Structural study of the Carbonic Anhydrase SazCA from the thermophilic bacterium Sulfurihydrogenibium Azorense

Purpose and Objective: ?-Carbonic anhydrases (?-CAs) are zinc-enzymes, which catalyze the reversible hydration of carbon dioxide to a bicarbonate ion and proton. Here we report the crystallographic structure of ?-CA isolated from the thermophilic bacterium Sulfurihydrogenibium Azorense (SazCA). The structural analysis of SazCA, the most catalitycally active CA known to date, is crucial to understand the structural determinants responsible of its higher catalytic efficiency. Moreover, the comparison with the homologous enzyme SspCA (from S. yellowstonensis), showing an exceptional thermal stability, could be useful to design SspCA variants possessing both higher stability and catalytic activity. Material and Methods: A recombinant SazCA was expressed in E.coli and purified by means of chromatographic techniques. Crystals were obtained for SazCA in complex with a strong inhibitor of ?-CAs, acetazolamide (AZM), using the hanging drop vapor diffusion method. Results: The three-dimensional structure of SazCA shows a dimeric arrangement typical of the bacterial ?-CAs. The higher activity of SazCA, compared to that of SspCA, is probably due to the presence of two different residues at the rim of the active site cavity. Conclusion: The structural study of SazCA identified the structural reasons why this enzyme is more catalytically active compared to SspCA. These results may be helpful for a possible employment of the enzyme as biocatalyst for the harsh conditions of carbon dioxide sequestration processes.