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Calcium supports loop closure but not catalysis in Rubisco
Saeid Karkehabadi1, Thomas C Taylor, Inger Andersson
1Department of Molecular Biosciences, Swedish University of Agricultural Sciences, BMC Box 590, S-751 24, Uppsala, Sweden.
Journal of Molecular Biology
|November 5, 2003
Summary
Calcium (Ca2+) activates Rubisco carbamylation but not catalysis, unlike magnesium (Mg2+). Structural studies reveal Ca2+ hinders catalysis by increasing proton hop distance in the active site, impacting CO2 assimilation.
Area of Science:
- Biochemistry
- Structural Biology
- Enzyme Catalysis
Background:
- Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is crucial for CO2 assimilation.
- Enzyme activation requires carbamylation of an active site lysine, stabilized by Mg2+.
- Different metal ions affect Rubisco's catalytic activity and carboxylation/oxygenation ratio.
Purpose of the Study:
- To investigate the role of calcium (Ca2+) in Rubisco catalysis.
- To determine the structural basis for Ca2+ promoting carbamylation but not catalysis.
Main Methods:
- Determined crystal structures of enzyme-Ca2+-2CABP complexes.
- Utilized soaking and co-crystallization techniques.
- Analyzed structural features and ligand interactions.
Main Results:
- Ca2+ complexes show carbamylation at Lys201 and active site loop closure, similar to Mg2+.
- Ca2+ binding increases the proton hop distance between C3 and Lys201 carbamate.
- Structural differences explain Ca2+'s inability to sustain Rubisco catalysis.
Conclusions:
- The larger radius and reduced Lewis-acid character of Ca2+ impede catalytic proton transfer.
- This structural impediment explains why Ca2+ fails to support Rubisco's catalytic cycle.
- Metal ion substitution significantly affects ligand dynamics and enzyme function.