Abstract
In mammals, B12 (or cobalamin) is an essential cofactor required by methionine synthase and methylmalonyl-CoA mutase. A complex intracellular pathway supports the assimilation of cobalamin into its active cofactor forms and delivery to its target enzymes. MMADHC (the methylmalonic aciduria and homocystinuria type D protein), commonly referred to as CblD, is a key chaperone involved in intracellular cobalamin trafficking, and mutations in CblD cause methylmalonic aciduria and/or homocystinuria. Herein, we report the first crystal structure of the globular C-terminal domain of human CblD, which is sufficient for its interaction with MMADHC (the methylmalonic aciduria and homocystinuria type C protein), or CblC, and for supporting the cytoplasmic cobalamin trafficking pathway. CblD contains an α+β fold that is structurally reminiscent of the nitro-FMN reductase superfamily. Two of the closest structural relatives of CblD are CblC, a multifunctional enzyme important for cobalamin trafficking, and the activation domain of methionine synthase. CblD, CblC, and the activation domain of methionine synthase share several distinguishing features and, together with two recently described corrinoid-dependent reductive dehalogenases, constitute a new subclass within the nitro-FMN reductase superfamily. We demonstrate that CblD enhances oxidation of cob(II)alamin bound to CblC and that disease-causing mutations in CblD impair the kinetics of this reaction. The striking structural similarity of CblD to CblC, believed to be contiguous in the cobalamin trafficking pathway, suggests the co-option of molecular mimicry as a strategy for achieving its function.
Highlights
Mutations in CblD, involved in B12 trafficking, lead to disease
We report the first crystal structure of the globular C-terminal domain of human CblD, which is sufficient for its interaction with MMADHC, or cobalamin directly but interact with proteins (CblC), and for supporting the cytoplasmic cobalamin trafficking pathway
We report the first structure of human CblD, a protein involved in intracellular cobalamin trafficking
Summary
Mutations in CblD, involved in B12 (or cobalamin) trafficking, lead to disease. Results: The first crystal structure of human CblD is reported. The structure reveals that CblD exhibits a nitro-FMN reductase (NFR)-like fold despite the lack of sequence similarity to other family members Within this family, CblD shares several distinguishing structural features with two other proteins involved in mammalian cobalamin metabolism, CblC, and the activation domain of methionine synthase (MSact) and with two recently described bacterial corrinoid-dependent dehalogenases [26, 27]. CblD shares several distinguishing structural features with two other proteins involved in mammalian cobalamin metabolism, CblC, and the activation domain of methionine synthase (MSact) and with two recently described bacterial corrinoid-dependent dehalogenases [26, 27] We propose that these proteins constitute a new subfamily within the NFR superfamily
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