4.1.3. Regulation of metabolic flux
Most microorganisms have developed different strategies to co-metabolize a mixture of simple carbohydrates, favoring the utilization of glucose as a carbon and energy source to sustain a higher growth rate. The metabolic plasticity allows them to obtain efficiently specific carbon sources and survive in competitive environments (Vinuselvi et al., 2012). Most bacteria regulate their metabolism via carbon catabolite repression (CCR), which involves a complex interplay between metabolism, signaling by proteins and metabolites, and the regulation of gene expression (Kremling et al., 2015). Other mechanisms can be used to modulate carbon flux at critical metabolic nodes without this regulatory control.
Post-translational modifications affect flux distribution between important metabolic branches, such as glycolysis and gluconeogenesis, TCA cycle and glyoxylate shunt, and glycolysis and TCA cycle. Wang et al. (2010) demonstrated that carbon source-associated acetylation modulates metabolic flux profiles in S. enterica . In the presence of glucose, acetylation increases the glycolysis/gluconeogenesis flux ratio 2.07-fold, while acetylation reduces the glyoxylate bypass/TCA flux ratio under a citrate-based carbon source.
The isocitrate node is an important regulation point of carbon flux between the TCA cycle and the glyoxylate shunt. Isocitrate, the substrate of isocitrate dehydrogenase (ICDH) and isocitrate lyase (AceA), is converted to α-ketoglutarate by ICDH or is cleaved to succinate and glyoxylate by AceA, directing the carbon source flow to TCA cycle or to glyoxylate shunt, respectively. In Mycobacterium tuberculosis, this metabolic node is regulated by the acetylation of the ICDH. Acetylation suppresses enzyme activity in the presence of fatty acids reducing carbon flow into the TCA cycle (Lee et al., 2017). The activation of glyoxylate bypass allows the conversion of acetyl-CoA to the metabolic intermediate succinate to support the growth in the presence of non-carbohydrates substrates such as fatty acids or acetate (Cronan & Laporte 2005; Lee et al., 2017). For S. enterica, it has been reported that the node is controlled by modulating the activity of the bifunctional isocitrate dehydrogenase phosphatase/kinase (AceK) (Wang et al., 2010). However, in the analysis of E. coliproteome, acetylation of AceK was not detected, and not change in the metabolic fluxes was quantified (Castaño-Cerezo et al., 2014). Acetylation affects, the activity of isocitrate lyase (AceA) in this bacterium. Acetylation led to a decrease in Ace-A specific activity, and with the proteomic data, several acetylation sites were detected in the protein (Castaño-Cerezo et al., 2014).
Recently it was demonstrated that glyceraldehyde 3-phosphate dehydrogenase (GapA) and 2,3-bisphosphoglycerate-dependent phosphoglycerate mutase (GpmA) were sensitive to non-enzymatic acetylation in vitro at physiological AcP concentrations. In both enzymes, acetylation reduced their activity, which could be reflected in reduced glycolytic/gluconeogenic flux in conditions with higher concentrations of AcP (Schastnaya et al., 2023).
The changing flux from glucose to glutamate is increased when the cell excretes glutamate. Factors like the depletion of biotin and the addition of detergents or antibiotics trigger glutamate overproduction and, therefore, a change in the flux of central carbon metabolism to favor glutamate production (Shirai et al., 2007). It has been proposed that in addition to the decrease in 2-oxoglutarate dehydrogenase complex (ODHC) activity, the regulation of phosphoenolpyruvate carboxylase (PEPC) activity by acetylation may be a mechanism involved in the change in metabolic flux during overproduction of glutamate. PEPC catalyzes the irreversible carboxylation of phosphoenolpyruvate to generate oxaloacetate, ensuring that the carbon flow is directed toward glutamate production via the Krebs cycle. Acetylation at the K653 site regulates enzyme activity and, therefore, the mechanism that maintains metabolic flux under glutamate-producing conditions (Mizuno et al., 2016; Nagano‐Shoji et al., 2017).
Hence, acetylation may provide a new strategy for regulating protein activity and improving the utilization of different carbon sources.