Purification and Properties of Phosphoenolpyruvate and Ribulose Diphosphate Carboxylases from C4 and C3 Plants

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Summary Ribulose diphosphate (RuDP) and phosphoenolpyruvate (PEP) carboxylases were partially purified from leaves of three C4 plants: Setaria, Pennisetum and Amaranthus and two C3 plants: Oryza and Rumex. RuDP carboxylase from C4 plants had a lower Km for RuDP and HCO3− than that of the enzyme from C3 type. Their reaction velocities were similar in all cases, reflecting the potential of RuDP carboxylase in C4 plants as well as in C3 plants. PEP carboxylase from C4 plants possessed higher Km values for PEP and Mg++ than that from C3 plants, but the Km for HCO3− was similar in both the categories. The maximum reaction velocities of PEP carboxylase from C4 plants were nearly 15–20 times higher than those from C3 plants, suggesting the basically different nature of PEP carboxylases from C3 and C4 plants. It is envisaged that in C4 plants PEP carboxylase helps in rapid fixation of CO2 into C4 acids at a faster rate under atmospheric CO2 concentration and these C4 acids move into bundle sheath and are decarboxylated creating a much higher concentration of CO2 than outside. At thus elevated levels of CO2 in bundle sheath of C4 plants RuDP carboxylase operates effectively.