A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2-grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhy drase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorp orates the observation that the expression of this enzyme is increased unde r growth conditions in which the expression of the carbon dioxide-concentra ting mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HC O3- and that there is negligible carbonic anhydrase activity in the cytosol . The role of the mitochondrial carbonic anhydrase is suggested to be the c onversion to HCO3- of the CO2 produced in the mitochondria in the light fro m tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppresse d by the carbon concentrating mechanism. If there is a HCO3- channel in the inner mitochondrial membrane then almost all of the inorganic carbon leave s the mitochondria as HCO3-, thus limiting the potential for CO2 leakage th rough the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic ex pense of less than 1% of the total ATP generation by plastids plus mitochon dria.