Catalyzes the reversible hydration of fumarate to (S)-malate. Functions as an aerobic enzyme in the direction of malate formation as part of the citric acid cycle. Accounts for about 80% of the fumarase activity when the bacteria grow aerobically. To a lesser extent, also displays D-tartrate dehydratase activity in vitro, but is not able to convert (R)-malate, L-tartrate or meso-tartrate. Can also catalyze the isomerization of enol- to keto-oxaloacetate. (S)-malate = fumarate + H2O oxaloacetate = enol-oxaloacetate Binds 1 [4Fe-4S] cluster per subunit. Is inactivated by oxygen, due to oxidation of the Fe-S cluster to its [3Fe-4S] form. Both the fumarase and the isomerase reactions are competitively inhibited by 3-hydroxy-2-nitropropionate. The isomerase reaction is also inhibited by fumarate and malate. kcat is 3000 sec(-1) for the hydration of fumarate and 300 sec(-1) for the isomerization of enol-oxaloacetate. Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1. Homodimer. Is expressed under aerobic conditions. Is repressed by glucose and anaerobiosis. Belongs to the class-I fumarase family.