MoBio
Contents

The Twenty Amino
Acids of Proteins

Chapter
2

A | B | C | D | E | F | G

Based on the physicochemical properties of R groups, the 20 amino acids
of proteins may be classified as follows.

1. Acidic: including aspartic acid
(aspartate) and glutamatic acid (glutamate).  In a neutral solution,
the R group of an acidic amino acid may lose a proton and become
negatively charged.

2. Basic: including lysine, arginine and
histidine.  In a neutral solution, the R group of a basic amino acid
may gain a proton and become positively charged.  Interaction between
positive and negative R groups may form a salt bridge, which is an
important stabilizing force in proteins.

3. Aromatic: including tyrosine, tryptophan
and phenylalanine.  Their R groups contain an aromatic
ring.

4. Sulfur: including cysteine and
methionine. Their R groups contain a sulfur atom (S).  The disulfide bond
formed between two cysteine residues provides a strong force for
stabilizing the globular structure.  A unique feature about
methionine is that the synthesis of all peptide chains
starts from methionine (Chapter 5 Section
C).

5. Uncharged hydrophilic: including serine,
threonine, asparagine and glutamine.  Their R groups are hydrophilic
and capable of forming hydrogen bonds.

6. Inactive hydrophobic: including glycine,
alanine, valine, leucine and isoleucine.  These amino acids are more
likely to be buried in the protein interior.  Their R groups do not
form hydrogen bonds and rarely participate in chemical reactions.

7. Special structure: including
proline.  In most amino acids, the R group and the amino group are
not directly connected.  Proline is the only exception among 20 amino
acids found in protein.  Due to this special feature, proline is
often located at the turn of a peptide chain in the three-dimensional
structure of a protein.

Figure 2-A-2.  Names, symbols, chemical structures and
hydrophobicity indices of the 20 amino acids found in proteins.  They
are arranged in the order as discussed above.

Note: The hydrophobicity index tells the relative hydrophobicity
among amino acids.  More positive value indicates stronger
hydrophobicity.  Hydrophilic amino acids have negative values.
In a protein, hydrophobic amino acids are more likely to be located in the
protein interior, whereas hydrophilic amino acids are more likely to face
the aqueous environment.