Tollinger M, Konrat R, Hilbert B H, Marsh E N, Kräutler B
Institute of Organic Chemistry, University of Innsbruck, Austria.
Structure. 1998 Aug 15;6(8):1021-33. doi: 10.1016/s0969-2126(98)00103-8.
Glutamate mutase is an adenosylcobamide (coenzyme B12) dependent enzyme that catalyzes the reversible rearrangement of (2S)-glutamate to (2S,3S)-3-methylaspartate. The enzyme from Clostridium tetanomorphum comprises two subunits (of 53.7 and 14.8 kDa) and in its active form appears to be an alpha 2 beta 2 tetramer. The smaller subunit, termed MutS, has been characterized as the B12-binding component. Knowledge on the structure of a B12-binding apoenzyme does not exist.
The solution structure and important dynamical aspects of MutS have been determined from a heteronuclear NMR study. The global fold of MutS in solution resembles that determined by X-ray crystallography for the B12-binding domains of Escherichia coli methionine synthase and Propionibacterium shermanii methylmalonyl CoA mutase. In these two proteins a histidine residue displaces the endogenous cobalt-coordinating ligand of the B12 cofactor. In MutS, however, the segment of the protein containing the conserved histidine residue forms part of an unstructured and mobile extended loop.
A comparison of the crystal structures of two B12-binding domains, with bound B12 cofactor, and the solution structure of the apoprotein MutS has helped to clarify the mechanism of B12 binding. The major part of MutS is preorganized for B12 binding, but the B12-binding site itself is only partially formed. Upon binding B12, important elements of the binding site appear to become structured, including an alpha helix that forms one side of the cleft accommodating the nucleotide 'tail' of the cofactor.
谷氨酸变位酶是一种依赖腺苷钴胺素（辅酶B12）的酶，可催化（2S）-谷氨酸可逆重排为（2S，3S）-3-甲基天冬氨酸。来自破伤风梭状芽孢杆菌的该酶由两个亚基（分别为53.7 kDa和14.8 kDa）组成，其活性形式似乎是一个α2β2四聚体。较小的亚基称为MutS，已被鉴定为B12结合成分。目前尚无关于B12结合脱辅基酶结构的相关知识。
通过异核核磁共振研究确定了MutS的溶液结构及重要动力学方面的信息。MutS在溶液中的整体折叠结构类似于通过X射线晶体学确定的大肠杆菌甲硫氨酸合酶和谢氏丙酸杆菌甲基丙二酰辅酶A变位酶的B12结合结构域。在这两种蛋白质中，一个组氨酸残基取代了B12辅因子的内源性钴配位配体。然而，在MutS中，含有保守组氨酸残基的蛋白质片段构成了一个无结构且可移动的延伸环的一部分。
对两个结合B12辅因子的B12结合结构域的晶体结构与脱辅基蛋白MutS的溶液结构进行比较，有助于阐明B12的结合机制。MutS的大部分结构预先为B12结合做好了准备，但B12结合位点本身仅部分形成。结合B12后，结合位点的重要元件似乎会形成结构，包括一个α螺旋，该螺旋形成容纳辅因子核苷酸“尾部”的裂隙的一侧。
