The Biology Project > Biochemistry > The Chemistry of Amino Acids
Sulfur-Containing Amino Acids
The sulfur-containing amino acids (cysteine and methionine) are generally considered to be nonpolar and hydrophobic. In fact, methionine is one of the most hydrophobic amino acids and is almost always found on the interior of proteins. Cysteine on the other hand does ionize to yield the thiolate anion. Even so, it is uncommon to find cysteine on the surface of a protein. There are several reasons. First, sulfur has a low propensity to hydrogen bond, unlike oxygen. A consequence of this fact is that H2S is a gas under conditions that H2O is a liquid. Second, the thiol group of cysteine can react with other thiol groups in an oxidation reaction that yields a disulfide bond. Perhaps as a consequence, cysteine residues are most frequently buried inside proteins.
The Biology Project > Biochemistry > The Chemistry of Amino Acids
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