Quantitative evaluation of the role of a putative CO₂-scavenging entity in the cyanobacterial CO₂-concentrating mechanism

This paper assesses the contribution of a postulated CO₂-scavenging system to the efficient operation of the CO₂ concentrating mechanism (CCM) in cyanobacteria. A quantitative model for the CCM is presented which incorporates an energy-dependent carbonic anhydrase-like entity located at or near the inner surface of the plasma membrane. This entity, which converts CO₂ to HCO₃^-, against the thermodynamic potential, scavenges CO₂ leaking outward from the carboxysomes, and, further, converts CO₂ entering from the medium to HCO_3-, thus maintaining an inward diffusion gradient along which CO₂ enters passively. The model resembles our earlier models in postulating that CO, and HCO_3- are not at equilibrium throughout the greater part of the cell, and that CO₂ is generated in high concentration at carbonic anhydrase sites within the carboxysomes. The model further takes into account the concentric thylakoid membranes which surround the carboxysomes, and events in the periplasmic space and the unstirred layer surrounding the cell. Implications of the predicted steady state fluxes of CO² and HCO₃^-, and of their steady state concentrations in various cellular compartments, are discussed. The plasma membrane carbonic anhydrase-like activity lowers the photosynthetic K(m) for external Ci, as well as decreasing the inorganic C 'leak', but it may not save on energy expenditure.