Wedler F C, Hoffmann F M, Kenney R, Carfi J
Experientia Suppl. 1976;26:187-97. doi: 10.1007/978-3-0348-7675-9_15.
Glutamine synthetase has been purified to homogeneity from B. subtilis (37 degrees) B. stearothermophilus (55 degrees), and B. caldolyticus (75 degrees). Those characteristics compared include size (6.0 +/- 0.3 X 10(5) daltons), quaternary structure (12 SU) amino acid content, substrate Km's and specificity for structural analogs, metal ion activation, number and kind of separate feedback modifier sites, and the complexity of modifier-substrate and modifier-modifier site interactions. Although the 37 degrees and 55 degrees systems are quite similar, the 75 degrees system shows important alterations in substrate specificity and modes of modifier action. Whereas at 37 degrees and 55 degrees AMP inhibits synergistically with amino acids (glycine, glutamine, histidine), the 75 degrees enzyme is inhibited directly by the products ADP, (which assumes the role of AMP) and glutamine, plus other ligands. Ligand binding domains are compared and found to be very different. Thermostabilization occurs by (a) protection by bound L-glutamate, (b) protein aggregation, (c) trends in the content of total polar residues, total Asx + Flx residues, the average hydrophobicity, and (d) disulfide bond cross-linking. Such studies provide insights to molecular evolution occurring with changes in environmental stress.
谷氨酰胺合成酶已从枯草芽孢杆菌（37摄氏度）、嗜热脂肪芽孢杆菌（55摄氏度）和嗜热解淀粉芽孢杆菌（75摄氏度）中纯化至同质。所比较的特性包括大小（6.0±0.3×10⁵道尔顿）、四级结构（12个亚基）、氨基酸含量、底物Km值和对结构类似物的特异性、金属离子激活、独立反馈调节位点的数量和种类，以及调节物-底物和调节物-调节物位点相互作用的复杂性。虽然37摄氏度和55摄氏度的系统非常相似，但75摄氏度的系统在底物特异性和调节作用模式上显示出重要变化。在37摄氏度和55摄氏度时，AMP与氨基酸（甘氨酸、谷氨酰胺、组氨酸）协同抑制，而75摄氏度的酶则直接被产物ADP（它承担了AMP的作用）、谷氨酰胺以及其他配体抑制。对配体结合结构域进行比较，发现它们非常不同。热稳定性通过以下方式实现：（a）结合的L-谷氨酸的保护作用，（b）蛋白质聚集，（c）总极性残基、总Asx + Flx残基含量、平均疏水性的趋势，以及（d）二硫键交联。此类研究为环境压力变化时发生的分子进化提供了见解。
