Impact of Post-Translational Modification on Serine Interactions

Consider a hypothetical protein consisting of a linear chain of amino acids, which folds into a three-dimensional conformation crucial for its biological function. The protein includes the amino acids Alanine, Serine, and Glutamic acid. During a study, the modification of a single amino acid residue through post-translational modification was observed, significantly impacting the protein's functionality and its interaction with substrate molecules. Understanding the specific roles of these amino acids in the protein's structure and function is essential for elucidating the mechanisms of enzyme activity and substrate specificity.

Which of the following modifications is most likely to enhance the interaction of Serine with a phosphate group, thus affecting the protein's function in a phosphorylation signaling pathway?