Protein phosphorylation is a post-translational modification that was first reported in 1906 and alters protein structure and function. It involves the addition of a covalently bound phosphate group to an amino acid residue on a protein by protein kinases. Phosphorylation can activate or deactivate proteins or modify their function. It is used in important cellular processes like metabolic enzyme regulation, receptor signaling, cell cycle control by cyclin-dependent kinases, and maintaining homeostasis.