At neutral pH, the ε-amino group is positively charged. This residue is deprotonated by a base (not shown in figure). (I) The lysine ε-amino group can now act as a nucleophile to attack the electrophilic carbonyl carbon of acetyl-CoA, (II) forming a tetrahedral intermediate. The electron dense oxygen expels the thiolate group (-SCoA). (III) Deprotonation of the amino group (IV) results in an acetylated lysine side chain and CoASH. Figure adapted from (Walsh, 2006).

Glycolysis (blue) metabolizes glucose to acetyl coenzyme A (acetyl-CoA) in an NAD⁺-dependent manner. GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PDH, pyruvate dehydrogenase. Acetate dissimilation requires the Pta-AckA pathway (red). The enzyme PTA (phosphotransacetylase) converts acetyl-CoA and inorganic phosphate (P_i) into coenzyme A (CoA) and the high-energy pathway intermediate acetyl phosphate (acetyl-P). The enzyme ACKA (acetate kinase) converts acetyl-P and ADP to acetate and ATP. The acetate freely diffuses across the cell envelope into the environment. Acetate assimilation (green) requires the high-affinity enzyme ACS (acetyl-CoA synthetase). In a two-step process that involves an enzyme-bound intermediate (acetyl-AMP), Acs converts acetate, ATP, and CoA into AMP, pyrophosphate (PP_i), and acetyl-CoA. The acetyl-CoA replenishes the NAD⁺-dependent tricarboxylic acid (TCA) cycle (orange). Mammalian ATP-citrate lyase (ACL) uses ATP and CoA to convert the TCA cycle intermediate citrate into oxaloacetate (OAA), ADP, P_i, and acetyl-CoA (purple). Figure adapted from (Wolfe, 2008).

The activity of ACS is regulated by reversible acetylation. In a form of feedback inhibition, a GNAT (either Pat or AcuA depending on the organism) uses acetyl-CoA as its acetyl donor to acetylate ACS (^AcACS), which is inactive. When the glycolytic carbon source is depleted, NAD⁺ is recycled (not shown). An NAD⁺-dependent sirtuin (CobB or SrtN, depending on the organism) can now deacetylate ACS, generating the by-product 2'-O-acetyl-ADP-ribose (2'-OAADPr). In B.subtilis, the class III KDAC AcuC also can catalyze deacetylation in an NAD⁺-independent manner (not shown). Deacetylated, and thus reactivated, ACS can now synthesize acetyl-CoA. The activity of CheY is also regulated by reversible acetylation. In this case, acetylation occurs by autoacetylation (auto), ACS, or some unknown acetyltransferase (AT). CobB deacetylates acetyl-CheY (^AcCheY).