Stereochemistry of phosphoenolpyruvate carboxylation catalyzed by phosphoenolpyruvate carboxykinase

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Abstract
The stereochemistry of the carboxylation of phosphoenolpyruvate to yield oxalacetate, catalyzed by chicken liver phosphoenolpyruvate carboxykinase and by Ascaris muscle phosphoenolpyruvate carboxykinase, was determined. The substrate (Z)-3-fluorophosphoenolpyruvate was used for the stereochemical analysis. The carboxylation reaction was coupled to malate dehydrogenase to yield 3-fluoromalate, and the stereochemistry of the products was identified by 19F NMR. In separate experiments, the enantiomeric tautomers of 3-fluorooxalacetate were shown to be utilized by malate dehydrogenase to yield (2R,3R)- and (2R,3S)-3-fluoromalate in nearly identical amounts. The products were determined by 19F NMR. When (Z)-3-fluorophosphoenolpyruvate was used as a substrate for phosphoenolpyruvate-carboxykinase from avian liver and from Ascaris, and malate dehydrogenase was used to trap the product, only a single diastereomer was observed. This product was shown to be (2R,3R)-3-fluoromalate in each case. The assignments were based on coupling constants taken from Keck et al. [Keck, R., Hess, H., and Retey, J. (1980) FEBS Lett. 114, 287]. These rsults indicate that the stereochemistry of carboxylation catalyzed by chicken phosphoenolpyruvate carboxykinase and by Ascaris phosphoenolpyruvate carboxykinase, is identical and takes place from the si side of the enzyme-bound phosphoenolpyruvate. The carboxylation reaction was run both in H2O and in D2O. No deuterium incoporation into fluoromalate was shown to occur. The product 3-fluorooxalacetate is thus released from phosphoenolpyruvate carboxykinase as the keto form and is reduced more rapidly by reduced nicotinamide adenine dinucleotide with malate dehydrogenase than by the occurrence of tautomerization.