{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nVitamin B12 (cobalamin) is an essential cofactor whose intracellular conversion into its active forms—methylcobalamin (MeCbl) in the cytoplasm and adenosylcobalamin (AdoCbl) in mitochondria—is critical for cellular metabolism. Early studies on the cblD defect, initially characterized by combined methylmalonic aciduria and homocystinuria due to an unidentified genetic lesion, later led to the discovery that MMADHC is responsible for this defect. MMADHC thereby emerges as a key trafficking chaperone that directs cobalamin along divergent metabolic routes, ensuring proper distribution of the coenzyme forms."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nBiochemical and mutational analyses have delineated distinct functional domains within MMADHC. Domain mapping studies reveal that specific regions of the protein differentially regulate the synthesis of AdoCbl and MeCbl—for example, residues downstream of Met116 are sufficient to support MeCbl production, whereas additional upstream regions are required for AdoCbl formation. Structural investigations, including crystallographic analyses, have shown that the C-terminal portion of MMADHC adopts a modified nitroreductase fold repurposed exclusively for protein–protein interactions, most notably with MMACHC, which processes cobalamin prior to its transfer. In addition, the dual subcellular localization of MMADHC to both the cytoplasm and mitochondria aligns well with its role as a branch point in intracellular cobalamin trafficking."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "7"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nFunctionally, MMADHC is integrated into a multiprotein complex that includes MMACHC, methionine synthase, and methionine synthase reductase, forming a coordinated network dedicated to vitamin B12 processing. This complex ensures the correct handoff and conversion of cobalamin, and disruptions to MMADHC—whether through missense or truncating mutations—lead to imbalances in coenzyme synthesis that underlie the variable clinical manifestations of the cblD defect. Thus, by orchestrating the intracellular trafficking and precise targeting of vitamin B12, MMADHC plays a critical role in maintaining cellular metabolic homeostasis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": ""}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "David Coelho, Terttu Suormala, Martin Stucki, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Gene identification for the cblD defect of vitamin B12 metabolism."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "N Engl J Med (2008)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1056/NEJMoa072200"}], "href": "https://doi.org/10.1056/NEJMoa072200"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "18385497"}], "href": "https://pubmed.ncbi.nlm.nih.gov/18385497"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Maria Plesa, Jaeseung Kim, Stéphane G Paquette, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Interaction between MMACHC and MMADHC, two human proteins participating in intracellular vitamin B₁₂ metabolism."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Genet Metab (2011)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.ymgme.2010.10.011"}], "href": "https://doi.org/10.1016/j.ymgme.2010.10.011"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "21071249"}], "href": "https://pubmed.ncbi.nlm.nih.gov/21071249"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Martin Stucki, David Coelho, Terttu Suormala, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Hum Mol Genet (2012)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1093/hmg/ddr579"}], "href": "https://doi.org/10.1093/hmg/ddr579"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "22156578"}], "href": "https://pubmed.ncbi.nlm.nih.gov/22156578"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Justin C Deme, Isabelle R Miousse, Maria Plesa, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Structural features of recombinant MMADHC isoforms and their interactions with MMACHC, proteins of mammalian vitamin B12 metabolism."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Genet Metab (2012)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.ymgme.2012.07.001"}], "href": "https://doi.org/10.1016/j.ymgme.2012.07.001"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "22832074"}], "href": "https://pubmed.ncbi.nlm.nih.gov/22832074"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "Wayne Mah, Justin C Deme, David Watkins, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Subcellular location of MMACHC and MMADHC, two human proteins central to intracellular vitamin B(12) metabolism."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Genet Metab (2013)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.ymgme.2012.11.284"}], "href": "https://doi.org/10.1016/j.ymgme.2012.11.284"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "23270877"}], "href": "https://pubmed.ncbi.nlm.nih.gov/23270877"}]}, {"type": "r", "ref": 6, "children": [{"type": "t", "text": "Jehona Jusufi, Terttu Suormala, Patricie Burda, et al. 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"}, {"type": "b", "children": [{"type": "t", "text": "Structure of Human B12 Trafficking Protein CblD Reveals Molecular Mimicry and Identifies a New Subfamily of Nitro-FMN Reductases."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M115.682435"}], "href": "https://doi.org/10.1074/jbc.M115.682435"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "26364851"}], "href": "https://pubmed.ncbi.nlm.nih.gov/26364851"}]}, {"type": "r", "ref": 8, "children": [{"type": "t", "text": "D Sean Froese, Jolanta Kopec, Fiona Fitzpatrick, et al. 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