CO2 Fixation Kinetics of Halothiobacillus neapolitanus Mutant Carboxysomes Lacking Carbonic Anhydrase Suggest the Shell Acts as a Diffusional Barrier for CO2

The widely accepted models for the role of carboxysomes in the carbon-concentrating mechanism of autotrophic bacteria predict the carboxysomal carbonic anhydrase to be a crucial component. The enzyme is thought to dehydrate abundant cytosolic bicarbonate and provide ribulose 1.5-bisphosphate carboxy...

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Bibliographic Details
Published in:The Journal of biological chemistry 2008-04, Vol.283 (16), p.10377-10384
Main Authors: Dou, Zhicheng, Heinhorst, Sabine, Williams, Eric B., Murin, C. Daniel, Shively, Jessup M., Cannon, Gordon C.
Format: Article
Language:English
Subjects:
Carbon - chemistry
Carbon Dioxide - chemistry
Carbonic Anhydrases - chemistry
Catalysis
Diffusion
Dimerization
Genetic Complementation Test
Halothiobacillus - genetics
Halothiobacillus - physiology
Hydrogen-Ion Concentration
Kinetics
Models, Biological
Mutation
Plasmids - metabolism
Ribulose-Bisphosphate Carboxylase - chemistry
Time Factors
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Summary:The widely accepted models for the role of carboxysomes in the carbon-concentrating mechanism of autotrophic bacteria predict the carboxysomal carbonic anhydrase to be a crucial component. The enzyme is thought to dehydrate abundant cytosolic bicarbonate and provide ribulose 1.5-bisphosphate carboxylase/oxygenase (RubisCO) sequestered within the carboxysome with sufficiently high concentrations of its substrate, CO2, to permit its efficient fixation onto ribulose 1,5-bisphosphate. In this study, structure and function of carboxysomes purified from wild type Halothiobacillus neapolitanus and from a high CO2-requiring mutant that is devoid of carboxysomal carbonic anhydrase were compared. The kinetic constants for the carbon fixation reaction confirmed the importance of a functional carboxysomal carbonic anhydrase for efficient catalysis by RubisCO. Furthermore, comparisons of the reaction in intact and broken microcompartments and by purified carboxysomal RubisCO implicated the protein shell of the microcompartment as impeding diffusion of CO2 into and out of the carboxysome interior.
ISSN:0021-9258
1083-351X
DOI:10.1074/jbc.M709285200