= Flavin mononucleotide
<Flavin mononucleotide> is <riboflavin> with a phosphate ester at its terminal primary hydroxy group. It functions as a <cofactor (biochemistry)> rather than as an <enzyme>. The neutral structure recorded by ChEBI has formula $\mathrm{C_{17}H_{21}N_4O_9P}$; ionic and redox forms require their own chemical description.<Flavin mononucleotide in ChEBI>[CHEBI:17621; Definition, Formula, Net Charge and cofactor role]{ref}
<Riboflavin kinase> produces <flavin mononucleotide> by phosphoryl transfer from <ATP>:
$$
\mathrm{ATP+riboflavin\longrightarrow ADP+FMN}.
$$
A different enzyme, <FAD synthase>, converts <flavin mononucleotide> into <flavin adenine dinucleotide> by transfer of an adenylyl group:
$$
\mathrm{ATP+FMN\longrightarrow diphosphate+FAD}.
$$
Here FMN and FAD abbreviate the respective flavin cofactors.<Enzyme Nomenclature by the IUBMB>[EC2.7.1.26 and EC2.7.7.2; accepted names and reactions]{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 $2\,\mathrm{\mu mol}$ <ATP> and $1\,\mathrm{\mu mol}$ <riboflavin> can make at most $1\,\mathrm{\mu mol}$ <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.
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