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Yoshihiro Shiraiwa, Munehiro Kikuyama, Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chora corallina, Plant and Cell Physiology, Volume 30, Issue 4, June 1989, Pages 581–587, https://doi.org/10.1093/oxfordjournals.pcp.a077778
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Abstract
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 O2 evolution by air-grown Chara corallina at pH 6.0 was twice that at pH 7.6, while the apparent Km for external inorganic carbon (Ci) at pH 7.6 was about 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)-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 14CO2 at 20 μM and of H14CO3− at 200 μM over the course of 5 s was very similar at pH 7.4. Addition of CA significantly suppressed the photosynthetic 14CO2-fixation but it stimulated the H14CO3−-fixation. This result indicates that free CO2 is an active species of Ci that is incorporated into the cell during photosynthesis.
These results together suggest the following: (1) Free CO2 is utilized for photosynthesis, (2) CA is mainly located inside the cell and functions to increase the affinity for CO2 in photosynthesis by facilitating the supply of CO2 from the plasmalemma to the site of CO2-fixation.
