Involved in the biosynthesis of the siderophore enterobactin (enterochelin), which is a macrocyclic trimeric lactone of N-(2,3-dihydroxybenzoyl)-serine. The serine trilactone serves as a scaffolding for the three catechol functionalities that provide hexadentate coordination for the tightly ligated iron(2+) atoms. EntB is a bifunctional protein that serves as an isochorismate lyase and an aryl carrier protein (ArCP). Catalyzes the conversion of isochorismate to 2,3-dihydro-2,3-dihydroxybenzoate (2,3-diDHB), the precursor of 2,3-dihydroxybenzoate (DHB). In the enterobactin assembly, EntB functions as an aryl carrier protein phosphopantetheinylated near the C terminus by EntD to yield holo-EntB, which is then acylated by EntE with 2,3-dihydroxybenzoyl-AMP to form DHB-holo-EntB. Then this product will serve in the formation of the amide bond between 2,3-dihydroxybenzoate (DHB) and L-serine. 3 2,3-dihydroxybenzoate + 6 ATP + 3 L-serine = 6 AMP + 6 diphosphate + enterobactin + 4 H(+) H2O + isochorismate = (2S,3S)-2,3-dihydroxy-2,3-dihydrobenzoate + pyruvate Inhibited by 3-[(carboxyethenyl)oxy]-6-hydroxy-1-benzoic acid and 3-[(carboxyethenyl)oxy]benzoic acid. kcat is 600 min(-1) for isochorismatase activity with isochorismate as substrate (at pH 7 and 37 degrees Celsius). kcat is 540 min(-1) for isochorismatase activity with 3-[(1-carboxylatoethenyl)oxy]-cyclohepta-1,6-diene-1-carboxylate as substrate (at pH 7 and 37 degrees Celsius). kcat is 310 min(-1) for isochorismatase activity with 4,5-dihydroisochorismate as substrate (at pH 7 and 37 degrees Celsius). Optimum pH is between 6.5 and 7.5. At pH 5.5, EntB retains 50% of isochorismatase activity. Siderophore biosynthesis; enterobactin biosynthesis. Proteins EntB, EntD, EntE, and EntF form a multienzyme complex called enterobactin synthase. Dimer. Under conditions of iron deficiency and by the fur protein. 4'-phosphopantetheine is transferred from CoA to a specific serine of apo-EntB by EntD. Holo-EntB so formed is then acylated with 2,3-dihydroxybenzoate in a reaction catalyzed by EntE. Cells lacking this gene are hypersensitive to the antimicrobial peptide wrwycr. In the N-terminal section; belongs to the isochorismatase family.