Carbonic Anhydrases (γ‐Class)

Abstract

The γ‐class carbonic anhydrase from the thermophilic archaeon Methanosarcina thermophila is involved in the aceticlastic pathway in which energy for growth is derived from the conversion of the methyl group of actetate to methane. This γ‐class carbonic anhydrase is arranged as a tightly packed trimer with each monomer folded into a left‐handed β‐helix. The catalytic active site zinc is located at the interface between subunits and is coordinated by three histidines, two from one monomer and the other from a second monomer. Interestingly, the arrangement of the histidine side chains ligating the metal show clear structural similarity to their counterparts in the well‐characterized α‐class carbonic anhydrases, despite the difference in overall fold. The conversion of CO 2 to bicarbonate has a maximal turnover number of 6 × 10 4 s −1 , and this reaction is believed to proceed by utilizing the zinc‐hydroxide mechanism previously proposed for α‐class carbonic anhydrases.