title: Unique holoenzyme dimers of the tetrameric enzyme Escherichia coli methylenetetrahydrofolate reductase: characterization of structural features associated with modulation of the enzyme's function.
abstract: Impaired functioning of methylenetetrahydrofolate reductase (MTHFR) can cause high levels of homocysteine in plasma or hyperhomocysteinemia, which is an independent risk factor for cardiovascular diseases and neural tube defects. We have studied in detail the effect of modulation of hydrophobic and electrostatic interactions of Escherichia coli MTHFR on its structure and function. Alterations in hydrophobic interactions of MTHFR, using urea, lead to dissociation of the native tetramer, resulting in stabilization of enzymatically active holoenzyme dimers followed by unfolding of the holoenzyme dimer to the denatured monomer along with dissociation of FAD from the enzyme. This is the first report of an enzymatically active dimer of E. coli MTHFR and suggests that the dimer rather than tetramer is the smallest functionally active unit of the enzyme. Furthermore, these results also demonstrate that dissociation of the FAD cofactor from the enzyme occurs only on unfolding of the dimer to denatured monomers. Modulation of electrostatic interactions, using NaCl, leads to dissociation of the native enzyme, resulting in stabilization of an enzymatically inactive partially unfolded holoenzyme dimer. Comparative analysis of loss of enzymatic activity and changes in structural features of MTHFR demonstrate a very good correlation between enhanced flexibility of the enzyme-bound FAD and loss of enzymatic activity, suggesting the importance of rigidity of the FAD cofactor in maintenance of the enzymatic activity of MTHFR.
authors.affiliation: Division of Molecular and Structural Biology, Central Drug Research Institute, Lucknow 226 001, India.
authors.forename: Sandeep K
authors.initials: SK
authors.lastname: Misra
authors.forename: Vinod
authors.initials: V
authors.lastname: Bhakuni
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D002704:Chlorates / Q000494:pharmacology; D002942:Circular Dichroism; D003432:Cross-Linking Reagents / Q000378:metabolism; D019281:Dimerization; D004795:Enzyme Stability / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology*; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D005976:Glutaral / Q000378:metabolism; D020035:Holoenzymes / Q000737:chemistry* / Q000378:metabolism; D006358:Hot Temperature; D006801:Humans; D042965:Methylenetetrahydrofolate Reductase (NADPH2); D009154:Mutation; D000587:Oxidoreductases Acting on CH-NH Group Donors / Q000737:chemistry* / Q000502:physiology*; D010957:Plasmids; D011487:Protein Conformation / Q000187:drug effects; D011489:Protein Denaturation; D017510:Protein Folding; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D014508:Urea / Q000494:pharmacology
chemical_list: D001426:Bacterial Proteins; D002704:Chlorates; D003432:Cross-Linking Reagents; D020035:Holoenzymes; D005182:Flavin-Adenine Dinucleotide; D014508:Urea; D000587:Oxidoreductases Acting on CH-NH Group Donors; D042965:Methylenetetrahydrofolate Reductase (NADPH2); D005976:Glutaral; C032706:sodium chlorate