Enzymatic Inactivation of Extracellular Carbonic Anhydrase and its Effect on <italic>K</italic><sub><italic>1/2</italic></sub> (CO<sup>2</sup>) for Photosynthesis in <italic>Chlorella ellipsoidea</italic> C-27

Abstract

The ratio of the extracellular to the intracellular activity of carbonic anhydrase (CA) in cells of Chlorella ellipsoidea C-27, adapted to low levels of CO2 for 24 h (low-CO2 cells), was about one to one. Treatment of intact cells with Pronase P inactivated about one-half of the extracellular CA activity without affecting photosynthetic activity. The CA activity in cell homogenates and in cell-wall ghosts liberated during cell division was completely inactivated by the same treatment. Pretreatment with Glycosidase mix, Chitosanase and Macerozyme enhanced the inactivation of the CA activity in intact cells. These results suggest that extracellular CA is evenly distributed throughout the whole cell-wall region. The apparent K1/2 for dissolved inorganic carbon (DIC) in low-CO2 cells doubled when extracellular CA was inactivated by treatment with Pronase P, but the K1/2 obtained was still one-half of that in high-CO2 cells. Photosynthetic 14CO2-fixation in low-CO2 cells was enhanced by acetazolamide, whereas H14CO3−-fixation was suppressed. The results suggest that CO2 is a dominant substrate utilized by cells and that HCO3− is utilized after conversion to CO2. The present results show that both intracellular and extracellular CA contribute to the increase in affinity for DIC during photosynthesis in low-CO2 cells of Chlorella ellipsoidea C-27.