An Interprotein Co-S Coordination Complex in the B₁₂-Trafficking Pathway

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Cobalamin (vitamin B12) is essential for numerous metabolic processes.
• CblC and CblD are chaperones involved in intracellular cobalamin trafficking.
• The precise role of CblD in this pathway remained largely unknown.

Purpose of the Study:

• To elucidate the function of the CblD chaperone in the cobalamin trafficking pathway.
• To characterize the interaction between CblC and CblD.
• To understand the mechanism of cobalamin processing involving CblD.

Main Methods:

• Cysteine scanning mutagenesis to identify the sulfur donor residue.
• Electron Paramagnetic Resonance (EPR) spectroscopy to study the cobalamin intermediate.
• X-ray absorption spectroscopy (XAS) to analyze the interprotein cobalt-sulfur bond.
• X-ray crystallography to visualize the complex structure.

Main Results:

• CblD provides a sulfur ligand to cob(II)alamin bound to CblC.
• Cysteine-261 (Cys-261) on CblD was identified as the sulfur donor.
• An unusual interprotein cobalt-sulfur (Co-S) bond was formed, leading to thiolato-cob(III)alamin.
• The crystal structure revealed the human CblD-thiolato-cob(III)alamin complex.

Conclusions:

• CblD acts as a sulfur donor, facilitating cobalamin processing via an interprotein coordination complex.
• This mechanism highlights the utilization of coordination chemistry for cofactor translocation.
• The findings provide critical insights into the cobalamin trafficking pathway.