Dou Zhicheng, Heinhorst Sabine, Williams Eric B, Murin C Daniel, Shively Jessup M, Cannon Gordon C
Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406-0001, USA.
J Biol Chem. 2008 Apr 18;283(16):10377-84. doi: 10.1074/jbc.M709285200. Epub 2008 Feb 7.
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.
关于羧酶体在自养细菌碳浓缩机制中作用的广泛接受的模型预测，羧酶体碳酸酐酶是一个关键成分。该酶被认为可使丰富的胞质碳酸氢盐脱水，并为羧酶体内隔离的1,5-二磷酸核酮糖羧化酶/加氧酶（RubisCO）提供足够高浓度的底物CO₂，以使其有效地固定在1,5-二磷酸核酮糖上。在本研究中，比较了从野生型那不勒斯嗜卤硫杆菌和一个缺乏羧酶体碳酸酐酶的高CO₂需求突变体中纯化的羧酶体的结构和功能。碳固定反应的动力学常数证实了功能性羧酶体碳酸酐酶对RubisCO高效催化的重要性。此外，对完整和破碎的微区室中反应以及纯化的羧酶体RubisCO的反应进行比较表明，微区室的蛋白质外壳阻碍了CO₂进出羧酶体内部的扩散。
