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). glycine betaine + L-homocysteine = L-methionine + N,N-dimethylglycine Binds 1 zinc ion per subunit. Amine and polyamine degradation; betaine degradation; sarcosine from betaine: step 1/2. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-methionine from L-homocysteine (BhmT route): step 1/1. Homotetramer.