Flavoenzymes: Covalent versus Noncovalent

Abstract The use of nonprotein cofactors by enzymes expands the range of biological chemistries supported in nature. Flavins, which are derivatives of vitamin B2, are highly conjugated rings that are particularly useful for oxidoreduction and group transfer reactions. Most flavins are noncovalently associated with their enzymes, but around 10% of flavoproteins have the flavin covalently attachedin vivo. Extensive research has investigated how the presence of the covalent bond between enzyme and flavin cofactor influences enzymatic catalysis. This work identified that the primary roles of the covalent flavin are to allow catalysis of more thermodynamically challenging reactions and to prevent the cofactor from disassociating from the enzyme. Major questions in the field now include the mechanism of covalent flavinylation. The earliest studies on a subset of covalent flavoproteins suggested that cofactor attachment could be an autocatalytic posttranslational process. However, the recent identification of assembly factors that promote covalent flavinylation identifies that ancillary proteins may be important for covalent flavinylationin vivo. Key Concepts Covalent flavin attachment increases stability of the holoenzyme and increases the enzyme's redox potential. Covalent flavinylation may occur either through an entirely autocatalytic mechanism, or be assisted by assembly factors. Enzyme‐associated flavin can promote a variety of chemistries. Flavoenzymes can have covalent or noncovalent flavin. Covalent flavinylation can occur on multiple sites of the protein and flavin molecule.