Protein Chemistry. Structure, Properties, and Research Methods - Shendryk A.N. 2022

Amino Acids
Amino Acids
Classification of Amino Acids

Based on the Chemical Structure of their R groups, Amino Acids are classified into the following categories:

1. aliphatic;

2. hydroxyl-containing;

3. sulfur-containing;

4. aromatic;

5. acidic and amides;

6. basic;

7. imino acids.

Aliphatic amino acids

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Amino acids with nonpolar R groups are less soluble in Water. These include amino acids with aliphatic side chains: Alanine, valine, leucine, and isoleucine, as well as phenylalanine, Tryptophan, and Methionine.

The polarity of hydroxyl-containing amino acids is due to their large dipole moment and the ability of the OH groups to form Hydrogen Bonds. All polar amino acids are readily soluble in water.

The hydroxyl group in Tyrosine behaves largely as nonpolar due to the presence of the phenyl ring.

Glycine and Proline are unique amino acids. Their structures differ significantly from other α-amino acids. Glycine lacks an asymmetric (chiral) carbon atom and is therefore not optically active. Proline is an imino acid, meaning its conformational parameters differ from those of all other R groups. In Proteins, proline most often behaves as a nonpolar residue.

Rare protein amino acids

In addition to the 20 most common amino acids discussed above, several others have been isolated from the hydrolysates of certain specialized proteins. As a rule, all of these are derivatives of standard amino acids, i.e., modified amino acids.

Here are a few Examples.

Desmosine and isodesmosine have been isolated from hydrolysates of the fibrous protein Elastin. It appears that these amino acids are unique to this protein. Their structure is unusual: four Lysine molecules are linked via their R groups to form a substituted pyridine ring. This unique structure presumably allows these amino acids to anchor four radially diverging peptide chains. As a result, unlike other Fibrous proteins, elastin is capable of undergoing deformation (stretching) in two mutually perpendicular directions.

Thyroxine (T4, T3) - tetra(tri)iodothyronine

T4 and T3 (where I* is replaced by H) are iodine-containing amino acids synthesized from tyrosine in The Thyroid Gland. They serve as essential Hormones that regulate GROWTH AND DEVELOPMENT processes in the body.

Non-protein amino acids

About 150 such amino acids are known. Most of them are derivatives of the α-amino acids found in proteins. ß-, γ-, and δ-amino acids also occur. Furthermore, they can have either an L- or a D-conformation. An example is D-glutamic acid, which is found in bacterial Cell walls. Some amino acids in this group act as crucial precursors or metabolic intermediates. For instance, ß-alanine serves as a precursor for pantothenic acid (a vitamin), while citrulline and Ornithine (see structures below) are intermediates in the synthesis of Arginine: Ornithine + CO₂ + NH₃ → citrulline (+NH₃) → arginine → urea + ornithine.

γ-Aminobutyric acid is a product of the decarboxylation of glutamic acid. It Functions as a chemical messenger in Nerve Impulse transmission.

A number of plant-derived non-protein amino acids, such as canavanine, djencolic acid, and ß-cyanoalanine, are toxic to other life forms. The structures of some of these amino acids are given as examples in Table 1.1.

Table 1.1 Structures of some non-protein amino acids.