FMN

FMN, or flavin mononucleotide, is a redox-active coenzyme derived from riboflavin (vitamin B2). It comprises the isoalloxazine ring attached to a ribityl phosphate moiety and exists in cells either as a covalently bound prosthetic group or as a loosely bound cofactor. FMN is the phosphorylated form of riboflavin and serves with FAD as a major class of flavin cofactors in metabolism.

FMN participates in a wide range of oxidation–reduction reactions. It accepts two electrons and two protons to form FMNH2 and then donates them to flavoproteins or other acceptors. In mitochondria, FMN is the initial electron carrier in complex I of the respiratory chain; many bacterial, plant, and fungal flavoenzymes rely on FMN for catalytic activity, including oxidases, reductases, and monooxygenases. Flavoproteins containing FMN are involved in energy production, fatty acid and amino acid metabolism, and antioxidant defenses.

Biosynthesis and cellular origin: The body obtains riboflavin from the diet and converts it to FMN via riboflavin kinase; FMN can be further converted to FAD by FMN adenylyltransferase. Cells maintain pools of FMN through salvage pathways and turnover, and FMN levels influence the activity of a broad set of flavoproteins.

Dietary and clinical context: As a vitamin, riboflavin deficiency reduces FMN availability, contributing to ariboflavinosis, characterized by sore throat, inflammation of mucous membranes, cheilosis, and dermatitis. Adequate riboflavin intake supports FMN synthesis and normal flavoprotein function.

Properties and applications: FMN is fluorescent, which enables its use in enzymatic assays and as a biochemical probe. In laboratory and industrial contexts, FMN and flavoproteins are studied to understand redox biology and to develop biosensors and biocatalysts.