Diversity of Microbial Carbonic Anhydrases, Their Physiological Role and Applications

Abstract

Carbonic anhydrase (CA) is a zinc metalloenzyme catalyzing the ­reversible hydration of CO₂ to bicarbonate, a reaction that supports various biochemical and physiological functions. Although ubiquitous in highly evolved organisms from the eukarya domain, the enzyme has received scant attention in prokaryotes from the bacteria and archaea domains. Unraveling of microbial genome sequences suggest that CA is widespread in metabolically and phylogenetically diverse prokaryotes. Evidence for the presence of carbonic anhydrase was obtained for freshwater, marine, mesophilic, thermophilic, aerobic, anaerobic, pathogenic, symbiotic, acetogenic, autotrophic, heterotrophic and photosynthetic species. In prokaryotes, carbonic anhydrases are involved in diverse biochemical and physiological processes, including photosynthesis, respiration, CO₂ and ion transport, and CO₂/bicarbonate balance required for biosynthetic reactions. Besides the biochemical and physiological importance of CA in CO₂ metabolism, this enzyme has found a new dimension in the field of biomimetic CO₂ sequestration. Anthropogenic CO₂ emission has led to adverse impact on climate and has been implicated in global warming. In the global effort to combat the predicted disaster, several CO₂ capture and storage technologies (CCS) are being considered. A novel biomimetic approach for CO₂ scrubbing using CA provides a viable means to accelerate CO₂-hydration reaction and has been found to be feasible for fixing large quantities of CO₂ into calcium carbonate in presence of suitable cations at moderate pH values in vitro. Thus, biomimetic CaCO₃ mineralization for carbon capture and storage offers potential as a stable CO₂ capture technology. Cost-efficient production of the enzyme by bacterial overexpression and production of value-added CO₂ by-product are critical for development of economically feasible CA based CO₂ capture processes.