在生物體內普遍存在著各種多胺分子（polyamines），例如腐胺（putrescine）、亞精胺（spermidine）與精胺（spermine）等。這些多胺分子帶正電的特性能平衡細胞內巨分子的電荷，包括DNA、RNA以及蛋白質分子都深受其影響。因此，當細胞進行分裂或分化時，多胺分子的調節是重要的關鍵。多胺分子的合成過程主要由三個酵素來調控；亞精胺合成酶是其中之一。該酵素的作用即是將decarboxylated S-adenosylmethionine的丙胺基(aminopropyl group)轉移到腐胺上，形成亞精胺分子。幽門螺旋桿菌的亞精胺合成酶具有262個氨基酸，其分子量約為30.5 kDa。我們利用X光晶體繞射實驗來決定幽門螺旋桿菌亞精胺合成酶的立體結構，以進一步瞭解它的生物功能。我們純化並結晶出含重金屬硒（selenium）的甲硫酸胺衍生物晶體，並透過多波長異常色散法（multiwavelength anomalous dispersion, MAD）來解決相位問題。經過分子模型的建立與修正後，於解析度為2.0 Å的情況下得到的R與Rfree值分別為23.6%和29.5%。仔細分析幽門螺旋桿菌亞精胺合成酶的立體結構，我們發現該酵素的四級結構為二元體（dimer）。每個結構單元（monomer）分成兩個區域：C端的Rossmann-like折疊結構，以及N端6股摺板區。其中，較大的C端區域是受質（substrates）進行催化、反應的空間；N端則扮演維持二元體結構的角色。

Polyamines, such as putrescine, spermidine, and spermine, are polycationic mediators of cell proliferation and differentiation in most organisms. Spermidine synthase catalyzes the transfer of the aminopropyl group from decarboxylated S-adenosylmethionine to putrescine in the biosynthesis of spermidine. Here we report the crystal structure of spermidine synthase from Helicobacter pylori, which specialized in the colonization of the human stomach. Spermidine synthase from H. pylori contains 262 amino acids with a molecular weight of 30.5 kDa. Its crystal structure has been determined to 2.5Å resolution by multiwavelength anomalous dispersion (MAD) from selenomethionine (Se-Met)-containing proteins and refined to a crystallographic R-factor of 23.6% and an Rfree value of 29.5% so far. The enzyme shows the architecture of a dimer, and each monomer consists of a C-terminal domain with a Rossmann-like fold and an N-terminal beta-stranded domain. The larger C-terminal is a catalytic site and the N-terminal plays a role in maintaining the dimer structure.

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