Efficient chemical and enzymatic syntheses of FAD nucleobase analogues and their analysis as enzyme cofactors

Flavin adenine dinucleotide (FAD), an essential cofactor in cellular metabolism, catalyses a wide range of redox reactions. The organic synthesis of FAD is typically conducted by coupling flavin mononucleotide (FMN) and adenosine monophosphate. The reported synthesis routes have certain limitations such as multiple reaction steps, low yields, and/or difficult-to-obtain starting materials. In this study, we report the synthesis of FAD nucleobase analogues using chemical and enzymatic methods with readily available starting materials achieved in 1-3 steps with moderate yields (10-51%). Further, we demonstrate that Escherichia coli glutathione reductase can use these analogues to catalyse the reduction of glutathione. Finally, we show that FAD nucleobase analogues can also be synthesized inside a cell from cellular substrates FMN and nucleoside triphosphates. This lays the foundation for their use in studying the molecular role of FAD in cellular metabolism and as biorthogonal reagents in biotechnology and synthetic biology applications.