Catalyzes the alpha,gamma-elimination of L-methionine to produce methanethiol, 2-oxobutanoate and ammonia, and that of L-homocysteine (By similarity). Can also use L-cysteine as substrate, catalyzing its alpha,beta-elimination; this activity seems to only minimally contribute to the production of hydrogen sulfide (H2S) by F.nucleatum in the oral cavity, which is toxic for a large variety of cells in periodontal regions (PubMed:21798982). H2O + L-methionine = 2-oxobutanoate + methanethiol + NH4(+) H2O + L-homocysteine = 2-oxobutanoate + H(+) + hydrogen sulfide + NH4(+) H2O + L-cysteine = H(+) + hydrogen sulfide + NH4(+) + pyruvate kcat is 0.69 sec(-1) for the production of H(2)S from L-cysteine. Homotetramer. Belongs to the trans-sulfuration enzymes family. L-methionine gamma-lyase subfamily.