Corynebacterium glutamicum - JUWEL - Forschungszentrum Jülich

5. L-Valine Production Process Development
the expression of the genes ilvBNC by an attenuation control mechanism (Morbach et al.,
2000). This is, however, probably of very limited importance in the used strain due to
the overexpression of these genes on the plasmid, which led to an approximately 15 times
increase in the enzyme activities (Cordes et al., 1992; Radmacher, 2001). Avoiding high
concentrations of L-isoleucine is also expected to be beneficial, since high concentrations
of the branched-chain amino acids have been shown to decrease the activity of the key
enzyme of branched chain amino acid synthesis: acetohydroxyacid synthase (AHAS, see
figure 5.2). This fact is, however, also expected to be of little importance in the final
production experiments, since this enzyme is also inhibited by the other branched chain
amino acids amongst which especially L-valine. This inhibition was shown to be maximal
about 50% (Eggeling et al., 1987). Furthermore, a recent study with the used strain
has indicated that the positive effect of L-isoleucine limitation on the biomass specific
AHAS activity was stronger than the inhibiting effect of high L-valine concentrations
(Lange et al., 2003).
The main aim of the deletion of the genes panBC, which are needed for the production
of D-pantothenate from ketoisovalerate, was to increase the availability of pyruvate as
the precursor of L-valine. Lack of D-pantothenate leads to a decrease in the production of
coenzyme A and a corresponding decrease in the pyruvate dehydrogenase activity. The
increased availability of pyruvate upon decreased pantothenic acid concentrations was
also shown in a L-lysine overproducing strain, where the reduced concentrations however
led to a decrease in L-valine and L-isoleucine and an increase in L-lysine production
(An et al., 1999). A second beneficial effect of the deletion of panBC is the increased
availability of ketoisovalerate for synthesis of L-valine when less is used for production
of pantothenate. Due to this deletion the strain is auxotrophic for D-pantothenic acid.
The overexpression of the genes ilvBNCD on a plasmid increases the flux from pyruvate
to ketoisovalerate and L-valine.
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