Escherichia coli fumarase A catalyzes the isomerization of enol and keto oxalacetic acid.

Fumarase A, a product of the fumA gene of Escherichia coli, has been found to catalyze the isomerization of enol to keto oxalacetic acid (OAA) in addition to catalyzing the fumarase reaction. The kcat/Km for the isomerization is almost identical to that for the fumarase reaction. The isomerization reaction apparently takes place at the same active site as the fumarase reaction since both reactions show a similar sensitivity to inactivation by O2, both reactions are strongly inhibited by 2-hydroxy-3-nitropropionate, and the isomerization reaction is inhibited by fumarate and malate. The isomerization requires the presence of a [4Fe-4S] or [3Fe-4S] cluster, perhaps for structural rather than catalytic reasons. Hydration of enol OAA to the gem diol has been ruled out as a possible mechanism of isomerization on the basis of the preservation of the oxygen on carbon 2 and the position of protonation on carbon 3. The isomerization is not stereospecific in the position of protonation at carbon 3 but appears to be stereoselective, with protonation preferentially occurring in the 3-pro-S position. Porcine fumarase, isopropyl malate isomerase, and dihydroxyacid dehydratase do not catalyze this isomerization. Fumarase A and aconitase, two enzymes with 4Fe-4S clusters that bind a linear 4-carbon dicarboxylic acid moiety in the trans conformation during their normal hydro-lyase reaction, do catalyze this isomerization.