Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C₄ Photosynthesis

Abstract

In C(4) plants carbonic anhydrase catalyzes the critical first step of C(4) photosynthesis, the hydration of CO(2) to bicarbonate. The maximum activity of this enzyme in C(4) leaf extracts, measured by H(+) production with saturating CO(2) and extrapolated to 25 degrees C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C(3) leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO(2) concentrations was apparently just sufficient to prevent the rate of conversion of CO(2) to HCO(3) (-) from limiting C(4) photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.