Aromatic amino acids and their derivatives are diverse primary and secondary metabolites with critical roles in protein synthesis, cell structure and integrity, defense and signaling. All de novo aromatic amino acid production relies on a set of ancient and highly conserved chemistries. Here we introduce a new enzymatic transformation for l-tyrosine synthesis by demonstrating that the β-subunit of tryptophan synthase—which natively couples indole and l-serine to form l-tryptophan—can act as a latent ‘tyrosine synthase’. A single substitution of a near-universally conserved catalytic residue unlocks activity toward simple phenol analogs and yields exclusive para carbon–carbon bond formation to furnish l-tyrosines. Structural and mechanistic studies show how a new active-site water molecule orients phenols for a nonnative mechanism of alkylation, with additional directed evolution resulting in a net >30,000-fold rate enhancement. This new biocatalyst can be used to efficiently prepare valuable l-tyrosine analogs at gram scales and provides the missing chemistry for a conceptually different pathway to l-tyrosine.

芳香氨基酸及其衍生物是多种初级和次级代谢物，在蛋白质合成、细胞结构和完整性、防御和信号传导中发挥着关键作用。所有芳香氨基酸的从头生产都依赖于一套古老且高度保守的化学反应。在这里，我们通过证明色氨酸合酶的 β 亚基（天然地耦合吲哚和L-丝氨酸形成L-色氨酸）可以充当潜在的“酪氨酸合酶”，介绍了用于L-酪氨酸合成的新酶促转化。几乎普遍保守的催化残基的单一取代可释放对简单苯酚类似物的活性，并产生独特的对位碳-碳键形成以提供l-酪氨酸。结构和机理研究表明，新的活性位点水分子如何定向酚类以实现非天然的烷基化机制，并通过额外的定向进化导致净速率提高 30,000 倍以上。这种新型生物催化剂可用于有效制备克级的有价值的L-酪氨酸类似物，并为概念上不同的L-酪氨酸途径提供缺失的化学物质。