Pathway identification combining metabolic flux and functional genomics analyses: acetate and propionate activation by Corynebacterium glutamicum.

Corynebacterium glutamicum can utilize acetic acid and propionic acid for growth and amino acid production. Growth on acetate as sole carbon source requires acetate activation by acetate kinase (AK) and phosphotransacetylase (PTA), encoded in the pta-ack operon. Genetic and enzymatic studies showed that these enzymes also catalyze propionate activation and were required for growth on propionate as sole carbon source. However, when glucose was present as a co-substrate strain lacking the AK-PTA pathway was still able to utilize acetate or propionate for growth indicating that an alternative activation pathway exists. As shown by (13)C-labelling experiments, the carbon skeleton of acetate is conserved during activation to acetyl-CoA in this pathway. Metabolic flux analysis during growth on an acetate-glucose mixture revealed that in the absence of the AK-PTA pathway carbon fluxes in glycolysis, the tricarboxylic acid (TCA) cycle and anaplerosis via PEP carboxylase and/or pyruvate carboxylase were increased, while the glyoxylate cycle flux was decreased. DNA microarray experiments identified cg2840 as a constitutively and highly expressed gene putatively encoding a CoA transferase. Purified His-tagged Cg2840 protein was active as CoA transferase interconverting acetyl-, propionyl- and succinyl-moieties as CoA acceptors and donors. Strains lacking both the CoA transferase and the AK-PTA pathway could neither activate acetate nor propionate in the presence or absence of glucose. Thus, when these short-chain fatty acids are co-metabolized with other carbon sources, CoA transferase and the AK-PTA pathway constitute a redundant system for activation of acetate and propionate.