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|Section2= |Section6= |Section7= }} Methionine ( or ; abbreviated as Met or M) is a non-polar α-amino acid with a straight side chain that possess a ''S''-methyl thioether (''i.e.'' C–S–C bonding) at the γ-carbon. It has the chemical formula HO2CCH(NH2)CH2CH2SCH3. It is an essential amino acid in all metazoa and is coded by the initiation codon AUG, which also indicates mRNA's coding region where translation into protein begins. ==Methionine: a proteinogenic amino acid== Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. Excluding the few exceptions where methionine may act as a redox sensor (''e.g.'' ), methionine residues do not have a catalytic role. This is in contrast to cysteine residues, where the thiol group has a catalytic role in many proteins.〔 The thioether does however have a minor structural role due to the stability effect of S/π interactions between the side chain sulfur atom and aromatic amino acids in one-third of all known protein structures.〔 This lack of a strong role is reflected in experiments where little effect is seen in proteins where methionine is replaced by norleucine, a straight hydrocarbon sidechain amino acid which lacks the thioether. It has been conjectured that norleucine was present in early versions of the genetic code, but methionine intruded into the final version of the genetic code due to the fact it is used in the cofactor ''S''-adenosyl methionine (SAM). This situation is not unique and may have occurred with ornithine and arginine. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「methionine」の詳細全文を読む スポンサード リンク
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