Aspartate transcarbamylase. Studies of the catalytic subunit by ultraviolet difference spectroscopy.
Aspartate transcarbamylase. Studies of the catalytic subunit by ultraviolet difference spectroscopy.
复制标题
天冬氨酸转氨甲酰酶。
DOI：
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发表时间：
1969
影响因子：
4.8
通讯作者：
G. Stark
中科院分区：
文献类型：
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作者：
K. D. Collins;G. Stark
用275m~310m的差光谱可以检测到底物和底物类似物与大肠杆菌天冬氨酸氨基转移酶催化亚基的结合，产生这些差异光谱的发色团被初步鉴定为被氨基甲酰磷酸结合扰动的色氨酸残基和被L-天冬氨酸结合扰动的酪氨酸残基。用差示光谱法测定了几种L-天冬氨酸类似物的离解常数。当氨基甲酸酯磷酸中心空置时，L-天冬氨酸、琥珀酸、L-苹果酸和D-苹果酸分别与L-天冬氨酸结合较弱。在氨基甲酰磷酸存在下，只有琥珀酸和D-苹果酸这两种缺乏L-α-取代基的二元酸与酶结合紧密。与之形成鲜明对比的是，当氨基甲酰磷酸被磷酸取代时，只有L-天冬氨酸和L-苹果酸这两种具有L-α-取代基的二元酸与酶结合紧密。只有当二元酸的结合增强时，三元络合物的差光谱的幅度才实质上大于每个类似物单独形成的二元络合物的幅度之和。我们认为，作为对这些观察的解释，酶将L-天冬氨酸的部分静电结合能转换为迫使底物结合在一起的机械行为的能量。基于本文和相关论文中提供的数据，提出了一个详细的催化机理模型。
Abstract The binding of substrates and substrate analogues to the catalytic subunit of aspartate transcarbamylase from Escherichia coli can be detected by difference spectroscopy between 275 and 310 mµ; the chromophores giving rise to these difference spectra are tentatively identified as a tryptophyl residue perturbed by the binding of carbamyl phosphate and a tyrosyl residue perturbed by the binding of l-aspartate. Dissociation constants for several l-aspartate analogues have been determined by difference spectroscopy. When the carbamyl phosphate site is unoccupied, l-aspartate, succinate, l-malate, and d-malate each bind weakly at the l-aspartate site. In the presence of carbamyl phosphate, only succinate and d-malate, the two dicarboxylic acids which lack l-α-substituents, bind tightly to the enzyme. In striking contrast, when carbamyl phosphate is replaced by phosphate, only l-aspartate and l-malate, the two dicarboxylic acids which have l-α-substituents, bind tightly to the enzyme. The magnitudes of the difference spectra for the ternary complexes are substantially greater than the sum of the magnitudes of the binary complexes formed by each analogue individually only when binding of the dicarboxylic acid is enhanced. We suggest, as an explanation for these observations, that the enzyme converts a portion of the electrostatic binding energy of l-aspartate into energy for the mechanical act of forcing the substrates together. A detailed model for the catalytic mechanism is presented, based on the data presented in this and the accompanying papers.