Amino acids are organic acids containing an amine group. The most common amino acids are -amino acids and the most common amino acids are the L--amino acids.

It is important to understand the following about amino acid structure:

1. Only 20 L--amino acids are used to make proteins (Figure 5.3 and Figure 5.5). Rare exceptions are bacterial membrane proteins, which contain a few D-Amino Acids, modified amino acids - primarily lysine (see here) and proline (see here), and occasional incorporation of the rare amino acid, selenocysteine).

2. Side groups (labelled "R" in Figure 5.4) are what distinguish the amino acids from each other.

3. Amino acids can exist as zwitterions - substances containing equal numbers of positive and negative charge - due to their carboxyl and amine groups, which can be negatively and positively charged, respectively.

Several common amino acids found in cells, such as ornithine and citrulline, are not used to make proteins.

Amino acids are the monomeric units or "building blocks" of proteins that are joined together covalently in peptide bonds.

Amino acids are built into proteins by the process of translation using the genetic code.

1. SECOST - Sequence-Conformation-Structure Database for Amino Acids in Proteins

2. IMB-Jena Amino Acid Repository

3. Introduction to Amino Acids