Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Water
Classification of amino acids based on their R-groups

Let us now examine the structures of the 20 Amino Acids found in Proteins. The task is simplified by the fact that all Amino acids can be grouped based on The properties of their R groups, specifically their polarity (Table 5–3), i.e., the ability of the R groups to interact with Water at physiological pH values (near pH 7.0). Based on the degree of polarity, all amino acid R groups can be arranged in a continuous spectrum, ranging from entirely nonpolar, or hydrophobic ("water-fearing"), R groups to highly polar, or hydrophilic ("water-loving"), R groups.

Class="center">Table 5–3. Classification of amino acids According to the polarity of their R groups (at pH 7)

Nonpolar

R groups

Alanine

Valine

Isoleucine

Leucine

Methionine

Proline

Tryptophan

Phenylalanine

Polar, uncharged R groups

Asparagine

Glycine

Glutamine

Serine

Tyrosine

Threonine

Cysteine

Negatively charged R groups

Aspartic acid

Glutamic acid

Positively charged R groups

Arginine

Histidine Lysine

The structures of the 20 standard Amino acids are shown in Fig. 5–6. There are four main classes of amino acids containing the following types of R groups: 1) nonpolar, or hydrophobic; 2) polar, but uncharged; 3) negatively charged; and 4) positively charged. Within each class, There is a certain gradation in the polarity, size, and shape of the R groups.



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