Crystal structure of the "cab"-type beta class carbonic anhydrase from thearchaeon Methanobacterium thermoautotrophicum

The structure of the "cab"-type beta class carbonic anhydrase from the arch aeon Methanobacterium thermoautotrophicum (Cab) has been determined to 2.1- Angstrom resolution using the multiwavelength anomalous diffraction phasing technique. Cab exists as a dimer with a subunit fold similar to that obser ved in "plant'-type beta class carbonic anhydrases. The active site zinc is coordinated by protein ligands Cys(32), His(87), and Cys(90), with the tet rahedral coordination completed by a water molecule. The major difference b etween plant- and cab-type beta class carbonic anhydrases is in the organiz ation of the hydrophobic pocket. The structure reveals a Hepes buffer molec ule bound 8 Angstrom away from the active site zinc, which suggests a possi ble proton transfer pathway from the active site to the solvent.