Molecular basis for dysfunction of some mutant forms of methylmalonyl-CoA mutase: deductions from the structure of methionine synthase.
Inherited defects in the gene for methylmalonyl-CoA mutase (EC 5.4.99.2) result in the mut forms of methylmalonic aciduria. mut- mutations lead to the absence of detectable mutase activity and are not corrected by excess cobalamin, whereas mut- mutations exhibit residual activity when exposed to excess cobalamin. Many of the mutations that cause methylmalonic aciduria in humans affect residues in the C-terminal region of the methylmalonyl-CoA mutase. This portion of the methylmalonyl-CoA mutase sequence can be aligned with regions in other B12 (cobalamin)-dependent enzymes, including the C-terminal portion of the cobalamin-binding region of methionine synthase. The alignments allow the mutations of human methylmalonyl-CoA mutase to be mapped onto the structure of the cobalamin-binding fragment of methionine synthase from Escherichia coli (EC 2.1.1.13), which has recently been determined by x-ray crystallography. In this structure, the dimethylbenzimidazole ligand to the cobalt in free cobalamin has been displaced by a histidine ligand, and the dimethylbenzimidazole nucleotide "tail" is thrust into a deep hydrophobic pocket in the protein. Previously identified mut0 and mut- mutations (Gly-623 --> Arg, Gly-626 --> Cys, and Gly-648 --> Asp) of the mutase are predicted to interfere with the structure and/or stability of the loop that carries His-627, the presumed lower axial ligand to the cobalt of adenosylcobalamin. Two mutants that lead to severe impairment (mut0) are Gly-630 --> Glu and Gly-703 --> Arg, which map to the binding site for the dimethylbenzimidazole nucleotide substituent of adenosylcobalamin. The substitution of larger residues for glycine is predicted to block the binding of adenosylcobalamin.
Drennan
C L
CL
Biophysics Research Division and Department of Biological Chemistry, The University of Michigan, Ann Arbor 48109, USA.
Matthews
R G
RG
Rosenblatt
D S
DS
Ledley
F D
FD
Fenton
W A
WA
Ludwig
M L
ML
D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000737:chemistry*; D000483:Alleles; D000595:Amino Acid Sequence; D000818:Animals; D001419:Bacteria / Q000201:enzymology; D001665:Binding Sites; D017173:Caenorhabditis elegans / Q000201:enzymology; D004926:Escherichia coli / Q000201:enzymology; D006579:Heterozygote; D006801:Humans; D008661:Metabolism, Inborn Errors / Q000201:enzymology; D008764:Methylmalonic Acid / Q000652:urine; D008765:Methylmalonyl-CoA Mutase / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009711:Nucleotides / Q000378:metabolism; D017354:Point Mutation; D011110:Polymorphism, Genetic; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D014805:Vitamin B 12 / Q000378:metabolism
D009711:Nucleotides; D008764:Methylmalonic Acid; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; D008765:Methylmalonyl-CoA Mutase; D014805:Vitamin B 12