One of the important chemical modifications that occur in histones is acetylation of lysine by an enzyme called histone acetyltransferase (HAT). The reverse of this reaction is deacetylation of lysine by an enzyme called histone deacetylase (HDAC). These reactions are depicted in the diagram and are important for regulating the state of chromatin. Lysine NH3+ CH2 COO- Acetylation by HAT CH3 Deacetylation by HDAC Based on the chemical modifications shown in the diagram, how are these modifications (acetylation versus deacetylation of lysine) likely to impact chromatin? Acetylation of lysine leads to stronger interactions between DNA and histones, which will increase the ionic bond formation between histones and the phosphate groups of DNA. Acetylation of lysine alters the ability of lysine to participate in ionic bonding. Acetylation is associated with increased accessibility for transcription and is generally prevalent in euchromatin rather than heterochromatin. Nucleosomes containing acetylated lysine.
The Correct Answer and Explanation is:
Acetylation of lysine residues in histones plays a crucial role in chromatin remodeling and gene regulation. The addition of an acetyl group (CH3CO) to the lysine side chain neutralizes its positive charge. Lysine normally contains an amino group (NH3+), which interacts with the negatively charged phosphate groups on the DNA backbone. When acetylated, the neutralized lysine is less able to form these ionic interactions, leading to a relaxation of the chromatin structure. This modification reduces the compaction of chromatin, allowing it to be more accessible for transcription factors and other regulatory proteins. Therefore, acetylation is generally associated with active gene expression and euchromatin, which is the less condensed form of chromatin.
Histone acetyltransferases (HATs) are the enzymes responsible for acetylating lysines on histones, facilitating a more open chromatin structure and allowing transcription to occur. In contrast, histone deacetylases (HDACs) remove these acetyl groups, restoring the positive charge on the lysine residue. This leads to stronger interactions between the histone and the DNA, promoting chromatin condensation and repressing gene expression. This process is often associated with heterochromatin, the more condensed, transcriptionally inactive form of chromatin.
In summary, acetylation of lysine residues weakens the interaction between histones and DNA, promoting a more open chromatin state that favors transcription. Deacetylation, on the other hand, strengthens these interactions, leading to chromatin condensation and transcriptional repression. Thus, acetylation and deacetylation of histones are key regulators of gene expression and chromatin dynamics.
