Methionine is an alpha amino acid also known as Met or M. It is classified as a non-polar and hydrophobic amino acid. It has a molecular weight of 149.21 and its molecular formula is C5H11NO2S . Methionine is an 'essential amino acid' as it is not produced naturally in humans, and therefore must be ingested. The triplet code for methionine is AUG, which is always the first codon to be translated in an open reading frame - thus, methionine is always the first amino acid in a polypeptide chain. However, methionine may be removed if the protein is edited in the Golgi Apparatus.