Involved in the regulation of homocysteine metabolism. Converts homocysteine to methionine using S-methylmethionine (SMM) as a methyl donor. 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. Expressed in fetal heart, lung, liver, kidney and eye.