Pepper (Capsicum annuum) serotonin N-hydroxycinnamoyltransferase (SHT) catalyzes the synthesis of N-hydroxycinnamic acid amides of serotonin, including feruloylserotonin and p-coumaroylserotonin. To elucidate the domain or the key amino acid that determines the amine substrate specificity, we isolated a tyramine N-hydroxycinnamoyltransferase (THT) gene from pepper. Purified recombinant THT protein catalyzed the synthesis of N-hydroxycinnamic acid amides of tyramine, including feruloyltyramine and p-coumaroyltyramine, but did not accept serotonin as a substrate. Both the SHT and THT mRNAs were found to be expressed constitutively in all pepper organs. Pepper SHT and THT, which have primary sequences that are 78% identical, were used as models to investigate the structural determinants responsible for their distinct substrate specificities and other enzymatic properties. A series of chimeric genes was constructed by reciprocal exchange of DNA segments between the SHT and THT cDNAs. Functional characterization of the recombinant chimeric proteins revealed that the amino acid residues 129 to 165 of SHT and the corresponding residues 125 to 160 in THT are critical structural determinants for amine substrate specificity. Several amino acids are strongly implicated in the determination of amine substrate specificity, in which glycine-158 is involved in catalysis and amine substrate binding and tyrosine-149 plays a pivotal role in controlling amine substrate specificity between serotonin and tyramine in SHT. Furthermore, the indisputable role of tyrosine is corroborated by the THT-F145Y mutant that uses serotonin as the acyl acceptor. The results from the chimeras and the kinetic measurements will direct the creation of additional novel N-hydroxycinnamoyltransferases from the various N-hydroxycinnamoyltransferases found in nature.
辣椒(Capsicum annuum)血清素N-羟基肉桂酰转移酶(SHT)催化合成血清素的N-羟基肉桂酰胺,包括香草酰血清素和对香豆酰血清素。为了阐明决定胺基底物特异性的区域或关键氨基酸,我们从辣椒中分离出一种酪胺N-羟基肉桂酰转移酶(THT)基因。纯化的重组THT蛋白催化合成酪胺的N-羟基肉桂酰胺,包括香草酰酪胺和对香豆酰酪胺,但不接受血清素作为底物。辣椒器官中均有SHT和THT mRNA的表达。利用主要序列相似度为78%的辣椒SHT和THT作为模型,研究了决定它们不同底物特异性和其他酶性质的结构因素。通过SHT和THT cDNA之间DNA片段的互换构建了一系列嵌合基因。重组嵌合蛋白的功能特性表明,SHT的129至165个氨基酸残基和THT中相应的125至160个残基是胺基底物特异性的关键结构因素。几个氨基酸强烈涉及胺基底物特异性的确定,其中甘氨酸-158参与催化和胺基底物结合,酪氨酸-149在SHT中在控制血清素和酪胺之间的胺基底物特异性方面起着关键作用。此外,酪氨酸的不可争议作用得到了THT-F145Y突变体的证实,该突变体使用血清素作为酰基受体。嵌合体和动力学测量的结果将指导从自然界中发现的各种N-羟基肉桂酰转移酶中创建额外的新型N-羟基肉桂酰转移酶。
Tyrosine decarboxylase (TDC) is a pyridoxal 5-phosphate (PLP)-dependent enzyme and is mainly responsible for the synthesis of tyramine, an important biogenic amine. In this study, the crystal structures of the apo and holo forms of