LOW-BARRIER HYDROGEN-BONDS AND LOW FRACTIONATION FACTOR BASES IN ENZYMATIC-REACTIONS
LOW-BARRIER HYDROGEN-BONDS AND LOW FRACTIONATION FACTOR BASES IN ENZYMATIC-REACTIONS
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DOI：
10.1021/bi00117a001
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发表时间：
1992-01-21
期刊：
影响因子：
2.9
通讯作者：
CLELAND, WW
中科院分区：
文献类型：
--
作者：
CLELAND, WW
分子中氢的氘分馏系数是相对于它在水中的含量，氘在这个位置富集的趋势。因此，一个分馏系数为0.5的可交换氢，只要只有微量的氘存在，其氘含量就只有溶剂水的一半。除非在反应中除去，否则不能交换的碳键氢，其分馏系数从乙炔的0.64到半缩醛的1.23，如葡萄糖的Cl中的氢（Cleland, 1980）。巯基是正常氨基酸侧链中唯一一个氢与溶剂交换且分馏因子小于1的部分（~ 0.5;Schowen, 1972）。羧基、咪唑基、氨基和羟基的分馏因子都接近于1。因此，当实验表明催化基团在酶上的分馏因子为~ 0.5时，假设这些是巯基。脯氨酸外消旋酶可能就是这种情况（Rudnick & Abeles, 1975; Belasco et al., 1986），相关的羟脯氨酸外消旋酶也可能是这种情况（Finlay & Adams, 1970），但由于x射线研究缺乏结构，仍然存在一些不确定性。
The deuterium fractionation factor of a hydrogen in a molecule is the tendency for deuterium to enrich in this position relative to its content in water. Thus, an exchangeable hy-drogen with a fractionation factor of 0.5 contains half the deuterium content of solvent water as long as only traces of deuterium are present. Carbon-bonded hydrogens that are not exchangeable except when removed during a reaction have fractionation factors ranging from 0.64 for acetylene to 1.23 for hemiacetals such as the hydrogen at Cl of glucose (Cleland, 1980). The sulfhydryl group is the only part of a normal amino acid side chain which has a hydrogen ex-changeable with the solvent with a fractionation factor less than unity (~ 0.5; Schowen, 1972). Carboxyl, imidazole, amino, and hydroxy groups all have fractionation factors close to unity.Thus, when experiments have indicated that the fractionation factors of catalytic groups on enzymes were~ 0.5, the supposition has been that these were sulfhydryl groups. This is probably the case with proline racemase (Rudnick & Abeles, 1975; Belasco et al., 1986) and possibly with the related hydroxyproline racemase(Finlay & Adams, 1970), but in the absence of structures from X-ray studies there is still some uncertainty.