The CblC and CblD chaperones are involved in early steps in the cobalamin trafficking pathway. Cobalamin derivatives en-tering the cytoplasm are converted by CblC to a common cob(II)alamin intermediate via glutathione-dependent alkyltransfer-ase or reductive elimination activities. Cob(II)alamin is subsequently converted to one of two biologically active alkylcobal-amins by downstream chaperones. The function of CblD has been elusive although it is known to form a complex with CblC under certain conditions. Here, we report that CblD provides a sulfur ligand to cob(II)alamin bound to CblC, forming an interprotein coordination complex that rapidly oxidizes to thiolato-cob(III)alamin. Cysteine scanning mutagenesis and EPR spectroscopy identified Cys-261 on CblD as the sulfur donor. The unusual interprotein Co-S bond was characterized by X-ray absorption spectroscopy and visualized in the crystal structure of the human CblD thiolato-cob(III)alamin complex. Our study provides insights into how cobalamin coordination chemistry could be utilized for cofactor translocation in the trafficking pathway.

中文翻译：

---

B12 运输途径中的蛋白间 Co-S 配位复合物

CblC 和 CblD 分子伴侣参与了钴胺素运输途径的早期步骤。进入细胞质的钴胺素衍生物由 CblC 通过依赖谷胱甘肽的烷基转移酶或还原消除活性转化为常见的钴胺素中间体。钴 (II) 胺随后被下游分子伴侣转化为两种具有生物活性的烷基钴胺之一。尽管已知在某些条件下与 CblC 形成复合物，但 CblD 的功能一直难以捉摸。在这里，我们报告了 CblD 为与 CblC 结合的 cob (II) alamin 提供了硫配体，形成了一种蛋白质间配位复合物，可快速氧化为硫醇基-cob (III) alamin。半胱氨酸扫描诱变和 EPR 光谱将 CblD 上的 Cys-261 鉴定为硫供体。不寻常的蛋白间 Co-S 键的特征在于 X 射线吸收光谱，并在人类 CblD 硫醇基-cob (III) alamin 复合物的晶体结构中进行了可视化。我们的研究提供了关于钴胺素配位化学如何用于运输途径中辅因子易位的见解。