title: Magnesium and phosphate ions enable NAD binding to methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase.
abstract: The mitochondrial NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) is believed to have evolved from a trifunctional NADP-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase-synthetase. It is unique in its absolute requirement for inorganic phosphate and magnesium ions to support dehydrogenase activity. To enable us to investigate the roles of these ions, a homology model of human NMDMC was constructed based on the structures of three homologous proteins. The model supports the hypothesis that the absolutely required Pi can bind in close proximity to the 2'-hydroxyl of NAD through interactions with Arg166 and Arg198. The characterization of mutants of Arg166, Asp190, and Arg198 show that Arg166 is primarily responsible for Pi binding, while Arg198 plays a secondary role, assisting in binding and properly orienting the ion in the cofactor binding site. Asp190 helps to properly position Arg166. Mutants of Asp133 suggest that the magnesium ion interacts with both Pi and the aspartate side chain and plays a role in positioning Pi and NAD. NMDMC uses Pi and magnesium to adapt an NADP binding site for NAD binding. This adaptation represents a novel variation of the classic Rossmann fold.
authors.affiliation: Department of Biochemistry, McGill University, Montreal, Canada.
authors.forename: Karen E
authors.initials: KE
authors.lastname: Christensen
authors.forename: I Ahmad
authors.initials: IA
authors.lastname: Mirza
authors.forename: Albert M
authors.initials: AM
authors.lastname: Berghuis
authors.forename: Robert E
authors.initials: RE
authors.lastname: Mackenzie
mesh_terms: D000595:Amino Acid Sequence; D000619:Aminohydrolases / Q000737:chemistry* / Q000378:metabolism*; D001120:Arginine; D001665:Binding Sites; D003067:Coenzymes; D004926:Escherichia coli / Q000201:enzymology; D006801:Humans; D007477:Ions; D008274:Magnesium / Q000737:chemistry / Q000502:physiology*; D008754:Methylenetetrahydrofolate Dehydrogenase (NADP) / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000378:metabolism*; D009243:NAD / Q000378:metabolism*; D010710:Phosphates / Q000737:chemistry / Q000502:physiology*; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D017385:Sequence Homology
chemical_list: D003067:Coenzymes; D007477:Ions; D009097:Multienzyme Complexes; D010710:Phosphates; C055342:methylene tetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase; D009243:NAD; D001120:Arginine; D008754:Methylenetetrahydrofolate Dehydrogenase (NADP); D000619:Aminohydrolases; D008274:Magnesium