Crystal structure of γ-carbonic anhydrase from the polyextremophilic bacterium Aeribacillus pallidus

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The polyextremophilic bacterium Aeribacillus pallidus produces a thermo- and alkali-stable γ-carbonic anhydrase (γ-apCA), a homotrimeric metalloenzyme containing a zinc ion in its active site that catalyzes the reversible hydration of carbon dioxide (CO2). Here, we present the first crystal structure of γ-apCA at 1.7 Å resolution, revealing two trimers in the asymmetric unit. The overall structure is consistent with other γ-CAs, where each monomer adopts a prism-like structure consisting of an N-terminal left-handed β-helix and a C-terminal α-helix. The active site, located at the interface between two monomers, coordinates the zinc ion with three histidine residues (H65, H82, and H87) and a water molecule in a tetrahedral configuration. Structural comparison indicates that the amino acid composition at the active site of γ-apCA differs significantly from the prototypic γ-CA from Methanosarcina thermophila. This variation likely accounts for the lack of measurable CO2 hydration activity in γ-apCA. Additionally, the structure reveals non-catalytic zinc and sulfate ions trapped at the trimer core and trimer-trimer non-crystallographic interfaces. These may contribute to stabilizing enzyme assembly and promoting crystal packing.