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. 2008 Apr 4;283(14):8939-45.
doi: 10.1074/jbc.M710449200. Epub 2008 Jan 29.

Betaine-homocysteine S-methyltransferase-2 Is an S-methylmethionine-homocysteine Methyltransferase

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Free PMC article

Betaine-homocysteine S-methyltransferase-2 Is an S-methylmethionine-homocysteine Methyltransferase

Sandra S Szegedi et al. J Biol Chem. .
Free PMC article

Abstract

We demonstrate that purified recombinant human betainehomocysteine methyltransferase-2 (BHMT-2) is a zinc metalloenzyme that uses S-methylmethionine (SMM) as a methyl donor for the methylation of homocysteine. Unlike the highly homologous betaine-homocysteine methyltransferase (BHMT), BHMT-2 cannot use betaine. The K(m) of BHMT-2 for SMM was determined to be 0.94 mm, and it has a turnover number similar to BHMT. Several compounds were tested as inhibitors of recombinant human BHMT and BHMT-2. The SMM-specific methyltransferase activity of BHMT-2 is not inhibited by dimethylglycine and betaine, whereas the former is a potent inhibitor of BHMT. Methionine is a stronger inhibitor of BHMT-2 than BHMT, and S-adenosylmethionine does not inhibit BHMT but is a weak inhibitor of BHMT-2. BHMT can use SMM as a methyl donor with a k(cat)/K(m) that is 5-fold lower than the k(cat)/K(m) for betaine. However, SMM does not inhibit BHMT activity when it is presented to the enzyme at concentrations that are 10-fold greater than the subsaturating amounts of betaine used in the assay. Based on these data, it is our current hypothesis that in vivo most if not all of the SMM-dependent methylation of homocysteine occurs via BHMT-2.

Figures

FIGURE 1.
FIGURE 1.
Alignment of human BHMT and human BHMT-2 amino acid sequences. Identical residues are shaded in black. The accession numbers are human BHMT, NP_001704; and human BHMT-2, NP_060084.
FIGURE 2.
FIGURE 2.
Structural comparison of human BHMT and human BHMT-2. A, monomer from the PDB 1LT8 of human BHMT. B, human BHMT-2 theoretical model generated from Swiss model 3.5, a protein structure homology-modeling server (35). This figure was generated using the PyMOL Molecular Visualization System (DeLano Scientific LLC, Palo Alto, CA).
FIGURE 3.
FIGURE 3.
Structural superposition of the active sites of structure 1LT8 (gray, human BHMT with CBHCy bound) and the model (blue, human BHMT-2). Positions of pertinent residues are indicated. This figure was generated using the PyMOL Molecular Visualization System.
FIGURE 4.
FIGURE 4.
Image of a Coomassie Blue R-250-stained discontinuous 10% SDS-PAGE gel containing 10 μg of recombinant human BHMT and recombinant human BHMT-2. The electrophoretic mobilities of molecular weight standards can be seen in lane 1. Sizes are as indicated and in kilodaltons. Lane 2 contains purified BHMT and lane 3 contains purified BHMT-2.

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