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Journal of Bacteriology, December 2006, p. 8087-8094, Vol. 188, No. 23
0021-9193/06/$08.00+0     doi:10.1128/JB.00990-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Characterization of the Carboxysomal Carbonic Anhydrase CsoSCA from Halothiobacillus neapolitanus

Sabine Heinhorst,¹ Eric B. Williams,¹ Fei Cai,¹ C. Daniel Murin,¹ Jessup M. Shively,^1,2 and Gordon C. Cannon^1*

Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406,¹ Department of Genetics and Biochemistry, Clemson University, Clemson, South Carolina 29634²

Received 6 July 2006/ Accepted 18 September 2006

In cyanobacteria and many chemolithotrophic bacteria, the CO₂-fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) is sequestered into polyhedral protein bodies called carboxysomes. The carboxysome is believed to function as a microcompartment that enhances the catalytic efficacy of RubisCO by providing the enzyme with its substrate, CO₂, through the action of the shell protein CsoSCA, which is a novel carbonic anhydrase. In the work reported here, the biochemical properties of purified, recombinant CsoSCA were studied, and the catalytic characteristics of the carbonic anhydrase for the CO₂ hydration and bicarbonate dehydration reactions were compared with those of intact and ruptured carboxysomes. The low apparent catalytic rates measured for CsoSCA in intact carboxysomes suggest that the protein shell acts as a barrier for the CO₂ that has been produced by CsoSCA through directional dehydration of cytoplasmic bicarbonate. This CO₂ trap provides the sequestered RubisCO with ample substrate for efficient fixation and constitutes a means by which microcompartmentalization enhances the catalytic efficiency of this enzyme.

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* Corresponding author. Mailing address: Department of Chemistry and Biochemistry, The University of Southern Mississippi, 118 College Dr. #5043, Hattiesburg, MS 39406-0001. Phone: (601) 266-4221. Fax: (601) 266-6075. E-mail: gordon.cannon{at}usm.edu.

Published ahead of print on 29 September 2006.

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Journal of Bacteriology, December 2006, p. 8087-8094, Vol. 188, No. 23
0021-9193/06/$08.00+0     doi:10.1128/JB.00990-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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