Metabolic pathway analysis of the methylcitrate cycle in bacteria and fungi identifies methylcitrate synthase as an antiinfective drug target

The tricarboxylic acid (TCA) cycle is well known as a crucial pathway in central metabolism in many organisms. A less known analogous pathway is the methylcitrate cycle (MCC). It is present in various fungi such as <i>Aspergillus</i> species and bacteria such as <i>Escherichia coli</i>, with some of them being pathogenic. The MCC catalyzes an alpha-oxidation of propionyl-CoA to pyruvate and is of interest in view of biotechnology and pharmacology. To elucidate the potential interaction of the MCC with other central metabolic pathways, we investigated the MCC by Elementary-flux-mode analysis. We first established a reaction network model, using information from both the KEGG database and literature. This reaction network contains enzymes of the MCC as well as of the TCA cycle, glyoxylate shunt, and carbon source-utilizing pathways, such as amino acid degradation. The network was then used to calculate the elementary flux modes (EFMs) by using the simulation software Metatool 4.3. We identified 76 EFMs, with 39 of them containing the MCC. In this way, some previously known pathways were confirmed theoretically and, additionally, some new EFMs were discovered. Among these, a different, but shorter version of the MCC was identified. The EFMs were systematically analyzed with respect to their ATP yield and the robustness of the network was computed. Predictions on the impact of enzyme deletion or inhibition on the network were made. From these analyses and based on the absence of the MCC in humans, we conclude that the methylcitrate synthase represents a promising drug target against various human pathogens.