Enzymes are renowned for their catalytic efficiency and selectivity, but many classical transformations in organic synthesis have no biocatalytic counterpart. Aldolases are prodigious C–C bond-forming enzymes, but their reactivity has only been extended past activated carbonyl electrophiles in special cases. To probe the mechanistic origins of this limitation, we use a pair of aldolases whose activity is dependent on pyridoxal phosphate. Our results reveal how aldolases are limited by kinetically favourable proton transfer with solvent, which undermines aldol addition into ketones. We show how a transaldolase can circumvent this limitation, enabling efficient addition into unactivated ketones. The resulting products are highly sought non-canonical amino acids with side chains that contain chiral tertiary alcohols. Mechanistic analysis reveals that transaldolase activity is an intrinsic feature of pyridoxal phosphate chemistry and identifies principles for extending aldolase catalysis beyond its previous limits to enable convergent, enantioselective C–C bond formation from simple starting materials.

酶以其催化效率和选择性而闻名，但有机合成中的许多经典转化没有生物催化对应物。醛缩酶是极好的 C-C 键形成酶，但在特殊情况下，它们的反应性仅扩展到活化羰基亲电试剂之外。为了探索这种限制的机制起源，我们使用了一对醛缩酶，其活性取决于磷酸吡哆醛。我们的结果揭示了醛醛酸酶如何受到溶剂的动力学有利质子转移的限制，这破坏了醛醇添加到酮中的作用。我们展示了转醛缩酶如何绕过这一限制，从而能够有效地添加到未活化的酮中。所得产品是备受追捧的非经典氨基酸，其侧链含有手性叔醇。机理分析表明，转羟醛酶活性是磷酸吡哆醛化学的内在特征，并确定了将醛缩酶催化扩展到其先前限制之外的原理，以实现从简单起始材料形成收敛的对映选择性 C-C 键。