Jakubowski H
Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry, New Jersey Medical School, Newark 07103.
Ann N Y Acad Sci. 1994 Nov 30;745:4-20. doi: 10.1111/j.1749-6632.1994.tb44360.x.
In many cases, the intrinsic binding energies of amino acids to aminoacyl-tRNA synthetases are inadequate to give the required accuracy of translation. This has necessitated the evolution of a second determinant of specificity, proofreading, or editing mechanisms that involve the expenditure of energy to remove errors. Studies of an error-editing function of bacterial methionyl-tRNA synthetase have led to the discovery of a distinct chemical mechanism of editing and to molecular dissection of the dual synthetic-editing function of the active site of the synthetase. Studies have also established the importance of proofreading in living cells and allowed direct measurements of energy costs associated with editing in vivo. An unexpected outcome of these studies was a discovery of functional and structural similarities between methionyl-tRNA synthetase and S-adenosylmethionine synthetase, suggesting an evolutionary relationship between the two proteins. The mechanism of editing involves a nucleophilic attack of a sulfur atom on the side chain of homocysteine in homocysteinyl adenylate on its carbonyl carbon, yielding homocysteine thiolactone. The model of the active site of methionyl-tRNA synthetase derived from structure-function studies explains how the active site partitions amino acids between synthetic and editing pathways. Hydrophobic and hydrogen bonding interactions of active site residues Trp305 and Tyr15 with the side chain of methionine prevent the cognate amino acid from entering the editing pathway. These interactions are missing in the case of the smaller side chain of the noncognate homocysteine, which therefore enters the editing pathway. Homocysteine thiolactone is formed as a result of editing of homocysteine by methionyl-tRNA synthetase in bacteria, yeast, and some cultured mammalian cells. In mammalian cells, enhanced synthesis of homocysteine thiolactone, is, thus far, associated with oncogenic transformation. In E. coli, most of the energy cost of proofreading by methionyl-tRNA synthetase is due to editing of the incorrect product, homocysteinyl adenylate.
在许多情况下，氨基酸与氨酰 - tRNA合成酶的内在结合能不足以实现所需的翻译准确性。这就促使了特异性的第二种决定因素——校对或编辑机制的进化，这些机制需要消耗能量来消除错误。对细菌甲硫氨酰 - tRNA合成酶的错误编辑功能的研究，导致了一种独特的编辑化学机制的发现，以及对该合成酶活性位点双重合成 - 编辑功能的分子剖析。研究还确立了校对在活细胞中的重要性，并能够直接测量体内与编辑相关的能量消耗。这些研究的一个意外结果是发现了甲硫氨酰 - tRNA合成酶和S - 腺苷甲硫氨酸合成酶之间的功能和结构相似性，这表明这两种蛋白质之间存在进化关系。编辑机制涉及硫原子对同型半胱氨酰腺苷酸中同型半胱氨酸侧链羰基碳的亲核攻击，产生同型半胱氨酸硫内酯。从结构 - 功能研究得出的甲硫氨酰 - tRNA合成酶活性位点模型，解释了活性位点如何在合成和编辑途径之间分配氨基酸。活性位点残基Trp305和Tyr15与甲硫氨酸侧链的疏水和氢键相互作用，阻止同源氨基酸进入编辑途径。在非同型半胱氨酸较小的侧链情况下，这些相互作用不存在，因此非同型半胱氨酸进入编辑途径。同型半胱氨酸硫内酯是细菌、酵母和一些培养的哺乳动物细胞中甲硫氨酰 - tRNA合成酶对同型半胱氨酸进行编辑的结果。在哺乳动物细胞中，到目前为止，同型半胱氨酸硫内酯合成的增加与致癌转化有关。在大肠杆菌中，甲硫氨酰 - tRNA合成酶校对的大部分能量消耗是由于对错误产物同型半胱氨酰腺苷酸的编辑。
