Characterization of the carboxysomal carbonic anhydrase CsoSCA from Halothiobacillus neapolitanus
In cyanobacteria and many chemolithotrophic bacteria, the CO2-fixing enzyme ribullose 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, CO., 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 CO, 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.