Drennan C L, Huang S, Drummond J T, Matthews R G, Ludwig M L
Biophysics Research Division, University of Michigan, Ann Arbor 48109-1055.
Science. 1994 Dec 9;266(5191):1669-74. doi: 10.1126/science.7992050.
The crystal structure of a 27-kilodalton methylcobalamin-containing fragment of methionine synthase from Escherichia coli was determined at 3.0 A resolution. This structure depicts cobalamin-protein interactions and reveals that the corrin macrocycle lies between a helical amino-terminal domain and an alpha/beta carboxyl-terminal domain that is a variant of the Rossmann fold. Methylcobalamin undergoes a conformational change on binding the protein; the dimethylbenzimidazole group, which is coordinated to the cobalt in the free cofactor, moves away from the corrin and is replaced by a histidine contributed by the protein. The sequence Asp-X-His-X-X-Gly, which contains this histidine ligand, is conserved in the adenosylcobalamin-dependent enzymes methylmalonyl-coenzyme A mutase and glutamate mutase, suggesting that displacement of the dimethylbenzimidazole will be a feature common to many cobalamin-binding proteins. Thus the cobalt ligand, His759, and the neighboring residues Asp757 and Ser810, may form a catalytic quartet, Co-His-Asp-Ser, that modulates the reactivity of the B12 prosthetic group in methionine synthase.
以3.0埃的分辨率测定了来自大肠杆菌的含27千道尔顿甲基钴胺素的甲硫氨酸合酶片段的晶体结构。该结构描绘了钴胺素与蛋白质的相互作用，并揭示了咕啉大环位于螺旋状的氨基末端结构域和α/β羧基末端结构域之间，后者是罗斯曼折叠的变体。甲基钴胺素在与蛋白质结合时会发生构象变化；在游离辅因子中与钴配位的二甲基苯并咪唑基团会远离咕啉，并被蛋白质提供的一个组氨酸取代。包含该组氨酸配体的序列Asp-X-His-X-X-Gly在依赖腺苷钴胺素的酶甲基丙二酰辅酶A变位酶和谷氨酸变位酶中是保守的，这表明二甲基苯并咪唑的取代将是许多钴胺素结合蛋白共有的特征。因此，钴配体His759以及相邻残基Asp757和Ser810可能形成一个催化四重奏，即Co-His-Asp-Ser，它调节甲硫氨酸合酶中B12辅基的反应活性。
