Multiple Forms of Plant Phosphoenolpyruvate Carboxylase Associated with Different Metabolic Pathways

The physical and kinetic properties of multiple forms of phosphoenolpyruvate carboxylase were studied in leaves of C(4) and C(3) species, their F(1) and F(3) hybrids, in greening maize leaves, in Crassulacean acid metabolism plants, and in nongreen root tissues. Four different forms are suggested: a C(4) photosynthetic phosphoenolpyruvate carboxylase with high Km for phosphoenolpyruvate ( approximately 0.59 mm), Km Mg ( approximately 0.5 mm), and V(max) ( approximately 29 micromoles per minute per milligram of chlorophyll); a C(3) photosynthetic phosphoenolpyruvate carboxylase with low Km for phosphoenolpyruvate ( approximately 0.14 mm), Km for Mg ( approximately 0.097 mm), and V(max) (1.5); a Crassulacean acid metabolism type with low Km for phosphoenolpyruvate (0.14 mm), and high V(max) (14 micromoles per minute per milligram of chlorophyll); and a nongreen or nonautotrophic type with low Km for phosphoenolpyruvate, Km for Mg, and low V(max). In closely related species or within species, the types can be differentiated by anion exchange column chromatography. Each of the four forms is associated with a different metabolic pathway: the phosphoenolpyruvate carboxylase of C(4) species for malate generation as a photosynthetic intermediate, the phosphoenolpyruvate carboxylase of C(3) species in malate generation as a photosynthetic product, the phosphoenolpyruvate carboxylase of Crassulacean acid metabolism species in malate generation as a CO(2) donor for photosynthesis during the subsequent light period, and a nongreen or root type producing malate for ionic balance and reduced nicotinamide adenine dinucleotide phosphate generation. The data in this paper in conjunction with published information support the notion of different molecular forms of a protein functioning in different metabolic pathways which have common enzymic steps.