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

Abstract

The polyextremophilic bacterium Aeribacillus pallidus produces a thermo- and alkali-stable gamma-carbonic anhydrase (gamma 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 gamma-apCA at 1.7-& Aring; resolution, revealing 2 trimers in the asymmetric unit. The overall structure is consistent with other gamma-CAs, where each monomer adopts a prism-like structure consisting of an N-terminal left-handed beta-helix and a C-terminal alpha-helix. The active site, located at the interface between 2 monomers, coordinates the zinc ion with 3 histidine residues (H65, H82, and H87) and a water molecule in a tetrahedral configuration. The structural comparison indicates that the amino acid composition at the active site of gamma-apCA differs significantly from the prototypic gamma-CA from Methanosarcina thermophila. This variation likely accounts for the lack of measurable CO2 hydration activity in gamma-apCA. Additionally, the structure reveals noncatalytic zinc and sulfate ions trapped at the trimer core and trimer-trimer noncrystallographic interfaces. These may contribute to stabilizing enzyme assembly and promoting crystal packing. (c) 2024 The Author(s). Published by Elsevier Inc. on behalf of Korean Society for Molecular and Cellular Biology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).