Enzyme modification is the alteration of an enzyme's molecular structure through covalent or other chemically defined changes that modify the structural or functional state of the enzyme. Within molecular enzymology, such modification encompasses changes to amino-acid side chains, protein termini, prosthetic or cofactor-associated groups, and other chemically accessible sites that can alter catalytic activity, substrate recognition, conformational behavior, stability, localization, molecular interactions, or susceptibility to inhibition and regulation ^[1][2]. Modification may arise through biologically controlled covalent processing, including phosphorylation, acetylation, glycosylation, ubiquitination, proteolytic processing, and related post-translational reactions, or through experimentally introduced chemical transformations such as residue-selective derivatization, conjugation, crosslinking, and incorporation of noncanonical chemical functionalities. A modification is characterized by the chemical identity of the structural change, its molecular site or sites, its stoichiometry and reversibility where applicable, and the resulting state of the enzyme molecule ^[1][2][3]. Functionally relevant enzyme modification consequently encompasses molecular changes capable of altering active-site properties, conformational equilibria, intermolecular interactions, or other structural determinants of enzymatic activity ^[2][3].