Catalyzes the reversible hydration of fumarate to (S)-malate. (S)-malate = fumarate + H2O Binds 1 [4Fe-4S] cluster per subunit. Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1. Homodimer. Belongs to the class-I fumarase family.