Carboxysomal carbonic anhydrases.

Matthew S Kimber

Sub-cellular biochemistry 2014

filter terms:

Cyanobacteria and some chemoautotrophic bacteria enhance their carbon fixation efficiency by actively concentrating bicarbonate within their cytosol. However, converting bicarbonate into carbon dioxide - the form required by RubisCO - would result in its rapid escape through cellular membranes. These organisms resolve this dilemma by sequestering RubisCO behind a semi-permeable protein shell; the resulting large insoluble bodies are known as carboxysomes. For the carbon concentrating mechanism to function, there is an absolute requirement for carbonic anhydrase activity within the carboxysome to convert the bicarbonate to cabon dioxide, and a simultaneous requirement that minimal carbonic anhydrase activity be found within the cystol. Carboxysomal carbomic anhydrases therefore contain additional motifs and domains that generally mediate protein-protein interactions, or encapsulation dependent activation mechanisms. Carboxysomes are found in two deeply divergent varieties. Alpha-Carboxysomes contain a β-carbonic anhydrase, CsoSCA, which is so divergent from canonical β-carbonic anhydrases that it was originally thought to be the founding member of a new class. Beta carboxysomes have CcmM whose N-terminal domain is an active γ-carbonic ahydrase in some strains, but in others has lost all activity and functions primarily as a protein complex assembly scaffold; in addition, a subset of β-carboxysomes also contain the β-carbonic anhydrase CcaA - either in addition to, or instead of, an active CcmM. Here we explore the structures, activities and interactions mediated by the three known carboxysomal carbonic anhydrases, and discuss the mechanisms by which they are recruited to the carboxysome.