CARBONIC-ANHYDRASE ACTIVITY IN LEAVES AND ITS ROLE IN THE FIRST STEP OF C-4 PHOTOSYNTHESIS

In C4 plants carbonic anhydrase catalyzes the critical first step of C4 photosynthesis, the hydration of CO2 to bicarbonate. The maximum activity of this enzyme in C4 leaf extracts, measured by H+ production with saturating CO2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO2 concentrations was apparently just sufficient to prevent the rate of conversion of CO2 to HCO3- from limiting C4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.