Kranz and single-cell forms of C4 plants in the subfamily Suaedoideae show kinetic C4 convergence for PEPC and Rubisco with divergent amino acid substitutions

The two carboxylation reactions performed by phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) are vital in the fixation of inorganic carbon for C4 plants. The abundance of PEPC is substantially elevated in C4 leaves, while the location of Rubisco is restricted to one of two chloroplast types. These differences compared with C3 leaves have been shown to result in convergent enzyme optimization in some C4 species. Investigation into the kinetic properties of PEPC and Rubisco from Kranz C4, single cell C4, and C3 species in Chenopodiaceae s. s. subfamily Suaedoideae showed that these major carboxylases in C4 Suaedoideae species lack the same mutations found in other C4 systems which have been examined; but still have similar convergent kinetic properties. Positive selection analysis on the N-terminus of PEPC identified residues 364 and 368 to be under positive selection with a posterior probability >0.99 using Bayes empirical Bayes. Compared with previous analyses on other C4 species, PEPC from C4 Suaedoideae species have different convergent amino acids that result in a higher K m for PEP and malate tolerance compared with C3 species. Kinetic analysis of Rubisco showed that C4 species have a higher catalytic efficiency of Rubisco (k catc in mol CO2 mol(-1) Rubisco active sites s(-1)), despite lacking convergent substitutions in the rbcL gene. The importance of kinetic changes to the two-carboxylation reactions in C4 leaves related to amino acid selection is discussed.