Quercetin 2,4‐Dioxygenase

Abstract Quercetin 2,4‐dioxygenase (QueD, EC 1.13.11.24) catalyzes the oxidative cleavage of the CC bond in quercetin, a flavonol, producing 2‐protocatechuoyl phloroglucinol carboxylic acid and carbon monoxide. Sequence and X‐ray crystal structural analyses classify QueD as a member of the cupin superfamily. Three QueDs from distinct bacterial or fungal sources have been extensively explored, revealing superimposable overall structures and active sites featuring nonheme mononuclear metal‐binding sites composed of three histidines and one glutamate. Despite their structural similarity, these QueDs incorporate metal cofactors, such as Mn, Ni, or Cu, significantly impacting protein folding or steady‐state catalytic activity. This unique metal specificity can be modulated by altering hydrophobic interactions between the secondary coordination spheres and quercetin, as demonstrated through directed evolution.