Kihira C, Hayashi Y, Azuma N, Noda S, Maeda S, Fukiya S, Wada M, Matsushita K, Yokota A
The effects of reduced efficiency of proton-motive force (pmf) generation on glucose metabolism were investigated in Escherichia coli respiratory-chain mutants. The respiratory chain of E. coli consists of two NADH dehydrogenases and three terminal oxidases, all with different abilities to generate a pmf. The genes for isozymes with the highest pmf-generating capacity (NADH dehydrogenase-1 and cytochrome bo₃ oxidase) were knocked out singly or in combination, using a wild-type strain as the parent. Analyses of glucose metabolism by jar-fermentation revealed that the glucose consumption rate per cell increased with decreasing efficiency of pmf generation, as determined from the growth parameters of the mutants. The highest rate of glucose metabolism was observed in the double mutant, and the lowest was observed in the wild-type strain. The respiration rates of the single-knockout mutants were comparable to that of the wild-type strain, and that of the double mutant was higher, apparently as a result of the upregulation of the remaining respiratory chain enzymes. All of the strains excreted 2-oxoglutaric acid as a product of glucose metabolism. Additionally, all of the mutants excreted pyruvic acid and/or acetic acid. Interestingly, the double mutant excreted L-glutamic acid. Alterations of the fermentation profiles provide clues regarding the metabolic regulation in each mutant.