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Methionine M (Met)

Chemical Properties:

(Sulfur containing group)

Physical Properties:
 Non polar (hydrophobic)

, an essential amino acid, is one of the two sulfur-containing amino acids. The side chain is quite hydrophobic and methionine is usually found buried within proteins. Unlike cysteine, the sulfur of methionine is not highly nucleophilic, although it will react with some electrophilic centers. It is generally not a participant in the covalent chemistry that occurs in the active centers of enzymes.

The chemical linkage of the sulfur in methionine is a thiol ether. Compare this terminology with that of the oxygen containing ethers. The sulfur of methionine, as with that of cysteine, is prone to oxidation. The first step, yielding methionine sulfoxide, can be reversed by standard thiol containing reducing agents. The second step yields methionine sulfone, and is effectively irreversible. It is thought that oxidation of the sulfur in a specific methionine of the elastase inhibitor in human lung tissue by agents in cigarette smoke is one of the causes of smoking-induced emphysema.

Methionine as the free amino acid plays several important roles in metabolism. It can react to form S-Adenosyl-L-Methionine (SAM) which servers at a methyl donor in reactions.

Methionine and cysteine are the only sulfur-containing amino acids.

Molecular structure of methionine CH3-S-(CH2)2-CH(NH3)-COO
Chemical structure for methionine

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The Biology Project > Biochemistry > The Chemistry of Amino Acids

The Biology Project
Department of Biochemistry and Molecular Biophysics
University of Arizona
August 25, 2003
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