 Flavin mononucleotide

Flavin mononucleotide is riboflavin with a phosphate ester at its terminal primary hydroxy group. It functions as a cofactor rather than as an enzyme. The neutral structure recorded by ChEBI has formula ${\mathrm{C}}_{17}{\mathrm{H}}_{21}{\mathrm{N}}_4{\mathrm{O}}_9\mathrm{P}$; ionic and redox forms require their own chemical description.^[ref]

Riboflavin kinase produces flavin mononucleotide by phosphoryl transfer from ATP:A different enzyme, FAD synthase, converts flavin mononucleotide into flavin adenine dinucleotide by transfer of an adenylyl group:Here FMN and FAD abbreviate the respective flavin cofactors.^[ref]

Phosphorus bookkeeping makes the distinction clear. The first reaction has $3+0=2+1$ phosphorus atoms across its reactants and products; the second has $3+1=2+2$. An ideal isolated mixture of $2\mu \mathrm{mol}$ ATP and $1\mu \mathrm{mol}$ riboflavin can make at most $1\mu \mathrm{mol}$ flavin mononucleotide through the first reaction. This is a stoichiometric ceiling, not a measured yield or an equilibrium prediction. Downstream consumption can also prevent FMN accumulation despite continued synthesis.