BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING SYSTEMS

3.4. Amino Acids

Protein molecules are built from Amino Acids. Although over 170 different Amino acids are found in Cells and Tissues, only 20 can be considered standard components of Proteins (Table 3.5).

Class="center">Table 3.5. The 20 standard amino acids found in proteins. Only the R-groups are shown. The grey square represents the invariant part of the molecule (shown in full only for Alanine). The amino acids are divided into four categories. Two acidic amino acids possess more than one carboxyl group, three basic ones possess more than one amino group, and all others are neutral. Two amino acids contain an additional element, sulfur. The label "essential" indicates that this amino acid must be obtained by The Human Body through diet. The R-groups of Three amino acids contain an aromatic ring.

Plants synthesize all the amino acids they need from simpler substances. In contrast, Animals are unable to synthesize all the amino acids they require; they must obtain some of them in a preformed state, i.e., through their diet. These latter are commonly referred to as Essential Amino Acids, while Other Amino Acids necessary for the animal body can be synthesized from them. It should be emphasized, however, that the term "essential" does not imply that these amino acids are in any way more important than the others as components of animal cells. They are "essential" solely in the sense that the animal body is incapable of synthesizing them.

3.4.1. Structure and Classification of Amino Acids

The general formula of amino acids is shown in Fig. 3.19. In an amino acid molecule, a central carbon atom (the α-carbon) is always bonded to one acidic group, —СООН (carboxyl), one basic group, —NH₂ (amino), and one hydrogen atom. Only the remainder of the molecule varies—the R-group. Its Structure differs significantly among various amino acids, and it is this group that determines the unique properties of each individual amino acid (Table 3.5).

In the simplest amino acid, Glycine (Fig. 3.20, A), The Role of R is played by a hydrogen atom H. In alanine, this role is fulfilled by the —СН₃ group (Fig. 3.20, B).

Fig. 3.19. General formula of amino acids.

Fig. 3.20. A. Glycine. B. Alanine.

Rare (Non-standard) Amino Acids

A small number of rare amino acids are found in the proteins of living organisms. These are derivatives of certain standard amino acids. For example, hydroxyproline is a derivative of Proline found in Collagen; In addition to hydroxyproline, collagen contains another rare amino acid, hydroxylysine, which is a derivative of Lysine.

The DNA triplet code contains no codons for these rare amino acids; they are formed through the Modification of the corresponding precursor amino acids after the latter have already been incorporated into the polypeptide chain.

Amino Acids Not Found in Proteins

Over 150 such amino acids are known. They occur in cells in a free or bound state, but are never detected as part of proteins. For instance, γ-aminobutyric acid (GABA) can be found exclusively in Nervous Tissue. It acts as a neurotransmitter inhibitor, playing a vital role in the Central Nervous system.