Premium
Assigning enzymatic function to betaine‐homocysteine S‐methyltransfearse‐2 (BHMT2) as an S‐methylmethionine (SMM)‐specific homocysteine (Hcy) methyltransferase
Author(s) -
Garrow Timothy A,
Szegedi Sandra S,
Castro Carmen
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a606-b
Subject(s) - betaine , methyltransferase , chemistry , methylation , enzyme , methionine , enzyme kinetics , biochemistry , homocysteine , amino acid , active site , dna
A human cDNA encoding BHMT2 was cloned into an E. coli expression vector and purified to homogeneity by affinity chromatography. Unlike BHMT, which the crystal structure shows to be a homotetramer, BHMT2 does not appear to form a stable quaternary structure and its activity is unstable without glycerol. The stabilized enzyme uses SMM as a methyl donor for the methylation of Hcy; has trace activity using S‐adenosylmethionine (AdoMet), but cannot use betaine. The Km 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 human recombinant BHMT and BHMT2. The SMM‐specific methyltransferase activity of BHMT2 is not inhibited by dimethylglycine and betaine, whereas the former is a potent inhibitor of BHMT. Methionine is a much stronger inhibitor of BHMT2 than BHMT, and AdoMet does not inhibit BHMT but is a weak inhibitor of BHMT2. BHMT can use SMM as a methyl donor with a Kcat/Km that is fivefold lower than the Kcat/Km for betaine. However, SMM is does not inhibit BHMT activity when it is presented to the enzyme at concentrations that are tenfold greater than the subsaturating amounts of betaine used in the assay. Based on these data, it is our current hypothesis that in vivo the betaine‐dependent methylation of Hcy occurs via BHMT, and most if not all of the SMM‐dependent methylation of Hcy occurs via BHMT2. Because of the significant homology between BHMT and BHMT2, it is possible that these enzymes form heterotetramers in vivo. This work was supported by NIDDK (DK52501).