Pyruvate overflow and carbon flux within the central metabolic pathways of Corynebacterium glutamicum during growth on lactate

Abstract

Growth of Corynebacterium glutamicum was characterized kinetically for chemostat cultures using lactate as the sole carbon substrate, and the concentration profile of various enzymes of central metabolism was established over a range of growth rates. Pyruvate overflow, together with incomplete lactate consumption, was observed under conditions for which the specific activity of pyruvate dehydrogenase no longer increased as a function of the growth rate. Biomass yields at such regimes were close to those obtained under exponential growth in batch cultures and were significantly higher than those associated with low growth rates. This shift in carbon conversion efficiency was correlated with increased malic enzyme and pyruvate carboxylase activities, suggesting the operation of a modified tricarboxylic acid cycle involving malate partitioning between malic enzyme (in conjunction with pyruvate carboxylation to replenish the C4 pool) and malate dehydrogenase. This metabolic deviation would provide the NADPH 2 necessary to sustain the observed biomass yields in the absence of NADH:NADP transhydrogenase activity. An analysis of the energetic potential of such a metabolic network indicates that NADH oxidase plays an important role in modulating the respiratory chain efficiency so as to avoid excessive production of adenosine triphosphate. Carbon flux through the central metabolic pathways of C. glutamicum is clearly a dynamic phenomenon whose regulatory complexity needs to be taken into account for future strain improvement strategies aimed at exploiting this organism's natural capacity to overproduce amino acids.