Role of Carbonic Anhydrase in Photosynthesis of Blue-Green Alga (Cyanobacterium) Anabaena variabilis ATCC 29413

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The affinity for NaHCO_3 (CO_2) in photosynthesis of Anabaena variabilis ATCC 29413 was much higher in the cells grown under ordinary air (low-CO_2 cells) than in those grown in air enriched with 2-4% CO_2 (high-CO_2 cells) (pH 8.0, 25℃). Ethoxyzolamide (50 μM) increased the Km(NaHCO_3) in low-CO_2 cells about nine times (from 14.3 to 125), while the maximum rate of photosynthesis decreased about 20%. When high-CO_2 cells were transferred to low-CO_2 conditions, carbonic anhydrase (CA) activity increased, while Km(NaHCO_3) in photosynthesis decreased from 140 to 30 μM within about 5 h. The addition of CA to the suspension of both high- and low-CO_2 cells enhanced the rates of photosynthetic O_2 evolution under CO_2-limiting conditions. The rate of ^<14>CO_2 fixation was much faster than that of H^<14>CO_3- fixation. The former reaction was greatly suppressed, while the latter was enhanced by the addition of CA. These results indicate that the active species of inorganic carbon utilized for photo-synthesis was free CO_2 Irrespective of the CO_2 concentration given during growth. It is suggested that CA plays an active role in increasing the affinity for CO_2 in photo-synthesis of low-CO_2 cells of this blue-green alga.
日本植物生理学会の論文