Wu Hong, Min Jinrong, Zeng Hong, McCloskey Diane E, Ikeguchi Yoshihiko, Loppnau Peter, Michael Anthony J, Pegg Anthony E, Plotnikov Alexander N
Structural Genomics Consortium, University of Toronto, Toronto, Ontario M5G 1L5, Canada.
J Biol Chem. 2008 Jun 6;283(23):16135-46. doi: 10.1074/jbc.M710323200. Epub 2008 Mar 26.
The crystal structures of two ternary complexes of human spermine synthase (EC 2.5.1.22), one with 5'-methylthioadenosine and spermidine and the other with 5'-methylthioadenosine and spermine, have been solved. They show that the enzyme is a dimer of two identical subunits. Each monomer has three domains: a C-terminal domain, which contains the active site and is similar in structure to spermidine synthase; a central domain made up of four beta-strands; and an N-terminal domain with remarkable structural similarity to S-adenosylmethionine decarboxylase, the enzyme that forms the aminopropyl donor substrate. Dimerization occurs mainly through interactions between the N-terminal domains. Deletion of the N-terminal domain led to a complete loss of spermine synthase activity, suggesting that dimerization may be required for activity. The structures provide an outline of the active site and a plausible model for catalysis. The active site is similar to those of spermidine synthases but has a larger substrate-binding pocket able to accommodate longer substrates. Two residues (Asp(201) and Asp(276)) that are conserved in aminopropyltransferases appear to play a key part in the catalytic mechanism, and this role was supported by the results of site-directed mutagenesis. The spermine synthase.5'-methylthioadenosine structure provides a plausible explanation for the potent inhibition of the reaction by this product and the stronger inhibition of spermine synthase compared with spermidine synthase. An analysis to trace possible evolutionary origins of spermine synthase is also described.
已解析出人类精胺合酶（EC 2.5.1.22）的两种三元复合物的晶体结构，一种是与5'-甲硫基腺苷和亚精胺形成的复合物，另一种是与5'-甲硫基腺苷和精胺形成的复合物。结果表明，该酶是由两个相同亚基组成的二聚体。每个单体有三个结构域：一个C端结构域，包含活性位点，其结构与亚精胺合酶相似；一个由四条β链组成的中央结构域；以及一个与S-腺苷甲硫氨酸脱羧酶结构显著相似的N端结构域，S-腺苷甲硫氨酸脱羧酶是形成氨丙基供体底物的酶。二聚化主要通过N端结构域之间的相互作用发生。删除N端结构域会导致精胺合酶活性完全丧失，这表明二聚化可能是活性所必需的。这些结构提供了活性位点的轮廓以及一个合理的催化模型。活性位点与亚精胺合酶的活性位点相似，但有一个更大的底物结合口袋，能够容纳更长的底物。在氨丙基转移酶中保守的两个残基（Asp(201)和Asp(276)）似乎在催化机制中起关键作用，定点诱变的结果支持了这一作用。精胺合酶.5'-甲硫基腺苷结构为该产物对反应的有效抑制以及与亚精胺合酶相比对精胺合酶更强的抑制提供了一个合理的解释。还描述了一项追溯精胺合酶可能进化起源的分析。
