L-valine production with pyruvate dehydrogenase complex-deficient Corynebacterium glutamicum

Corynebacterium glutamicum was engineered for the production of L-valine from glucose by deletion of the aceE gene encoding the E1p enzyme of the pyruvate dehydrogenase complex and additional overexpression of the ilvBNCE genes encoding the L-valine biosynthetic enzymes acetohydroxyacid synthase, isomeroreductase, and transaminase B. In the absence of cellular growth, C. glutamicum DeltaaceE showed a relatively high intracellular concentration of pyruvate (25.9 mM) and produced significant amounts of pyruvate, L-alanine, and L-valine from glucose as the sole carbon source. Lactate or acetate was not formed. Plasmid-bound overexpression of ilvBNCE in C. glutamicum DeltaaceE resulted in an approximately 10-fold-lower intracellular pyruvate concentration (2.3 mM) and a shift of the extracellular product pattern from pyruvate and L-alanine towards L-valine. In fed-batch fermentations at high cell densities and an excess of glucose, C. glutamicum DeltaaceE(pJC4ilvBNCE) produced up to 210 mM L-valine with a volumetric productivity of 10.0 mM h(-1) (1.17 g l(-1) h(-1)) and a maximum yield of about 0.6 mol per mol (0.4 g per g) of glucose.

E1 enzyme of the pyruvate dehydrogenase complex in Corynebacterium glutamicum: molecular analysis of the gene and phylogenetic aspects

The E1p enzyme is an essential part of the pyruvate dehydrogenase complex (PDHC) and catalyzes the oxidative decarboxylation of pyruvate with concomitant acetylation of the E2p enzyme within the complex. We analyzed the Corynebacterium glutamicum aceE gene, encoding the E1p enzyme, and constructed and characterized an E1p-deficient mutant. Sequence analysis of the C. glutamicum aceE gene and adjacent regions revealed that aceE is not flanked by genes encoding other enzymes of the PDHC. Transcriptional analysis revealed that aceE from C. glutamicum is monocistronic and that its transcription is initiated 121 nucleotides upstream of the translational start site. Inactivation of the chromosomal aceE gene led to the inability to grow on glucose and to the absence of PDHC and E1p activities, indicating that only a single E1p enzyme is present in C. glutamicum and that the PDHC is essential for the growth of this organism on carbohydrate substrates. Surprisingly, the E1p enzyme of C. glutamicum showed up to 51% identity to homodimeric E1p proteins from gram-negative bacteria but no similarity to E1 alpha- or beta-subunits of heterotetrameric E1p enzymes which are generally assumed to be typical for gram-positives. To investigate the distribution of E1p enzymes in bacteria, we compiled and analyzed the phylogeny of 46 homodimeric E1p proteins and of 58 alpha-subunits of heterotetrameric E1p proteins deposited in public databases. The results revealed that the distribution of homodimeric and heterotetrameric E1p subunits in bacteria is not in accordance with the rRNA-based phylogeny of bacteria and is more heterogeneous than previously assumed.

Feedback-resistant acetohydroxy acid synthase increases valine production in Corynebacterium glutamicum

Acetohydroxy acid synthase (AHAS), which catalyzes the key reactions in the biosynthesis pathways of branched-chain amino acids (valine, isoleucine, and leucine), is regulated by the end products of these pathways. The whole Corynebacterium glutamicum ilvBNC operon, coding for acetohydroxy acid synthase (ilvBN) and aceto hydroxy acid isomeroreductase (ilvC), was cloned in the newly constructed Escherichia coli-C. glutamicum shuttle vector pECKA (5.4 kb, Km(r)). By using site-directed mutagenesis, one to three amino acid alterations (mutations M8, M11, and M13) were introduced into the small (regulatory) AHAS subunit encoded by ilvN. The activity of AHAS and its inhibition by valine, isoleucine, and leucine were measured in strains carrying the ilvBNC operon with mutations on the plasmid or the ilvNM13 mutation within the chromosome. The enzyme containing the M13 mutation was feedback resistant to all three amino acids. Different combinations of branched-chain amino acids did not inhibit wild-type AHAS to a greater extent than was measured in the presence of 5 mM valine alone (about 57%). We infer from these results that there is a single binding (allosteric) site for all three amino acids in the enzyme molecule. The strains carrying the ilvNM13 mutation in the chromosome produced more valine than their wild-type counterparts. The plasmid-free C. glutamicum DeltailvA DeltapanB ilvNM13 strain formed 90 mM valine within 48 h of cultivation in minimal medium. The same strain harboring the plasmid pECKAilvBNC produced as much as 130 mM valine under the same conditions.