Zappia V, Cacciapuoti G, Pontoni G, Oliva A
J Biol Chem. 1980 Aug 10;255(15):7276-80.
Spermidine synthase (EC 2.5.1.16) purified from Escherichia coli has been subjected to a kinetic analysis including initial velocity and substrate analogs inhibition studies. Evidence is reported for a ping-pong mechanism, indicating that a propylaminated form of the enzyme is an obligatory intermediate in the reaction mechanism. S-Adenosyl(5')-3-methylthiopropylamine exerts a competitive substrate inhibition by combining with the improper stable enzyme form, while putrescine does not show any inhibitory effect. In order to investigate the substrate binding sites, new sulfonium-deaminated analogs of S-adenosyl(5')-3-methylthiopropylamine have been synthesized and assayed as substrates and as inhibitors of the reaction. The replacement of the amino group of adenine, or propylamine moiety of the sulfonium compound by the hydroxyl group, or both, resulted in a complete loss of activity as substrate. On the other hand, the deaminated analogs exert a competitive inhibition with respect to putrescine. On the basis of these results and in analogy with methyltransfer reactions, three recognition sites for S-adenosyl(5')-3-methylthiopropylamine on propylamine transfer enzymes are proposed.
从大肠杆菌中纯化得到的亚精胺合酶（EC 2.5.1.16）已进行了动力学分析，包括初速度和底物类似物抑制研究。报道了乒乓机制的证据，表明该酶的丙胺化形式是反应机制中的一个必需中间体。S-腺苷基（5'）-3-甲基硫丙胺通过与不稳定的酶形式结合发挥竞争性底物抑制作用，而腐胺则未显示出任何抑制作用。为了研究底物结合位点，已合成了S-腺苷基（5'）-3-甲基硫丙胺的新型锍脱氨基类似物，并作为底物和反应抑制剂进行了测定。用羟基取代腺嘌呤的氨基或锍化合物的丙胺部分，或两者都取代，导致作为底物的活性完全丧失。另一方面，脱氨基类似物对腐胺发挥竞争性抑制作用。基于这些结果并与甲基转移反应类比，提出了丙胺转移酶上S-腺苷基（5'）-3-甲基硫丙胺的三个识别位点。
