Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chora corallina |
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Authors: | Shiraiwa, Yoshihiro Kikuyama, Munehiro |
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Affiliation: | 1Department of Biology, Faculty of Science, Niigata University Ikarashi 2-8050, Niigata, 950-21 Japan 2Niigata College of Pharmacy Kamishin-ei-cho, Niigata, 950-21 Japan |
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Abstract: | Carbonic anhydrase (CA, EC. 4.2.1.1[EC]) activity in air-grown Characorallina 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 cellsap and the cytoplasmic sol. The maximum rate of photosynthetic O2 evolution by air-grownChara corallina at pH 6.0 was twice that at pH 7.6, while theapparent Km for external inorganic carbon (Ci) at pH 7.6 wasabout three times that at pH 6.0. However, the apparent Km(CO2)was about three times larger at pH 6.0 than at pH 7.6. The Km(Ci)-valueat pH 7.6 increased severalfold in the presence of acetazolamide(AZA), an inhibitor of CA, but no inhibition was observed atpH 6.0. The pH-dependence may be due to differences in the permeabilityof AZA at the given pH values. Fixation of 14CO2 at 20 µMand of H14CO3 at 200 µM over the course of 5 swas very similar at pH 7.4. Addition of CA significantly suppressedthe photosynthetic 14CO2-fixation but it stimulated the H14CO3-fixation.This result indicates that free CO2 is an active species ofCi that is incorporated into the cell during photosynthesis. These results together suggest the following: (1) Free CO2 isutilized for photosynthesis, (2) CA is mainly located insidethe cell and functions to increase the affinity for CO2 in photosynthesisby facilitating the supply of CO2 from the plasmalemma to thesite of CO2-fixation. 3Present address: Biological Laboratory, The University of theAir, Wakaba 2-11, Chiba, 260 Japan. (Received December 9, 1988; Accepted March 22, 1989) |
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