Enhancement of ribulose 1,5-bisphosphate carboxylation reaction by carbonic anhydrase

Abstract

It has been established that algal cells grown with ordinary air (low-CO2 cells) have a much lower Km value for COZ in photosynthesis than those grown with COz-enriched air (high-CO, cells) [l-4]. Carbonic anhydrase (EC 4.2.1 .I) is almost exclusively located in low-CO2 cells of Chlorella and its activity in high-CO2 cells is marginal, if any [S ,6]. COZ concentration during growth does not affect ribulose l$bisphosphate (Ru-P,) carboxylase (EC 4.1.1.39) activity [3,6,7]. The app. Km (CO*) in low-CO2 cells is greatly increased by Diamox, an inhibitor of CA [3,6]. These results indicate the possibility that the transport of CO? from outside the algal cells to the site of Ru-P2 carboxylase may be accelerated by CA and that the observed difference in Km (COZ) may be due to the difference in CA activity. We have found [8] that, irrespective of CO2 concentration during growth, the rate of photosynthetic CO* fixation in Chlorella vulgaris 11 h cells in the presence of low NaHCOa concentrations was greatly enhanced by the addition of CA, while the initial rate of COZ fixation immediately after bubbling of ordinary air was suppressed by this enzyme. Since CA catalyzes the dehydration of bicarbonate and the