Biochemical characterization of a homocysteine S-methyltransferase from Leuconostoc suionicum

We identified and characterized a homocysteine S-methyltransferase (HMT) from the lactic acid bacterium Leuconostoc suionicum LT-38, representing the first characterization of an HMT from lactic acid bacteria. The enzyme exhibited a kcat/Km value of 15.7 mM-1 s-1 toward L-homocysteine. It showed optimal activity at 30 °C and pH 7.5 and remained stable over a broad range of temperatures (30-60 °C) and pH values (5-9). The enzyme accepted both L- and D-homocysteine as methyl acceptors but displayed markedly higher catalytic efficiency toward the L-form (~30-fold). In contrast, L- and D-cysteine were not utilized. Both S-methyl-L-methionine and S-methyl-D-methionine were consumed in reactions containing racemic S-methyl-DL-methionine, and S-adenosyl-L-methionine was also accepted, whereas betaine and related compounds were not utilized. Gel filtration analysis indicated a monomeric structure (~32 kDa). Phylogenetic analysis showed that Ls-HMT clustered with bacterial HMT homologs and was distinct from mammalian BHMT/BHMT2 proteins. Inductively coupled plasma-mass spectrometry revealed approximately one Zn atom per enzyme molecule, and the Zn-depleted apo enzyme showed no detectable activity. These findings support that Ls-HMT is a Zn-dependent HMT with relatively high activity and expand the functional diversity of microbial HMTs.