Fixation of carbon dioxide by plant roots through phosphoenolpyruvate carboxylase

Summary Extracts of snap-bean roots are capable of fixing large amounts of CO₂ through carboxylation of phosphoenolpyruvate. Fixation was dependent upon a supply of PEP or substrate capable of being transformed to PEP. 3-PGA served as substrate, but when its conversion to PEP was blocked by fluoride, CO₂ fixation was eliminated. Acetate or pyruvate were not effective as substrates. The amount of CO₂ fixed was closely associated with production of inorganic phosphorus. From these characteristics and the unfavorable equilibria of the other known carboxylating reactions associated with the Krebs cycle, it is concluded that PEP carboxylase accounts for the major entrance of CO₂ into the Krebs cycle.Magnesium stimulated the reaction, while Ca inhibited it. Potassium additions to the assay mixture were found to have no direct effect on stimulation of CO₂ fixation. In the presence of NH₄, amino acids rather than malate were found to contain the majority of the added CO₂.Some implications of the importance of this enzyme system in metabolism of the root and on the soil-root relationship are discussed.Contribution from the Department of Soils, North Carolina Agricultural Experiment Station, Raleigh, North Carolina. Submitted as Paper No. 908 of the Journal Series.