This family corresponds to the N terminal domain of the bifunctional enzyme riboflavin kinase / FAD synthetase. These enzymes have both ATP:riboflavin 5'-phospho transferase and ATP:FMN-adenylyltransferase activity [1]. They catalyse the 5'-phosphorylation of riboflavin to FMN and the adenylylation of FMN to FAD [1]. This domain is thought to have the flavin mononucleotide (FMN) adenylyltransferase activity [2].

1: Purification and characterization of FAD synthetase from Brevibacterium ammoniagenes. Manstein DJ, Pai EF; J Biol Chem 1986;261:16169-16173. PMID:3023344

2: Crystal structure of flavin binding to FAD synthetase of Thermotoga maritima. Wang W, Kim R, Yokota H, Kim SH; Proteins 2005;58:246-248. PMID:15468322