The cell’s on/off switch—adding a phosphate group to a protein changes its shape and activates (or deactivates) it.Phosphorylation is the enzymatic addition of a phosphate group (PO₄³⁻) to serine, threonine, or tyrosine residues on proteins, catalyzed by kinases. This post-translational modification alters protein conformation, activity, localization, or interaction partners. Dephosphorylation by phosphatases reverses the signal. Most peptide receptor signaling cascades—GPCR/cAMP/PKA, receptor tyrosine kinase/MAPK, JAK-STAT—propagate through sequential phosphorylation events. Understanding which phosphorylation cascades a peptide activates is essential to predicting both therapeutic effects and off-target risks.