Crystal Structure of the "cab"-type β Class Carbonic Anhydrase from the Archaeon Methanobacterium thermoautotrophicum

The structure of the "cab"-type β class carbonic anhydrase from the archaeon Methanobacterium thermoautotrophicum (Cab) has been determined to 2.1-Å resolution using the multiwavelength anomalous diffraction phasing technique. Cab exists as a dimer with a subunit fold similar to that observed in "plant"-type β class carbonic anhydrases. The active site zinc is coordinated by protein ligands Cys ³², His⁸⁷, and Cys⁹⁰, with the tetrahedral coordination completed by a water molecule. The major difference between plant- and cab-type β class carbonic anhydrases is in the organization of the hydrophobic pocket. The structure reveals a Hepes buffer molecule bound 8 Å away from the active site zinc, which suggests a possible proton transfer pathway from the active site to the solvent.