= FAD synthase
{c}
<FAD synthase> is the <enzyme> activity that converts <flavin mononucleotide> and <ATP> into <flavin adenine dinucleotide> and <diphosphate>. Its official classification is EC 2.7.7.2. The reaction transfers an <adenylyl group>, extending the flavin-containing molecule rather than simply adding another free <phosphate> group.<Enzyme Nomenclature by the IUBMB>[EC 2.7.7.2, Reaction and Systematic name]{ref}
Using the standard flavin abbreviations, the substrate–product balance is
$$
\mathrm{ATP+FMN\longrightarrow FAD+PP_i}.
$$
An ideal mixture containing 12 nanomoles of <flavin mononucleotide> and excess <ATP> can therefore form at most 12 nanomoles each of <flavin adenine dinucleotide> and <diphosphate> through the forward reaction. If a coupled <inorganic diphosphatase> then completely hydrolyses that <diphosphate>, it produces 24 nanomoles of <phosphate>. The factor of two belongs to the reporter reaction; it does not mean two FAD molecules formed per original turnover.
The phosphorus inventory clarifies the chemistry. One <ATP> contributes three phosphorus atoms and one <flavin mononucleotide> contributes one; the two-phosphorus <flavin adenine dinucleotide> and two-phosphorus <diphosphate> retain the same total of four. Measuring total phosphorus alone would miss the change of molecular organization.
The activity requires <magnesium> ions. Some organisms express it as a separate <enzyme>, whereas many <bacteria> have a bifunctional <protein> that also carries <riboflavin kinase> activity.<Enzyme Nomenclature by the IUBMB>[EC 2.7.7.2, Comments]{ref} A shared <protein> can therefore perform two distinguishable chemical reactions, and assaying one activity does not by itself quantify the other.
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