Why Superoxide Imposes an Aromatic Amino Acid Auxotrophy onEscherichia coli
Journal of Biological Chemistry
The lack of superoxide dismutase and the consequent elevation of [O 2 ؊ ] imposes, on Escherichia coli, auxotrophies for branched chain, sulfur-containing, and aromatic amino acids. The former two classes of auxotrophies have already been explained, whereas the third is explained herein. Thus O 2 ؊ is shown to interfere with the production of erythrose-4-phosphate, which is essential for the first step of the aromatic biosynthetic pathway. It does so by oxidizing the 1,2-dihydroxyethyl thiamine
... pyrophosphate intermediate of transketolase and inactivating this enzyme. Escherichia coli mutants, which are unable to express both the iron-and the manganese-containing superoxide dismutases, exhibit several phenotypic deficits, among which are oxygendependent auxotrophies for branched chain, sulfur-containing (1), and aromatic amino acids (2). The requirement for branched chain amino acids was explained on the basis of the oxidative inactivation of the dihydroxy acid dehydratase (3-6), which catalyzes the penultimate step in the relevant biosynthetic pathway. The requirement for sulfur-containing amino acids was attributable to leakage of sulfite from the cells (7, 8). We have now investigated the aromatic amino acid auxotrophy of the sodA sodB strain and find an explanation quite different from those that rationalized the other amino acid auxotrophies. Thus, the aromatic biosynthetic pathway begins with the condensation of erythrose-4-phosphate (E-4-P) 1 with phosphoenol pyruvate (PEP) to yield 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP), and the production of E-4-P in turn is dependent on the sequential actions of transketolase (TK) and transaldolase. It has been shown that the intermediate of the TK reaction, which is 1,2-dihydroxyethyl thiamine pyrophosphate, is oxidized by O 2 Ϫ (9, 10). We now report that this oxidation interferes with the production of E-4-P and thus accounts for the decrease of aromatic biosynthesis in aerobic sodA sodB E. coli.