Practice question

Histone acetylation neutralizes positive charge on lysine residues in flexible N-terminal tails, weakening interaction with negatively charged DNA and favoring open euchromatin. Histone deacetylases reverse this activation mark by hydrolyzing the N-acetyl amide bond, regenerating unmodified lysine. Loss of acetyl restores electrostatic attraction, tightening wrap around octamer and recruiting repressive complexes such as Sin3, NuRD and CoREST. Humans possess 18 HDAC enzymes divided into Zn2+-dependent classes I, II, IV and NAD+-dependent sirtuins, targeting both histone and non-histone substrates for transcriptional control.