Carbon dioxide, the substrate for phosphoenolpyruvate carboxylase from leaves of maize

Using a spectrophotometric technique it has been shown that CO2 is the substrate for phosphoenolpyruvate carboxylase (EC 4.1.1.31) from leaves of Zea mays. The Km (CO2) is approximately 7.5 × 10−6 M, which is the order of the half saturation value for CO2 in natural photosynthesis. Studies indicate also that carbonic anhydrase is localized in chloroplasts, isolated by the 'laceration technique' from leaves of Spinacea oleracea, but is absent from leaves of Zea mays. It is speculated that carbonic anhydrase catalyses a 'trap' for the CO2 escaping into the environment via photorespiration.