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Covalent modification is a process in which amino acids of protein residues are modified in the presence of enzymes.The reaction involves adding or eliminating certain chemical species from the amino acid.

Some of the examples of covalent modifications include:

• Phosphorylation:Phosphorylation is the most common modification to convert -OH group attached amino acids to an amino acid derivative. The enzyme protein kinases catalyze the phosphoryl attached amino acid and convert it to specific amino acid residue. An example is O-Phosphoserine is derived through phosphorylation by a serine amino acid.
• Acetylation: The acetylase enzyme carries out the reaction in which the acetyl or ethanoyl group is added to synthesize the amino acid derivative. Acetylation is usually done on amino acid lysine.
• Prenylation: As in other cases, this modification reaction is also carried with enzyme geranyl transferase, in which the addition of hydrocarbon side chain is done in cysteine amino acid.
• Methylation: DNA methylation is an important example of a covalent modification within the 5th position of the cytosine ring. It doesn’t change gene structure but affects gene activity.