= Riboflavin kinase
<Riboflavin kinase> is an <enzyme> that phosphorylates <riboflavin> to form <flavin mononucleotide>, using <ATP> and producing <ADP>. It is EC 2.7.1.26, with systematic activity name ATP:riboflavin 5′-phosphotransferase. The reaction supplies a flavin <cofactor (biochemistry)> and also a substrate for <FAD synthase>.<Enzyme Nomenclature by the IUBMB>[EC 2.7.1.26, Reaction, Systematic name and Comments]{ref}
The two reactions can be kept chemically distinct:
$$
\begin{aligned}
\mathrm{riboflavin+ATP}&\longrightarrow\mathrm{FMN+ADP},\\
\mathrm{FMN+ATP}&\longrightarrow\mathrm{FAD+PP_i}.
\end{aligned}
$$
Their sum requires two <ATP> molecules per <flavin adenine dinucleotide> made from <riboflavin>, producing one <ADP> and one <diphosphate>. The <flavin mononucleotide> intermediate cancels in the net equation, but must still be formed during the pathway.<Enzyme Nomenclature by the IUBMB>[EC 2.7.1.26 and EC 2.7.7.2, Reactions]{ref}
A constructed inventory makes the limitation explicit. With 8 nanomoles of <riboflavin> and 12 nanomoles of <ATP>, no more than 6 nanomoles of <flavin adenine dinucleotide> can be formed through the complete two-step route if there are no other ATP sources. This is a stoichiometric ceiling. If the first reaction instead converts all eight nanomoles before the second proceeds, only four nanomoles of <ATP> remain, allowing four nanomoles of <flavin adenine dinucleotide> and leaving four of <flavin mononucleotide>. The same initial inventory permits different intermediate distributions.
A divalent metal ion is required for <riboflavin kinase> activity, with preferences depending on the biological source. Some <bacteria> place this activity and <FAD synthase> in one bifunctional <protein>.<Enzyme Nomenclature by the IUBMB>[EC 2.7.1.26, Comments]{ref} The activity names distinguish reactions, while protein architecture describes how an organism implements them.
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