Hofmann G E, Glaunsinger W S
J Biochem. 1978 Jun;83(6):1779-82. doi: 10.1093/oxfordjournals.jbchem.a132093.
The nature of the intermediate-affinity (n2) Mn(II) binding sites in glutamine synthetase [EC 6.3.1.2] has been studied as a function of adenylylation in a variety of enzyme-metal complexes by EPR. In the absence of nucleotide the n2 Mn(II) environment is nearly isotropic, the Mn(II) bonds are highly ionic, and the interaction distance R greater than or equal to 12-14 A. Nucleotide binding at the n2 Mn(II) site renders the n2 Mn(II) signal unobservable and causes a reduction in signal amplitude (approximately 30%) and line broadening (approximately 6 G) at the high-affinity (n1) Mn(II) site. This behavior indicates that nucleotide binding induces a conformational change in the enzyme which brings the previously distant n1 and n2 sites into closer proximity (R less than or equal to 8-11 A), possibly for the purpose of activating the nucleotide for direct phosphoryl transfer to L-glutamate. In line with this suggestion, the broad, unresolved resonances in complexes containing both L-methionine SR-sulfoximine (MSOX) and nucleotide may result from the phosphorylation of MSOX. The n2 Mn(II) site is not affected by adenylylation in all the enzyme-metal complexes studied, which suggests that the regulatory effects of adenylylation may only act at the n1 Mn(II) sites.
通过电子顺磁共振（EPR）研究了谷氨酰胺合成酶[EC 6.3.1.2]中中等亲和力（n2）锰（II）结合位点的性质，该性质是多种酶-金属复合物中腺苷酸化作用的函数。在不存在核苷酸的情况下，n2锰（II）环境几乎是各向同性的，锰（II）键具有高度离子性，相互作用距离R大于或等于12 - 14埃。核苷酸在n2锰（II）位点结合会使n2锰（II）信号无法观测到，并导致高亲和力（n1）锰（II）位点的信号幅度降低（约30%）和线宽增加（约6 G）。这种行为表明核苷酸结合会诱导酶发生构象变化，使先前距离较远的n1和n2位点靠得更近（R小于或等于8 - 11埃），可能是为了激活核苷酸以便将磷酸基团直接转移至L - 谷氨酸。与此建议一致，同时含有L - 蛋氨酸SR - 亚砜亚胺（MSOX）和核苷酸的复合物中宽泛、未解析的共振可能是由MSOX的磷酸化引起的。在所研究的所有酶-金属复合物中，n2锰（II）位点不受腺苷酸化的影响，这表明腺苷酸化的调节作用可能仅作用于n1锰（II）位点。
