Involved in the biosynthesis of the siderophore enterobactin (macrocyclic trimeric lactone of N-(2,3-dihydroxybenzoyl)-serine). Catalyzes the reversible conversion of chorismate to isochorismate. chorismate = isochorismate kcat is 290 min(-1) for mutase activity with chorismate. kcat is 173 min(-1) for mutase activity with chorismate (at pH 7.8 and 37 degrees Celsius). kcat is 108 min(-1) for mutase activity with isochorismate (at pH 7.8 and 37 degrees Celsius). kcat is 37 min(-1) for mutase activity with chorismate. Siderophore biosynthesis; enterobactin biosynthesis. Monomer. Under conditions of iron deficiency and by the fur protein. Cells lacking this gene are unable to produce enterobactin and are hypersensitive to the antimicrobial peptide wrwycr. Belongs to the isochorismate synthase family. Extended N-terminus.