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.