Involved in the regulation of homocysteine metabolism. Converts betaine and homocysteine to dimethylglycine and methionine, respectively. This reaction is also required for the irreversible oxidation of choline (By similarity). L-homocysteine + S-methyl-L-methionine = H(+) + 2 L-methionine Binds 1 zinc ion per subunit. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-methionine from L-homocysteine (BhmT route): step 1/1. Homotetramer.