5.1. Characteristics of lysine residues sensitive to chemical acetylation
As previously mentioned, in mitochondria, non-enzymatic acetylation is favored by its physicochemical conditions, where the pH value affects the protonation state of the lysine side chain; at basic pH, lysine is deprotonated, which increases its nucleophilicity (Figure 4). However, the cytoplasmatic pH is maintained at values close to neutrality in bacteria, e.g., in neutrophilic bacteria, the cytoplasmic pH is in a narrow range of ~7.5–7.7, alkaliphilic bacteria maintain a constant internal basic pH value of 7.5–8.3, and the pH of acidophilic bacteria is close to 6.5 (Krulwich et al., 2011; Slonczewski et al., 2009). Under these conditions, lysine is not reactive since it has a high pKa, and the epsilon amino groups of most lysine side chains are protonated.
The analysis of different chemically acetylated proteins has shown that non-enzymatic acetylation in these organisms depends on the microenvironment of the protein that influences lysine reactivity toward AcCoA or AcP (Figure 4). The process preferentially occurs on lysine flanked by residues with positive charges (Lys or Arg), hydroxyls (Ser, Thr, or Tyr), or amides (Gln or Asn), which coordinate the AcP binding through ionic and hydrogen bonds (Hebert et al., 2013; Kuhn et al., 2014; Wagner and Payne, 2013). Also, a tendency of negatively charged glutamate (E) and/or aspartate (D) residues near the -1 or +1 position to an acetylated lysine, which reduces the pKa of lysine and enhancement its reactivity, has been observed (Christensen et al., 2019; Kuhn et al., 2014; Post et al., 2017).