Identification and characterization of two carbonic anhydrases from the hydrothermal vent tubeworm Riftia pachyptila Jones

Abstract

The giant hydrothermal vent tubeworm Riftia pachyptila inhabits regions of geothermal activity associated with sea floor spreading centers. In lieu of external feeding apparatus, the tubeworms contain intracellular sulfur-oxidizing bacterial symbionts that fix CO sub(2) from sea water. High activities of carbonic anhydrase (CA) were found in the respiratory organ (the obturacular plume) and in the trophosome, where the symbionts are housed. CA from plume tissue (CAp) had a molecular mass of 27 kd (as determined by SDS-PAGE), and a Km of 13.9 mmol/L. Trophosome CA (CAt) was slightly larger, at 28 kd, and exhibited a Km of 7.2 mM. Both proteins appear to function as monomers, similar to mammalian CA isozymes, although neither exhibited esterase activity associated with mammalian forms. Both proteins also cross-react with heterologous polyclonal antibodies raised against chicken CAII, suggesting that some portion of the enzyme is highly conserved across a wide phylogenetic range. These findings are consistent with the hypothesis that CA may facilitate the process of inorganic carbon uptake this symbiosis.