Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chara corallina

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Carbonic anhydrase (CA, EC. 4.2.1.1) activity in air-grown Chara corallina was detected mainly in the intracellular fraction, most of which composed of chloroplasts and cytoplasmic gel, and not on the cell surface. Only minor levels of CA activity, on the basis of equivalent volumes, were detected in the cell sap and the cytoplasmic sol. The maximum rate of photosynthetic O_2 evolution by air-grown Chara corallina at pH 6.0 was twice that at pH 7.6, while the apparent K_m for external inorganic carbon (C_i) at pH 7.6 was about three times that at pH 6.0. However, the apparent K_m(CO_2) was about three times larger at pH 6.0 than at pH 7.6. The K_m(C_i)-value at pH 7.6 increased severalfold in the presence of acetazolamide (AZA), an inhibitor of CA, but no inhibition was observed at pH 6.0. The pH-dependence may be due to differences in the permeability of AZA at the given pH values. Fixation of ^<14>CO_2 at 20 μM and of H^<14>CO_3^- at 200 μM over the course of 5 s was very similar at pH 7.4. Addition of CA significantly suppressed the photosynthetic ^<14>CO_2-fixation but it stimulated the H^<14>CO_3^--fixation. This result indicates that free CO_2 is an active species of C_i that is incorporated into the cell during photosynthesis. These results together suggest the following: (1) Free CO_2 is utilized for photosynthesis, (2) CA is mainly located inside the cell and functions to increase the affinity for CO_2 in photosynthesis by facilitating the supply of CO_2 from the plasmalemma to the site of CO_2-fixation.
日本植物生理学会の論文

Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chara corallina

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