Biosynthesis of 4-ethylphenol via an enzyme cascade with an engineered fatty acid photodecarboxylase

4-Ethylphenol (4-EP) is a key platform molecule for the synthesis of flavors, polymers, and pharmaceuticals. The enzyme responsible for its biosynthesis, postulated to be a phloretic acid decarboxylase, has not yet been identified in nature. This study unveils that fatty acid photodecarboxylase (FAP) effectively substitutes for this "missing" enzyme. Upon photoexcitation, the FAP cofactor directly decarboxylates p -hydroxyphenyl propionic acid (HPPA) without prerequisite activation, thereby introducing a novel paradigm of light-driven decarboxylation. A semi-rational design yielded a triple FAP mutant (Y466V/G462V/V453M) with 3.7-fold enhanced activity. Leveraging this discovery, we developed an integrated fermentation-photocatalysis process. Engineered E. coli first produced tyrosine from glucose, followed by a two-step whole-cell biotransformation that achieved a record 4-EP titer of 4.02 mM (0.49 g L⁻¹). This cofactor- and toxin-free route establishes a plug-and-play platform for the sustainable synthesis of aromatic compounds.