Role of carbonic anhydrase in photosynthesis in Chlorella derived from kinetic analysis of ^<14>CO_2 fixation

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Time courses of photosynthetic ^<14>CO_2 fixation and its simulation are presented for Chlorella cells grown under low CO_2 concentration (low-CO_2 cells) and subsequently exposed to 0.2 mM NaH^<14>CO_3 or 130 ppm ^<14>CO_2 in the presence or absence of carbonic anhydrase (CA) in the suspending medium. It was shown that Chlorella cells utilized only free CO_2 when NaHCO_3 was given in the presence or absence of CA, or when CO_2 was bubbled in the absence of CA. How-ever, the present simulation indicated that both CO_2 and HCO_3^- were utilized when CO_2 was given in the presence of CA. Based on these results, we concluded that 1) Chlorella cells absorb only free CO_2 and 2) this gas is provided to algal cells in two ways, i.e., by direct and indirect CO_2 supply. Usually, the dissolved CO_2 is directly utilized by the algal cells (direct supply of CO_2). However, when the concentration of dissolved CO_2 is extremely low and when there is CA, CO_2 reconverted from HCO_3^- is also utilized by Chlorella cells (indirect supply of CO_2). The utilization of HCO_3^- indicated by the above simulation was explained by the indirect supply of CO_2. We further assumed that the indirect supply of CO_2 to ribulose 1,5-bisphosphate carboxylase occurs mainly in the chloroplasts of low-CO_2 cells containing high CA. Thus, under low CO_2 con-centrations, low-CO_2 cells can carry out more efficient CO_2 fixation than high-CO_2 cells, resulting in the lower apparent Km(CO_2).
日本植物生理学会の論文

Role of carbonic anhydrase in photosynthesis in Chlorella derived from kinetic analysis of ^<14>CO_2 fixation

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