Anapleurotic CO2 Fixation by Phosphoenolpyruvate Carboxylase in C3 Plants

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Abstract
The role of phosphoenolpyruvate carboxylase in photosynthesis in the C3 plant Nicotiana tabacum has been probed by measurement of the 13C content of various materials. Whole leaf and purified ribulose bisphosphate carboxylase are within the range expected for C3 plants. Aspartic acid purified following acid hydrolysis of this ribulose bisphosphate carboxylase is enriched in 13C compared to whole protein. Carbons 1-3 of this aspartic acid are in the normal C3 range, but carbon-4 (obtained by treatment of the aspartic acid with aspartate β-decarboxylase) has an isotopic composition in the range expected for products of C4 photosynthesis (−5‰), and it appears that more than half of the aspartic acid is synthesized by phosphoenolpyruvate carboxylase using atmospheric CO2/HCO3−. Thus, a primary role of phosphoenolpyruvate carboxylase in C3 plants appears to be the anapleurotic synthesis of four-carbon acids.