FAD synthase

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.^[ref]

Using the standard flavin abbreviations, the substrate–product balance isAn 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.^[ref] A shared protein can therefore perform two distinguishable chemical reactions, and assaying one activity does not by itself quantify the other.