This enzyme has two activities: FAD diphosphatase activity and FAD synthase activity (PubMed:16643857, PubMed:21924249, PubMed:21951714, PubMed:23443125, PubMed:25135855, PubMed:26277395, PubMed:27259049, PubMed:31351152, PubMed:38688286). FAD diphosphatase acts on FAD and NADH to produce FMN and NMNH(2-), respectively (PubMed:26277395, PubMed:31351152, PubMed:38688286). FAD synthase catalyzes the adenylation of flavin mononucleotide (FMN) to form flavin adenine dinucleotide (FAD) coenzyme (PubMed:16643857, PubMed:21924249, PubMed:21951714, PubMed:23443125, PubMed:27259049, PubMed:38688286). In addition to its catalytic activities, the protein also facilitates the delivery of FAD to client apo-flavoproteins (PubMed:25954742). The balance between FAD synthesis and hydrolysis may be regulated by redox- sensing cysteine residues (PubMed:25135855, PubMed:26277395). At a much lower rate, FAD synthase catalyzes the reverse pyrophosphorolytic reaction (PubMed:21951714, PubMed:23443125, PubMed:25135855, PubMed:26277395). FAD synthase can also convert roseoflavin mononucleotide (RoFMN) to roseoflavin adenine dinucleotide (RoFAD); RoFMN is produced by riboflavin kinase when acting on the antibiotic roseoflavin (RoF) (PubMed:21924249). FAD synthase cannot convert 8- demethyl-8-amino-riboflavin mononucleotide (AFMN) to 8-demethyl-8- amino-riboflavin adenine dinucleotide (AFAD); AFMN is produced by riboflavin kinase when acting on the antibiotic 8-demethyl-8-amino- riboflavin (AF) (PubMed:21924249). {ECO:0000269|PubMed:16643857, ECO:0000269|PubMed:21924249, ECO:0000269|PubMed:21951714, ECO:0000269|PubMed:23443125, ECO:0000269|PubMed:25135855, ECO:0000269|PubMed:25954742, ECO:0000269|PubMed:26277395, ECO:0000269|PubMed:27259049, ECO:0000269|PubMed:31351152, ECO:0000269|PubMed:38688286}. This is the function of FLAD1 (flavin adenine dinucleotide synthetase 1, Ensembl gene identifier ENSG00000160688).