Notes on the amino acids commonly found in proteins

Glycine is the only amino acid without an asymmetric carbon atom. Hence it does not have D or L forms. The ‘side chain’ consists of hydrogen.

Alanine has a methyl group as its side chain.

Valine has two methyl groups in the side chains.

Leucine has a side chain similar to that of valine except that it has an additional CH2 group.

Isoleucine is an isomer of leucine but has a second asymmetric carbon atom.

Serine. The replacement of β hydrogen of alanine by a hydroxyl group (-OH) gives serine. Thus serine has a hydroxyl group in the side chain.

Threonine is the other aliphatic amino acid with a hydroxyl group in the side chain. It has a second asymmetric centre.

Cysteine can be derived from alanine by replacement of the hydrogen with an –SH group. Thus the side chain of cysteine contains of sulphur atom. The sulphur atom is important in the formation of a disulphide linkage (-S-S). The double amino acid cystine consists of consists of two cysteine residues linked by a disulphide bond. The disulphide linkage has an important role in determining the tertiary degree configuration in proteins.

Methionine is the other amino acid with a sulphur atom in the side chain. The starting amino acid in the synthesis of polypeptide chains is methinonine in eukaryotes. In prokaryotes it in N-formyl methoinonine, a formulated from of methionine.

Aspartic acid is a dicarboxylic amino acid. The carboxyl group of the side chin dissociated at physiological pH to given the negatively charged side chain. It will be called aspartate to emphasize its negative charge at physiological pH.

Asparagine is the uncharged derivative of asparate. It contains a terminal amide group instead of a carboxylate. Although the amide group is not acidic it is polar, and is able to take part in hydrogen bonding.

Glutamic acid is the other dicarboxylic amino acid. Like in aspartic acid, the carboxyl group of the side chain dissociates at physiological pH to give a negatively charged side chain. It will therefore be called glutamate.

Arginine is positively charged at neutral pH and is therefore a basic amino acid.

Lysine is also positively charged at neutral pH.

Hydroxylsine is a hydroxylated derivative of lysine. It has a second asymmetric carbon atom, as in isoleucine and threonine. It occurs only in collagen and gelatin.

Histidine may be positively charged or neutral, depending on the local environment. It is the only amino acid having a proton that dissociates in the neutral pH range. The imidazole group in the histidine side chain is a part of the active site of many enzymes. It may bind to a metal ion, like other basic groups in proteins.

Phenylalanine is an aromatic amino acid. Replacement of β hydrogen of alanine by phenyl gives phenylalanine.

Tyrosine is an aromatic amino acid. Structurally it is a phenolic hydroxyl. It is a hydroxylated derivative of phenylalanine.

Proline has an imino group (NH2+) instead of a NH8+ group attached to the α carbon and the NH2+ group, thus forming a cyclic structure.

4-Hydroxyplroline is a hydroxylated derivated of proline. The hydroxyl (-OH) group is added to proline after the protein has been synthesized. It has two asymmetric carbon atoms. It is found only in collagen and gelatin.

Thyroxin occurs only in the protein thymoglobulin formed in the thyroid gland. It contains four atoms of iodine.