Benzalacetone synthase (BAS), a plant-specific type III polyketide synthase (PKS), catalyzes a one-step decarboxylative condensation of malonyl-CoA and 4-coumaroyl-CoA to produce the diketide benzalacetone. We solved the crystal structures of both the wild-type and chalcone-producing I207L/L208F mutant of
Rheum palmatum
BAS at 1.8 Å resolution. In addition, we solved the crystal structure of the wild-type enzyme, in which a monoketide coumarate intermediate is covalently bound to the catalytic cysteine residue, at 1.6 Å resolution. This is the first direct evidence that type III PKS utilizes the cysteine as the nucleophile and as the attachment site for the polyketide intermediate. The crystal structures revealed that BAS utilizes an alternative, novel active-site pocket for locking the aromatic moiety of the coumarate, instead of the chalcone synthase’s coumaroyl-binding pocket, which is lost in the active-site of the wild-type enzyme and restored in the I207L/L208F mutant. Furthermore, the crystal structures indicated the presence of a putative nucleophilic water molecule which forms hydrogen bond networks with the Cys-His-Asn catalytic triad. This suggested that BAS employs novel catalytic machinery for the thioester bond cleavage of the enzyme-bound diketide intermediate and the final decarboxylation reaction to produce benzalacetone. These findings provided a structural basis for the functional diversity of the type III PKS enzymes.
苯甲酮合酶(Benzalacetone synthase,BAS)是一种植物特异性的III型聚酮合酶(polyketide synthase,PKS),能催化马来酰辅酶A和4-香豆酰辅酶的一步脱羧缩合反应,生成二酮苯甲酮。我们解析了中药大黄(Rheum palmatum)BAS野生型和合成染料嘧啶酮(chalcone)的I207L/L208F突变体的晶体结构,分别在1.8 Å和1.6 Å的分辨率下得到了结果。此外,我们还解析了野生型酶的晶体结构,其中单酮香豆酸中间体与催化半胱氨酸残基共价结合。这是第一次直接证明III型PKS利用半胱氨酸作为亲核试剂和聚酮中间体的连接点。晶体结构揭示BAS利用另一种新型活性位点来锁定香豆酸的芳香基团,而不是失活酶的活性位点中的香豆酰结合口袋,这在I207L/L208F突变体中得到了修复。此外,晶体结构表明存在一个潜在的亲核水分子,与Cys-His-Asn催化三元组形成氢键网络。这表明BAS采用新的催化机制进行酶结合的二酮中间体的硫酯键裂解和最终的脱羧反应,生成苯甲酮。这些发现为III型PKS酶的功能多样性提供了结构基础。