Enzymatic fractionation of carbon isotopes by phosphoenolpyruvate carboxylase from c(4) plants

The carbon atoms of glucose and malate in C₄ plants are 2 to 3‰ enriched in ¹²C with respect to atmospheric CO₂; whereas these intermediates in C₃ plants are 15 to 18‰ enriched with ¹²C with respect to atmospheric CO₂. The enzymatic synthesis of malate from phosphoenolpyruvate and bicarbonate in preparations of leaves of Sorghum bicolor, Haygrazer result in a carbon isotope fractionation of about 3‰. The enzymatic synthesis of phosphoglyceric acid from ribulose 1,5-diP and CO₂ in these preparations (contaminated with carbonic anhydrase) at 24 C and 37 C result in a carbon isotope fractionation of 33.7‰ and 18.3‰, respectively. These data are consistent with the conclusion that the small enrichment of ¹²C in the carbon atoms of malate and glucose (with respect to atmospheric CO₂) in leaves of Sorghum bicolor, Haygrazer occurs at the phosphoenolpyruvate carboxylase step.