Lowe G, Sproat B S, Tansley G
Eur J Biochem. 1983 Feb 1;130(2):341-5. doi: 10.1111/j.1432-1033.1983.tb07158.x.
Methionyl-tRNA synthetase from Escherichia coli catalyses the activation of [18O2]methionine by adenosine 5'-[(R)-alpha 17O]triphosphate with inversion of configuration at P alpha. Furthermore methionyl-tRNA synthetase does not catalyse positional isotope exchange in adenosine 5'-[beta-18O2]triphosphate in the absence of methionine or in the presence of the competitive inhibitor, methioninol, which eliminates the possibility of either adenylyl-enzyme or adenosine metaphosphate intermediates being involved. These observations require that methionyl-tRNA synthetase catalyses the activation of methionine by an associative 'in-line' nucleotidyl transfer mechanism. A kinetic study of positional isotope exchange in adenosine 5'-[beta-18O2]triphosphate in the presence of methionine, Mg2+ and methionyl-tRNA synthetase showed that torsional equilibration (18O exchange into the P alpha--O--P beta bridge) occurs faster than tumbling (18O exchange into P gamma by rotation about the C2 axis of Mg[18O2]PPi), demonstratings that the positional isotope exchange occurs at least in part in the E X Met-AMP X Mg[18O2]PPi complex.
来自大肠杆菌的甲硫氨酰 - tRNA合成酶催化[18O2]甲硫氨酸被5'-[(R)-α 17O]三磷酸腺苷激活，同时Pα构型发生翻转。此外，在不存在甲硫氨酸或存在竞争性抑制剂甲硫氨醇的情况下，甲硫氨酰 - tRNA合成酶不会催化5'-[β - 18O2]三磷酸腺苷中的位置同位素交换，这排除了涉及腺苷酰 - 酶或腺苷偏磷酸中间体的可能性。这些观察结果表明，甲硫氨酰 - tRNA合成酶通过缔合性的“线性”核苷酸转移机制催化甲硫氨酸的激活。对在甲硫氨酸、Mg2+和甲硫氨酰 - tRNA合成酶存在下5'-[β - 18O2]三磷酸腺苷中位置同位素交换的动力学研究表明，扭转平衡（18O交换到Pα - O - Pβ桥中）比翻滚（18O通过围绕Mg[18O2]PPi的C2轴旋转交换到Pγ中）发生得更快，这表明位置同位素交换至少部分发生在E X Met - AMP X Mg[18O2]PPi复合物中。
