FIGURE 5.

Ac(et)ylation is a modification of molecules that is tightly connected to the cellular metabolic state. (A) Left panel: GNATs depend on ac(et)yl-CoA as donor molecule for the transfer of the ac(et)yl group to the substrate amino group. Shown is only acetyl-CoA but dependent on the specificity of the GNATs for different acyl-CoAs, also other acyl groups can be transferred. Acetyl-CoA is generated in the metabolism of all major nutrient classes. Dependent on the metabolic state, the intracellular concentration of ac(et)yl-CoA fluctuates. The catalytic efficiency of the GNATs depend on the intracellular acetyl-CoA/CoA ratio rather than the acetyl-CoA concentration. This is reflected by the similar K_M-values of GNATs for ac(et)yl-CoA and CoA. Moreover, several GNATs are regulated by binding of metabolic molecules such as cAMP, NADP⁺, acetyl-CoA and amino acids to accessory, allosteric sites. Binding of the ligands to the allosteric site modulates GNAT activity constituting another layer for regulation of GNAT activity. Further regulatory systems include post-translational modifications of GNATs, not shown here. Right panel: sirtuin activity is also tightly connected to the cellular metabolic state. Sirtuins use NAD⁺ as stoichiometric co-substrate for catalysis and other endogenous regulators such as c-di-GMP for CobB also exist. As described for GNATs, also sirtuins are regulated by post-translational modifications which are not shown here. (B) Acetyl-phosphate is the major factor for non-enzymatic acetylation in bacteria. Acetyl-CoA is formed during metabolism of all major nutrient classes. Shown is the formation of acetyl-CoA through glycolysis starting from glucose to form pyruvate, which is oxidatively decarboxylated to form acetyl-CoA by the pyruvate dehydrogenase complex (PDH). Acetyl-CoA can subsequently further metabolized either by oxidative phosphorylation under aerobic conditions to drive formation of ATP and regeneration of NAD⁺ or it is converted to acetate by phosphotransacetylase (Pta) catalyzing the formation of acetyl-phosphate and acetate kinase (AK) yielding acetate. Both reactions are reversible, i.e., AK and Pta can also convert acetate to acetyl-CoA under consumption of ATP. Alternatively, the acetate can be converted by acetyl-CoA synthetase (Acs) to form acetyl-CoA. Due to the production of pyrophosphate (PP_i) in the first half reaction yielding acetyl-adenylate (acetyl-AMP), this reaction is almost irreversible. Under conditions of carbon overflow, the intracellular acetate concentration (>5 mM) increases so that also the reaction catalyzed by AK and Pta to produce acetyl-CoA from acetate becomes important. This results in accumulation of acetyl-phosphate under conditions of carbon overflow causing systemic non-enzymatic acetylation.