Difference Between Acetylation and Methylation

Main Difference – Acetylation vs Methylation

Acetylation and methylation are two types of chemical reactions. Both of them are used to add a chemical group to a chemical compound. Acetylation is the process of adding an acetyl group, forming an acetylated compound. Methylation is the process of adding a methyl group, forming a methylated compound. These reactions are very important in organic synthesis. Both these reactions can be found in biological systems as well. The main difference between acetylation and methylation is that acetylation introduces an acetyl group to a chemical compound as a functional group whereas methylation introduces a methyl group to a chemical compound.

Key Areas Covered

1. What is Acetylation
      – Definition, Reaction Mechanism
2. What is Methylation
      – Definition, Reaction Mechanism
3. What is the Difference Between Acetylation and Methylation
      – Comparison of Key Differences

Key Terms: Acetylation, Acetyl Group, Alkylation, Methylation, Methyl Group, N-terminal Acetylation, Protein Acetylation

What is Acetylation

Acetylation is the process of introducing an acetyl group to a molecule. Acetyl group has the chemical formula CH₃CO. It is represented as Ac. This acetyl group is attached to a compound through a replacement or a substitution reaction.

The substitution of an acetyl group occurs through an active hydrogen atom. An active hydrogen atom is a hydrogen atom that can be easily removed from a molecule. For example, -OH groups are composed of a reactive hydrogen atom attached to an oxygen atom. By substituting this hydrogen atom with an acetyl group, an ester can be produced. This ester is named as acetate.

Figure 1: Protein Acetylation

Acetylation is more common in protein molecules. Here, it is called protein acetylation. N-terminal acetylation is the most common protein acetylation method. This acetylation reaction is catalyzed by enzymes. N-terminal is one of the two ends of a polypeptide chain where the ammonium group is located. In this ammonium group, there are active hydrogen atoms attached to the nitrogen atom. One of these hydrogen atoms can be substituted with an acetyl group.

What is Methylation

Methylation is the process of adding a methyl group to a molecule. Here, a reactive atom is substituted with a methyl group. It is a form of alkylation. Alkylation is the substitution of an alkyl group into a molecule. When alkylation is done with a methyl group, it is called methylation.

Methylation can be done in an electrophilic mechanism or a nucleophilic mechanism. The most common method is electrophilic methylation. Although nucleophilic methylation is not that much common, it is used for the Grignard reaction. Here, a methyl group attached with either a Li⁺ ion or a Mg⁺² ion is used as the Grignard reagent. Then this Grignard reagent delivers this methyl group into an aldehyde or a ketone. This is an electrophilic addition reaction.

Figure 2: Grignard Reaction for Methylation of a Ketone Molecule

In addition, methylation can take place as biological methylation. Here, the DNA or protein can be methylated. In DNA methylation, a methyl group is attached to a nitrogenous base of the DNA. In protein methylation, some amino acids in a polypeptide chain can get methylated.

Difference Between Acetylation and Methylation

Definition

Acetylation: Acetylation is the process of introducing an acetyl group to a molecule.

Methylation: Methylation is the process of adding a methyl group to a molecule.

Functional Group

Acetylation: Acetylation attaches an acetyl group as a functional group to a molecule.

Methylation: Methylation attaches a methyl group as a functional group to a molecule.

Mechanism

Acetylation: Acetylation is done as a substitution reaction.

Methylation: Methylation can be done as either a substitution reaction or an addition reaction.

Applications

Acetylation: Protein acetylation is a common acetylation method.

Methylation: DNA methylation and protein methylation are common biological applications.

Conclusion

Acetylation and methylation are two chemical reactions that are used as organic synthesis reactions. Both acetylation and methylation are found in chemical applications as well as in biological systems. The main difference between acetylation and methylation is that acetylation introduces an acetyl group to a chemical compound as a functional group whereas methylation introduces a methyl group to a chemical compound.

References:

1. “Acetylation.” Wikipedia, Wikimedia Foundation, 2 Sept. 2017, Available here. Accessed 8 Sept. 2017.
2. Gladwin, Rahul. “Methylation.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 11 July 2016, Available here. Accessed 8 Sept. 2017. 

Image Courtesy:

1. “Protein-Acetylation Figure2” By Ybs.Umich – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “MeLi on acetone” (Public Domain) via Commons Wikimedia