mg2+, Co2+) to study intermediate partitioning.

Main Results:

• The intermediate exists predominantly as a beta-keto acid in solution, not its hydrate form.
• Non-activated RuBisCO catalyzes slow decarboxylation, while activated enzyme favors hydrolysis to glycerate 3-phosphate.
• Enzyme partitioning showed a strong preference for product formation, with variations in specificity linked to pre-intermediate interactions.

Conclusions:

• The 6-carbon intermediate demonstrates a strong commitment to product formation in both carboxylase and oxygenase reactions.
• Variations in RuBisCO's specificity for CO2 and O2 are likely determined by enzyme-substrate interactions preceding intermediate formation.
• This suggests that optimizing RuBisCO's catalytic efficiency may involve targeting these early interactions.