Biosynthesis of 4-Ethylphenol via an Enzyme Cascade with an Engineered Fatty Acid Photodecarboxylase

4-Ethylphenol (4-EP) is an important aromatic fine chemical used as a flavor compound and as an intermediate in the synthesis of polymers and pharmaceuticals. Although 4-EP occurs naturally, the decarboxylase responsible for its formation from phloretic acid has not yet been identified. To enable the biobased production of 4-EP, we designed an enzymatic cascade employing fatty acid photodecarboxylase (FAP) as the missing decarboxylase. Using structure-guided directed evolution, we engineered FAP for the decarboxylation of 3-(4-hydroxyphenyl)propanoic acid (HPPA), achieving a 3.7-fold increase in productivity. In addition, an Escherichia coli strain was engineered to produce l-tyrosine from glucose, and a whole-cell biocatalyst was developed to convert l-tyrosine into HPPA. By integrating tyrosine fermentation, multienzyme biotransformation, and photodecarboxylation, we achieved a 4-EP titer of 4.02 mM (490 mg/L)─the highest reported to date. This work demonstrates the significant potential of photoenzymes in the sustainable bioproduction of aromatic fine chemicals, particularly natural flavors and fragrances.