The enzyme strictosidine synthase (STR1) from the Indian medicinal plant Rauvolfia serpentina is of primary importance for the biosynthetic pathway of the indole alkaloid ajmaline. Moreover, STR1 initiates all biosynthetic pathways leading to the entire monoterpenoid indole alkaloid family representing an enormous structural variety of ∼2000 compounds in higher plants. The crystal structures of STR1 in complex with its natural substrates tryptamine and secologanin provide structural understanding of the observed substrate preference and identify residues lining the active site surface that contact the substrates. STR1 catalyzes a Pictet-Spengler–type reaction and represents a novel six-bladed β-propeller fold in plant proteins. Structure-based sequence alignment revealed a common repetitive sequence motif (three hydrophobic residues are followed by a small residue and a hydrophilic residue), indicating a possible evolutionary relationship between STR1 and several sequence-unrelated six-bladed β-propeller structures. Structural analysis and site-directed mutagenesis experiments demonstrate the essential role of Glu-309 in catalysis. The data will aid in deciphering the details of the reaction mechanism of STR1 as well as other members of this enzyme family.
来自印度草药植物Rauvolfia serpentina的酶STrictosidine synthase (STR1) 对于吲哚生物碱ajmaline的生物合成途径至关重要。此外,STR1启动了所有导致整个单萜吲哚生物碱家族的生物合成途径,代表了植物中大约2000种化合物的巨大结构变化。STR1与其天然底物tryptamine和secologanin结合的晶体结构提供了对观察到的底物偏好的结构理解,并确定了接触底物的活性位点表面上的残基。STR1催化Pictet-Spengler型反应,并代表了植物蛋白中的一种新的六叶片β-螺旋桨折叠。基于结构的序列比对揭示了一个常见的重复序列模体(三个疏水残基后跟一个小残基和一个亲水残基),表明STR1和几个序列无关的六叶片β-螺旋桨结构之间可能存在进化关系。结构分析和位点定向突变实验表明Glu-309在催化中起着必不可少的作用。这些数据将有助于解密STR1以及这种酶家族的其他成员的反应机制的细节。