Cofactor F₄₂₀-dependent enzymes: An under-explored resource for asymmetric redox biocatalysis

The asymmetric reduction of enoates, imines and ketones are among the most important reactions in biocatalysis. These reactions are routinely conducted using enzymes that use nicotinamide cofactors as reductants. The deazaflavin cofactor F₄₂₀ also has electrochemical properties that make it suitable as an alternative to nicotinamide cofactors for use in asymmetric reduction reactions. However, cofactor F₄₂₀-dependent enzymes remain under-explored as a resource for biocatalysis. This review considers the cofactor F₄₂₀-dependent enzyme families with the greatest potential for the discovery of new biocatalysts: the flavin/deazaflavin-dependent oxidoreductases (FDORs) and the luciferase-like hydride transferases (LLHTs). The characterized F₄₂₀-dependent reductions that have the potential for adaptation for biocatalysis are discussed, and the enzymes best suited for use in the reduction of oxidized cofactor F₄₂₀ to allow cofactor recycling in situ are considered. Further discussed are the recent advances in the production of cofactor F₄₂₀ and its functional analog F_O-5^′-phosphate, which remains an impediment to the adoption of this family of enzymes for industrial biocatalytic processes. Finally, the prospects for the use of this cofactor and dependent enzymes as a resource for industrial biocatalysis are discussed.