The effect of pfl gene knockout on the metabolism for optically pure d-lactate production by Escherichia coli

Abstract

The effect of gene knockout on metabolism in the pflA⁻, pflB⁻, pflC⁻, and pflD⁻ mutants of Escherichia coli was investigated. Batch cultivations of the pfl ⁻ mutants and their parent strain were conducted using glucose as a carbon source. It was found that pflA⁻ and pflB⁻ mutants, but not pflC⁻ and pflD⁻ mutants, produced large amounts of d-lactate from glucose under the microaerobic condition, and the maximum yield was 73%. In order to investigate the metabolic regulation mechanism, we measured enzyme activities for the following eight enzymes: glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), pyruvate kinase, lactate dehydrogenase (LDH), phosphoenolpyruvate carboxylase, acetate kinase, and alcohol dehydrogenase. Intracellular metabolite concentrations of glucose 6-phosphate, fructose 1,6-bisphosphate, phosphoenolpyruvate, pyruvate, acetyl coenzyme A as well as ATP, ADP, AMP, NADH, and NAD⁺ were also measured. It was shown that the GAPDH and LDH activities were considerably higher in pflA⁻ and pflB⁻ mutants, which implies coupling between NADH production and consumption between the two corresponding reactions. The urgent energy requirement was shown by the lower ATP/AMP level due to both oxygen limitation and pfl gene knockout, which promoted significant stepping-up of glycolysis when using glucose as a carbon source. It was shown that the demand for energy is more important than intracellular redox balance, thus excess NADH produced through GAPDH resulted in a significantly higher intracellular NADH/NAD⁺ ratio in pfl ⁻ mutants. Consequently, the homolactate production was achieved to meet the requirements of the redox balance and the energy production through glycolysis. The effect of using different carbon sources such as gluconate, pyruvate, fructose, and glycerol was investigated.