5. ACP is the donor molecule in chemical acetylation
Many studies on protein acetylation in bacteria have focused on describing enzymatic acetylation, its implication in cellular physiology, pathogenesis, in bacterial response to environmental conditions, etc. However, evidence shows that non-enzymatic acetylation is also possible (Table 2).
The acetylation by AcP has been mainly described by proteomic studies, in which a comparative analysis of the acetylome of different strains allows us to determine if there is any difference in the protein acetylation levels and infer the acetylation mechanism. This has been achieved in E. coli, N. gonorrhoeae and Borrelia burgdorferi (Table 2) (Bontemps-Gallo et al., 2018; Kosono et al., 2015; Kuhn et al., 2014; Post et al., 2017; Reverdy et al., 2018; Schilling et al., 2015; Weinert et al., 2013). These studies have focused on comparing the proteomic data of the acetylated proteins of the wild-type strain with different isogenic mutant strains. The acetylome data using Escherichia coli as a model demonstrated that acetylation depends on acetyl-phosphate (AcP) formation and occurs at a low level in growth-arrested cells. Mutant cells unable to synthesize (pta ackA  mutant) or metabolize (ackA  mutant) AcP had the opposite behavior, while in the first one, significantly reduced acetylation levels were observed, and the accumulation of AcP significantly elevated acetylation levels. Also, the authors demonstrated that the AcP acetylate lysine residues in vitro at a concentration comparable to that found in vivo . These data establish AcP as a critical regulator of acetylation and suggest that AcP acts nonenzymatically to regulate acetylation levels in response to glucose (Table 2) (Kuhn et al., 2014; Schilling et al., 2015; Weinert et al., 2013).
Since AcCoA and AcP are derived from multiple metabolic pathways in other microorganisms, it is difficult to establish whether both or only one of these molecules is the acetyl group donor. The bacterium B. burgdorferi is characterized by producing AcCoA and AcP from a unique metabolic pathway, the acetate/mevalonate pathway. In this pathway, acetate is converted to Ac-P by acetate kinase (AckA), which is metabolized to acetyl-CoA by phosphotransacetylase (Pta) (Bontemps-Gallo et al., 2018; Richards et al., 2015). The acetylome analysis of different mutant strains and their respective complements showed that no acetylation is observed in the strain that does not synthesize either AcP nor AcCoA (ΔackA mutant). In the ackA complemented strain, an increase in acetylation was detected, and the Δpta complemented strain displayed similar levels of acetylation as the wild-type. Remarkably, hyper-lysine acetylation levels were detected in the Δpta due to the AcP accumulation. Together these results demonstrated that this molecule is the primary source of acetylation (Bontemps-Gallo et al., 2018) (Table 2).
To establish the metabolic processes that AcP acetylation regulates, the modified proteins can be analyzed with different software (PANTHER, DAVID, ERGO, and KEEG). From the proteomic data of E. coli, the functional analysis reveals that the elongation factors, most of the ribosomal subunits and aminoacyl-tRNA ligases, are acetylated in an AcP-dependent manner (Christensen et al., 2019; Kuhn et al., 2014; Post et al., 2017). In Borrelia burgdorferi , the acetylated proteins were involved in genetic information, metabolism and transport, protein folding and degradation, detoxification, motility, and chemotaxis (Bontemps-Gallo et al., 2018). Similarly, some metabolic pathways related to carbohydrate metabolisms (glycolysis/gluconeogenesis, pentose phosphate pathway, pyruvate metabolism, and the TCA cycle), fatty acid metabolism, and pantothenate metabolism are sensitive to AcP-dependent acetylation (Kuhn et al., 2014; Schilling et al., 2015).