The final step of the pathway involves a familiar phosphotransferase reaction involving the formation of a low-energy phosphate bond. Every step in is this pathway is unidirectional except the adenylyl transfer reaction. Hydrolysis of PPi, however, pulls the adenylyl transfer reaction forward. FMN and FAD We now examine the pathway for the biosynthesis of the flavin coenzymes: flavin mononucleotide (FMN) and flavine adenine dinucleotide (FAD). We begin with riboflavin, an essential nutrient or vitamin. The first step involves the phosphorylation of riboflavin by ATP to yield the energy-poor FMN (Figure 14-22). In the next reaction, ATP functions as active adenylate to yield FAD. This reaction, which occurs without a loss in the number of high-energy bonds, is functionally isoergonic. Hydrolysis of PPi, however, serves to pull the adenylyl transfer reaction forward