Ethionine
Ethionine is a non-proteinogenic amino acid structurally related to methionine, with an ethyl group in place of the methyl group.
Ethionine is an antimetabolite and methionine antagonist. It is a substrate for methionyl-tRNA synthetase and is incorporated into proteins.
Ethionine has been found to naturally occur in the edible pulp of the durian fruit, and postulated to be a biosynthetic precursor for ethanethiol and other strong odorants found in the fruit.