Structural Principles of Covalent Flavin Modification in Oxidoreductases.

Flavin-dependent oxidoreductases participate in a remarkably diverse array of biochemical transformations, enabled by the redox and covalent versatility of the isoalloxazine cofactor. The N5 and C4a atoms of the flavin are central to catalysis and, in selected cases, serve as loci of irreversible covalent modification. This review focuses on structurally validated examples of covalent inactivation of flavoenzymes in which the chemical nature of the flavin adduct has been established by X-ray crystallography, beginning with the historically important reversible N5 sulfite adduct and extending to mechanism-based irreversible inactivation. Mining of the Protein Data Bank reveals that covalent flavin modification occurs across multiple enzyme families, including monoamine oxidases, lysine-specific demethylase 1, proline dehydrogenase, spermine/polyamine oxidases, and cytokinin oxidase/dehydrogenase. These systems illustrate a limited but recurring set of structural outcomes, most commonly N5 alkylation or C4a alkylation, frequently accompanied by flavin reduction and characteristic butterfly bending of the isoalloxazine ring. A theme emerging from the structural record is the susceptibility of amine oxidases and dehydrogenases among covalently inactivated flavoenzymes. This prevalence reflects intrinsic mechanistic features of C-N bond oxidation─particularly iminium formation proximal to reduced flavin─that predispose these enzymes to irreversible flavin modification. In contrast, other flavoenzyme classes rarely generate electrophiles positioned for flavin attack, rendering stable covalent modification less common. By integrating structural, mechanistic, and inhibitor-design perspectives, this review highlights both the chemical disposition underlying covalent flavin inactivation and the constraints that shape its distribution across flavoprotein biochemistry.