title: CO2 fixation kinetics of Halothiobacillus neapolitanus mutant carboxysomes lacking carbonic anhydrase suggest the shell acts as a diffusional barrier for CO2.
abstract: 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, CO(2), 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 CO(2)-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 CO(2) into and out of the carboxysome interior.
authors.affiliation: Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406-0001, USA.
authors.forename: Zhicheng
authors.initials: Z
authors.lastname: Dou
authors.forename: Sabine
authors.initials: S
authors.lastname: Heinhorst
authors.forename: Eric B
authors.initials: EB
authors.lastname: Williams
authors.forename: C Daniel
authors.initials: CD
authors.lastname: Murin
authors.forename: Jessup M
authors.initials: JM
authors.lastname: Shively
authors.forename: Gordon C
authors.initials: GC
authors.lastname: Cannon
mesh_terms: D002244:Carbon / Q000737:chemistry; D002245:Carbon Dioxide / Q000737:chemistry*; D002256:Carbonic Anhydrases / Q000737:chemistry*; D002384:Catalysis; D004058:Diffusion; D019281:Dimerization; D005816:Genetic Complementation Test; D044222:Halothiobacillus / Q000235:genetics* / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008954:Models, Biological; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000737:chemistry; D013997:Time Factors
chemical_list: D002245:Carbon Dioxide; D002244:Carbon; D012273:Ribulose-Bisphosphate Carboxylase; D002256:Carbonic Anhydrases