2. Non-enzymatic acetylation
Acetyl-CoA is synthesized by different reactions: oxidative decarboxylation of pyruvate during glycolysis; catabolism of some amino acids (isoleucine, leucine, and threonine); and β-oxidation of fatty acids. Several lysine acetyltransferases use this molecule to acetylate proteins (previously mentioned) and can modify the amino group of lysine side chains non-enzymatically. The chemical acetylation was first mentioned in histone proteins by Phillips in 1963. A few years later, it was demonstrated that purified histones, albumin, and synthetic lysine homopolymer are acetylated in vitro at a pH ~9 and in the presence of acetyl-CoA without additional enzymes (Paik et al., 1970). In eukaryotic cells, specifically in the mitochondria, the absence of acetyltransferases and the unique conditions, such as the high concentration of acetyl-CoA (1.5 mM steady-state) and alkaline pH (7.9–8.0), suggests that lysine acetylation occurs chemically. For example, many mitochondrial proteins are acetylated under conditions mimicking those of the mitochondria matrix in a mechanism that is generally pH- and acyl-CoA concentration-dependent (Wagner & Payne, 2013). The proposed mechanism for mitochondrial non-enzymatic acetylation appears to occur by a nucleophilic attack on the acetyl carbonyl by the side-chain amine of a lysine residue (Wagner & Payne, 2013). However, under the physiological characteristic of the mitochondrial matrix, the reaction would be slow, so it has been proposed that protein acetylation occurs via S-acetylation cysteine thiol with the subsequent transfer of the acetyl group to a nearby deprotonated lysine residue (Christensen et al., 2019; James et al., 2017).
Direct acetylation has also been observed in bacteria, where at elevated concentrations, acetyl phosphate (AcP) can non-enzymatically acetylate lysine residues on many proteins. AcP is a highly reactive molecule susceptible to hydrolyzing or acetylating without enzymes. It undergoes rapid hydrolysis under basic or acid conditions and in the presence of some ions (Mg2+ or Ca2+). However, in the presence of nucleophilic reagents such as thiols, hydroxyl groups, ε-amino or α-amino groups of lysine side chains or proteins, the chemical acetylation of these groups is favored (Kuhn et al., 2014).