Which of the following is NOT considered a covalent modification?

Covalent modifications are chemical changes to a molecule that involve the forming or breaking of covalent bonds. These modifications often play crucial roles in regulating protein function, interactions, and stability.

Phosphorylation, ubiquitination, and glycosylation are all examples of covalent modifications. Phosphorylation involves the addition of a phosphate group to an amino acid residue in a protein, which can alter the protein's activity or interaction with other molecules. Ubiquitination refers to the attachment of ubiquitin, a small protein, to target proteins for degradation or regulation of their activity. Glycosylation involves the addition of sugar moieties to proteins, which can affect their stability, localization, and interactions.

In contrast, hydrophobic interactions are not covalent modifications; they are non-covalent forces that occur due to the tendency of non-polar molecules to avoid interaction with water. These interactions play an important role in the folding and stability of proteins but do not involve the formation of covalent bonds. Hence, hydrophobic interactions are fundamentally different from the other processes listed.