Methionine is nonpolar, but cysteine is polar. Cysteine can form weak hydrogen bonds with oxygen and nitrogen; it is also weakly acidic and is sometimes found at the active site of enzymes. Cysteine also acts as a reducing agent within the cell, both as the free amino acid and in the form of the antioxidant tripeptide glutathione. The sulfydril groups of two cysteine residues can be oxidized to form the double amino acid cystine, and this is the predominant form of the amino acid in extracellular fluid. When the same reaction occurs between cysteine residues in adjacent polypeptide chains, a strong, covalent disulfide bond is formed that gives the protein a rigid structure. This appears to be particularly important in stabilizing extracellular or secreted proteins. Methionine can be converted to S-adenosyl-methionine, the donor of methyl groups in transmethylation reactions. Methionine can be converted to cysteine in the body, but not vice versa. Selenium can replace sulfur in some cysteine and methionine residues, particularly when selenium intake is high. The antioxidant protein glutathione peroxidase requires a selenocysteine residue at its active site.
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