Figure 1.

Schematic representation of the allosteric regulation of ZmPEPC-C₄. In C₄ plants, the first carboxylation reaction that incorporates the atmospheric CO₂ in an organic compound is catalyzed by PEPC-C₄ isozymes located in the mesophyll cells. The product of this reaction, oxaloacetate, is then reduced to malate, as in maize shown here, or converted to aspartate in other plants; both compounds are important allosteric inhibitors of most PEPC enzymes. During the illumination period, l-malate accumulates to high concentrations in the mesophyll cells from where it passively diffuses to the bundle-sheath cells where it is decarboxylated to yield CO₂, which is then incorporated in the Calvin-Benson cycle by Rubisco, and pyruvate, which moves back to the mesophyll cells, regenerating PEP. The allosteric activators trioses-P and hexoses-P are mainly produced in the bundle-sheath cells from the 3-phosphoglycerate (PGA) formed in the Rubisco-catalyzed reaction but also may be produced in mesophyll cells given that 3-phosphoglycerate may diffuse from the bundle-sheath cells. Neutral amino acids (glycine and serine) are intermediates of the photorespiration pathway; they may also be transported to the mesophyll cell where they activate PEPC-C₄ isozymes from monocot grasses, thus counteracting malate inhibition.