Pathway Analysis and Metabolic Engineering in Corynebacterium glutamicum

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Abstract
The gram-positive bacterium Corynebacterium glutamicum is used for the industrial production of amino acids, e.g. of L-glutamate and L-lysine. During the last 15 years, genetic engineering and amplification of genes have become fascinating methods for studying metabolic pathways in greater detail and for the construction of strains with the desired genotypes. In order to obtain a better understanding of the central metabolism and to quantify the in vivo fluxes in C. glutamicum, the [13C]-labelling technique was combined with metabolite balancing to achieve a unifying comprehensive pathway analysis. These methods can determine the flux distribution at the branch point between glycolysis and the pentose phosphate pathway. The in vivo fluxes in the oxidative part of the pentose phosphate pathway calculated on the basis of intracellular metabolite concentrations and the kinetic constants of the purified glucose-6-phosphate and 6-phosphogluconate dehydrogenases determined in vitro were in full accordance with the fluxes measured by the [13C]-labelling technique. These data indicate that the oxidative pentose phosphate pathway in C. glutamicum is mainly regulated by the ratio of NADPH/NADP concentrations and the specific activity of glucose-6-phosphate dehydrogenase. The carbon flux via the oxidative pentose phosphate pathway correlated with the NADPH demand for L-lysine synthesis. Although it has generally been accepted that phosphoenolpyruvate carboxylase fulfills a main anaplerotic function in C. glutamicum, we recently detected that a biotin-dependent pyruvate carboxylase exists as a further anaplerotic enzyme in this bacterium. In addition to the activities of these two carboxylases three enzymes catalysing the decarboxylation of the C4 metabolites oxaloacetate or malate are also present in this bacterium. The individual flux rates at this complex anaplerotic node were investigated by using [13C]-labelled substrates. The results indicate that both carboxylation and decarboxylation occur simultaneously in C. glutamicum so that a high cyclic flux of oxaloacetate via phosphoenolpyruvate to pyruvate was found. Furthermore, we detected that in C. glutamicum two biosynthetic pathways exist for the synthesis of DL-diaminopimelate and L-lysine. As shown by NMR spectroscopy the relative use of both pathways in vivo is dependent on the ammonium concentration in the culture medium. Mutants defective in one pathway are still able to synthesise enough L-lysine for growth, but the L-lysine yields with overproducers were reduced. The luxury of having these two pathways gives C. glutamicum an increased flexibility in response to changing environmental conditions and is also related to the essential need for DL-diaminopimelate as a building block for the synthesis of the murein sacculus.