Cometabolism of a Nongrowth Substrate: l-Serine Utilization by Corynebacterium glutamicum

摘要

Despite its key position in central metabolism, l-serine does not support the growth of Corynebacterium glutamicum. Nevertheless, during growth on glucose, l-serine is consumed at rates up to 19.4 ± 4.0 nmol min?1 (mg [dry weight])?1, resulting in the complete consumption of 100 mM l-serine in the presence of 100 mM glucose and an increased growth yield of about 20%. Use of 13C-labeled l-serine and analysis of cellularly derived metabolites by nuclear magnetic resonance spectroscopy revealed that the carbon skeleton of l-serine is mainly converted to pyruvate-derived metabolites such as l-alanine. The sdaA gene was identified in the genome of C. glutamicum, and overexpression of sdaA resulted in (i) functional l-serine dehydratase (l-SerDH) activity, and therefore conversion of l-serine to pyruvate, and (ii) growth of the recombinant strain on l-serine as the single substrate. In contrast, deletion of sdaA decreased the l-serine cometabolism rate with glucose by 47% but still resulted in degradation of l-serine to pyruvate. Cystathionine β-lyase was additionally found to convert l-serine to pyruvate, and the respective metC gene was induced 2.4-fold under high internal l-serine concentrations. Upon sdaA overexpression, the growth rate on glucose is reduced 36% from that of the wild type, illustrating that even with glucose as a single substrate, intracellular l-serine conversion to pyruvate might occur, although probably the weak affinity of l-SerDH (apparent Km, 11 mM) prevents substantial l-serine degradation.