Regulation of α-Ketoglutarate Dehydrogenase Formation in Escherichia coli

In the aerobic tricarboxylate cycle succinate is formed via oxidation of a-ketoglutarate, but anaerobically various bacteria form it via reduction of fumarate. The latter pathway was initially inferred to account for the large amounts of succinate and its derivative, propionate, produced in the fermentation of glycerol by propionic bacteria (l-4). Furthermore, fumarate was later shown to serve as electron acceptor in Escherichiu coli growing anaerobically on glycerol plus fumarate (4, 5). More recently, studies with mutant 309-l of E. coli, which lacks the a-ketoglutarate dehydrogenase system (referred to in this paper as a-ketoglutarate dehydrogenase) (6, 7), provided evidence that the reductive pathway to succinate is active under anaerobic but not at all under aerobic conditions. Anaerobically this mutant grew on glucose as well as the wild type, but aerobically it required succinate (or the amino acids methionine, lysine, and diaminopimelate) (6). ' Finally, it has been shown that under anaerobic conditions E. coli forms an inducible fumarate reductase, distinct from succinate dehydrogenase. A mutant lacking the reductase had lost the ability to grow anaerobically on glycerol plus fumarate but could still grow aerobically on succinate (12) It thus appears that, while the tricarboxylate pathway serves in a cyclic manner under aerobic conditions, in anaerobic growth it is modified to provide a branched, noncyclic pathway. Oxaloacetate +2 (H) d Acetyl-CoA Malate Citrate --Hz0 -1 4 Fumsrate Isocitrate +2 (H) 1 1 --2 (H) -co* Succinate