Which chemical change is considered a covalent modification of proteins?

Covalent modifications of proteins involve the formation or breaking of covalent bonds within the protein structure, resulting in a permanent change to the protein’s chemical composition. Phosphorylation is a quintessential example of this process, where a phosphate group is added to an amino acid residue, typically serine, threonine, or tyrosine, by a kinase enzyme. This modification can significantly alter the protein's function, activity, or interaction with other molecules.

Phosphorylation can affect protein conformation, activity, and signaling pathways, playing a critical role in cellular processes such as cell growth, apoptosis, and metabolism. Since it involves the addition of a chemically distinct group and alters the protein’s behavior, it exemplifies how covalent modifications can regulate protein function in a dynamic and reversible manner.

In contrast, hydrolysis involves the breakdown of molecules through the addition of water and does not create new covalent bonds; oxidation and reduction are forms of redox reactions that do not involve the addition of functional groups to proteins in the same manner as phosphorylation. Therefore, phosphorylation stands out as the clear choice for a covalent modification of proteins.