Divergent mechanisms of suicide inactivation for ethanolamine ammonia-lyase
Divergent mechanisms of suicide inactivation for ethanolamine ammonia-lyase
复制标题
DOI：
10.1021/ja051527k
复制
发表时间：
2005-06-22
影响因子：
15
通讯作者：
Radom, L
中科院分区：
文献类型：
--
作者：
Sandala, GM;Smith, DM;Radom, L
采用从头算分子轨道计算方法研究了三种不同底物类似物诱导乙醇胺解氨酶自杀失活的机制。以2-氨基乙醇（乙醇胺）为底物的正常催化机理的分析预测，涉及B-12-辅因子的氢提取和氢再提取步骤都可能是放热的。另一方面，第一种底物类似物乙醇醛的拟议灭活机制导致高度稳定的自由基，导致非常吸热的（约为100%）。90 kJ mol（-1））的氢再提取步骤，这被认为是停止酶的正常功能。奇怪的是，在第二种底物类似物2-羟乙基肼（HEH）的情况下，催化作用机制的能量要求与催化底物的能量要求平行，尽管HEH在实验上被发现是可降解的。然而，进一步的分析揭示了HEH的低能量途径的存在，导致高度稳定的肼鎓自由基阳离子的形成。以类似于当乙醇醛是底物类似物时的方式，这导致氢再提取步骤的吸热性非常大。与这些相关的失活机制相反，第三种底物类似物2-氨基乙醛显然以完全不同的方式完成EAL的失活。实验观察到的乙酸和铵阳离子形成的途径已被确定，并且在5 '-脱氧腺苷自由基再生的意义上似乎具有催化作用。然而，解释随后形成4 '，5'-脱水腺苷和辅因子的类可啉环降解的机制尚未阐明。
Ab initio molecular orbital calculations have been used to study the mechanism of suicide inactivation of ethanolamine ammonia-lyase induced by three different substrate analogues. Analysis of the normal catalytic mechanism with 2-aminoethanol (ethanolamine) as substrate predicts that both the hydrogen-abstraction and hydrogen-reabstraction steps involving the B-12-cofactor are likely to be exothermic. On the other hand, the proposed inactivation mechanism for the first substrate analogue, glycolaldehyde, leads to a highly stabilized radical that results in a very endothermic (by ca. 90 kJ mol(-1)) hydrogenreabstraction step, which is thought to halt the normal function of the enzyme. Curiously, the energy requirements for a catalytically imposed mechanism in the case of the second substrate analogue, 2-hydroxyethylhydrazine (HEH), parallel those for the catalytic substrate, despite the fact that HEH is found to inactivate EAL experimentally. However, further analysis reveals the presence of a lower energy pathway for HEH that leads to the formation of the highly stabilized hydrazinium radical cation. In a manner similar to when glycolalclehyde is the substrate analogue, this results in an endothermicity for the hydrogenreabstraction step that is prohibitively large. In contrast to these related inactivation mechanisms, the third substrate analogue, 2-aminoacetaldehyde, apparently accomplishes the inactivation of EAL in an entirely different manner. A pathway for the experimentally observed formation of acetic acid and ammonium cation has been identified and appears catalytic in the sense that 5'-deoxyadenosyl radical is regenerated. However, mechanisms to account for the subsequent formation of 4',5'-anhydroadenosine and degradation of the corrinoid ring of the cofactor have not been elucidated.