1. L-Amino acids are the building blocks of proteins. They are frequently grouped according to the chemical nature of their side chains. Common groupings of amino acids are aliphatic, hydroxyl/sulfur, cyclic, aromatic, basic, acidic and acid amides. Links to individual amino acids are given below:

Alanine	Arginine	Asparagine	Aspartic Acid
Cysteine	Glutamine	Glutamic Acid	Glycine
Histidine	Isoleucine	Leucine	Lysine
Methionine	Phenylalanine	Proline	Serine
Threonine	Tryptophan	Tyrosine	Valine

A rare amino acid, selenocysteine, is sometimes incorporated into proteins at UGA, which is normally a normal stop codon.

2. Figure 5.3 shows the structures of the 20 amino acids in proteins. Table 5.1 lists the 20 amino acids used to make proteins, the one-letter code used to designate them, and some of their chemical properties.

2. The genetic code for each of the 20 amino acids above is shown here. Links and 1/3 letter codes are shown here.

3. Amino acids not found in proteins are shown here.

4. Modified amino acids are sometimes found in proteins.

5. Virtually all amino acids in proteins are in the L configuration.

6. During times of starvation or low food supply, some amino acids (called glucogenic) can serve as precursors of glucose via gluconeogenesis.

7. Dietary amino acids are classified as being essential (must be in the diet) or non-essential (can be synthesized by the organism).