Kinetic Modifications of C₄ PEPC Are Qualitatively Convergent, but Larger in Panicum Than in Flaveria

C₄ photosynthesis results from a set of anatomical features and biochemical components that act together to concentrate CO₂ within the leaf and boost productivity. This complex trait evolved independently many times, resulting in various realizations of the phenotype, but in all C₄ plants the primary fixation of atmospheric carbon is catalyzed by phosphoenolpyruvate carboxylase. Comparisons of C₄ and non-C₄ PEPC from a few closely related species suggested that the enzyme was modified to meet the demands of the C₄ cycle. However, very few C₄ groups have been investigated, hampering general conclusions. To test the hypothesis that distant C₄ lineages underwent convergent biochemical changes, we compare the kinetic variation between C₄ and non-C₄ PEPC from a previously assessed young lineage (Flaveria, Asteraceae) with those from an older lineage found within the distantly related grass family (Panicum). Despite the evolutionary distance, the kinetic changes between the non-C₄ and C₄ PEPC are qualitatively similar, with a decrease in sensitivity for inhibitors, an increased specificity (k _cat/K _m) for bicarbonate, and a decreased specificity (k _cat/K _m) for PEP. The differences are more pronounced in the older lineage Panicum, which might indicate that optimization of PEPC for the C₄ context increases with evolutionary time.