Microaerobic cultivation conditions have been shown experimentally and theoretically to improve the performance of a number of bioproduction systems. However, under these conditions, the production of l-valine by Escherichia coli is decreased mainly because of a redox cofactor imbalance and a decreased l-glutamate supply. The synthesis of one mole of l-valine from one mole of glucose generates two moles of NADH via glycolysis but consumes a total of two moles of NADPH, one in the ketol-acid reductoisomerase (KARI) reaction and the other in the regeneration of l-glutamate as an amino group donor for the branched-chain amino acid aminotransferase (BCAT) reaction. The improvement of l-valine synthesis under oxygen deprivation may be due to solving these problems. Increased l-valine synthesis under oxygen deprivation conditions was previously shown in Corynebacterium glutamicum (Hasegawa et al., 2012). In this study, we have proposed the use of NADH-dependent leucine dehydrogenase (LeuDH; EC 1.4.1.9) Bcd from B. subtilis instead of the native NADPH-dependent pathway including aminotransferase encoded by ilvE to improve l-valine production in E. coli under microaerobic conditions. We have created l-valine-producing strains on the base of the aminotransferase B-deficient strain V1 (B-7 ΔilvBN ΔilvIH ΔilvGME::PL -ilvBN N17K DA) by introducing one chromosomal copy of the bcd gene or the ilvE gene. Evaluation of the l-valine production by the obtained strains under microaerobic and aerobic conditions revealed that leucine dehydrogenase Bcd had a higher potential for l-valine production under microaerobic conditions. The Bcd-possessing strain exhibited 2.2-fold higher l-valine accumulation (up to 9.1 g/L) and 2.0-fold higher yield (up to 35.3%) under microaerobic conditions than the IlvE-possessing strain. The obtained results could be interpreted as follows: an altering of redox cofactor balance in the l-valine biosynthesis pathway increased the production and yield by E. coli cells under microaerobic conditions. Thus, the effective synthesis of l-valine by means of "valine fermentation" was shown in E. coli. This methodology has the advantages of being an economical and environmentally friendly process.
Keywords: Biochemistry; Genetics; Microbiology.