ABSTRACT
In industrial fermentation of amino acids the cells are often forced to synthesize the biochemicals excessive of their physiological needs. The knowledge of metabolic networks and their regulation relevant usually come from biochemical research, especially from enzymology, not from engineering study. To enrich the knowledge of metabolic sub-network of L-valine syntheses for higher production of L-valine, Corynebacterium glutamicum AS1.495 and its genetic derivatives AA361, AAT231, AATV341 were used for metabolic flux analysis. AS1.495 is a leucine auxotrophic (Leu-), and the three derivatives carry additional mutations. AA361 contains D-aspartic acid-beta-hydroxamate supersensitive marker (Leu-, L-AAHss), AAT231 (Leu-, L-AAHss, 2-TAr) is D-aspartic acid-beta-hydroxamate supersensitive and 2-thiazole alanine resistant, and AAT341 (Leu-, L-AAHss, 2-TAr, Vd-) is a D-aspartic acid-beta-hydroxamate supersensitive, 2-thiazole alanine resistant and valine-decompose-ability imperfect (Vd-). The concentrations of extra-cellular metabolites were determined under sub-steady-state of the batch culture. The metabolic flux distribution maps of the four strains were obtained, compared and analyzed. Our analysis showed that the flux ratio of EMP and HMP from the glucose-6-phosphate had increased from 0.205 in the parental strain AS1.495 to 0.321 in the multiple-mutation strain AATV341; the flux ratio of L-valine synthesis branch and the rest branches from the pyruvate node increased from 0.188 in AS1.495 to 3.29 in AATV341; the flux of lactic acid synthesis branch decreased from 11.1 in AS1.495 to 1.16 in AATV341; the flux of L-valine synthesis branch increased from 5.37 in AS1.495 to 37.3 in AATV341; and the productivity of L-valine correspondently increased from 4 g/L in AS1.495 to 24.5 g/L in AATV341. These results indicate that the introduction of analog supersensitive marker L-AAH55 and/or analog resistant marker 2-TAr skew the metabolic flux towards the formation of L-valine. This study revealed the usefulness of the metabolic flux analysis as a tool for verification of existing production strains. The analysis may play an important role in helping us b to rationally re-design metabolism for further improvement of fermentation process.