The adhE gene of Escherichia coli encodes a multifunctional ethanol oxidoreductase (AdhE) that catalyzes successive reductions of acetyl-CoA to acetaldehyde and then to ethanol reversibly at the expense of NADH. Mutant JE52, serially selected for acquired and improved ability to grow aerobically on ethanol, synthesized an AdhE A267T/E568K with two amino acid substitutions that sequentially conferred improved catalytic properties and stability. Here we show that the aerobic growth ability on ethanol depends also on protection of the mutant AdhE against metal-catalyzed oxidation by the chaperone DnaK (a member of the Hsp70 family). No DnaK protection of the enzyme is evident during anaerobic growth on glucose. Synthesis of DnaK also protected E. coli from H2O2 killing under conditions when functional AdhE is not required. Our results therefore suggest that, in addition to the known role of protecting cells against heat stress, DnaK also protects numerous kinds of proteins from oxidative damage. enzyme evolution ͉ Hsp70 chaperone E thanol oxidoreductase (AdhE) of Escherichia coli is a multifunctional enzyme that catalyzes fermentative production of ethanol by two sequential NADH-dependent reductions of acetyl-CoA, as well as deactivation of pyruvate formate-lyase, which cleaves pyruvate to acetyl-CoA and formate. This large protein of 891 aa apparently emerged as a result of gene fusion. The NH 2 -terminal region of this protein is highly homologous to the family of aldehyde:NAD ϩ oxidoreductases, whereas the COOH-terminal region is homologous to the family of Fe 2ϩ -dependent alcohol:NAD ϩ oxidoreductases. The single Fe 2ϩ bound to the alcohol oxidoreductase moiety of the AdhE protomer is also required for the deactivase activity (1-4). The estimated 3 ϫ 10 4 AdhE molecules in an anaerobically grown cell are assembled into helical rod-like structures, or spirosomes, most of which contain 20-60 protomers. Residues 1-449 and 763-890 of AdhE are involved in its polymerization and may also be responsible for the propensity of the protein to aggregate amorphously. The biological significance of spirosomes, however, remains enigmatic (3,5,6).Despite the reversibility of the two NADH-coupled reactions catalyzed by AdhE, wild-type E. coli is unable to grow on ethanol as a sole source of carbon and energy, because the adhE gene is transcribed aerobically at lowered levels (7-9) and the half-life of AdhE activity is shortened during aerobic metabolism by metal-catalyzed oxidation (MCO). During this process, the hydroxyl radicals locally generated by the Fe 2ϩ bound to AdhE covalently attack the amino acid side chains near the active site (10, 11).Mutants of E. coli capable of aerobic growth on ethanol as sole carbon and energy source have been isolated and characterized (7,12,13). All 16 independent first-stage mutants studied by us, typified by JE46, grew on ethanol with a doubling time of 240 min at 37°C and synthesized an AdhE with an Ala-267 3 Thr substitution in the acetaldehyde:NAD ϩ oxidoreductase domain (AdhE A267T ...