Cells of the zona fasciculata and zona reticularis lack aldosterone synthase (CYP11B2) that converts corticosterone to aldosterone, and thus these tissues produce only the weak mineralocorticoid corticosterone. However, both these zones do contain the CYP17A1 missing in zona glomerulosa and thus produce the major glucocorticoid, cortisol. Zona fasciculata and zona reticularis cells also contain CYP17A1, whose 17,20-lyase activity is responsible for producing the androgens, dehydroepiandosterone (DHEA) and androstenedione. Thus, fasciculata and reticularis cells can make corticosteroids and the adrenal androgens, but not aldosterone.
FMN is synthesized from riboflavin via the ATP-dependent enzyme riboflavin kinase (RFK). RFK introduces a phosphate group onto the terminal hydroxyl of riboflavin. The RFK gene is located on chromosome and is composed of 4 exons that encode a 155 amino acid protein. FMN is then converted to FAD via the attachment of AMP (derived from ATP) though the action of flavin adenine dinucleotide synthetase 1 which is encoded by the FLAD1 gene. The FLAD1 gene is located on chromosome and is composed of 7 exons that generate four alternatively spliced mRNAs each of which encode distinct isoforms of the enzyme.