Just as in one train each wagon is engaged to the next, in one protein each amino acid is linked to another by a peptide bond. By this bonding, the amino group of one amino acid joins the carboxyl group of the other, releasing a molecule of water. The two joined amino acids form a dipeptide.
Binding of a third amino acid to dipeptide yields a tripeptide which then contains two peptide bonds. If a fourth amino acid binds to the previous three, we will have a tetrapeptide with three peptide bonds. As the number of amino acids in the chain increases, a polypetide is formed, a name used up to 70 amino acids. From this number the compound formed is considered to be a protein.
Essential and Natural Amino Acids
All living things produce proteins. However, not all produce the twenty types of amino acids required for protein construction. Man, for example, is able to synthesize in the liver only eleven of the twenty types of amino acids. These eleven amino acids are considered natural to our species. They are: alanine, asparagine, cysteine, glycine, glutamine, histidine, proline, thyroxine, aspartic acid, glutamic acid.
The other nine types, the ones we do not synthesize, are the essentials and must be obtained from those who produce them (plants or animals). They are: arginine, phenylalanine, isoleucine, leucine, lysine, methionine, serine, threonine, tryptophan and valine.
Remember that a particular amino acid can be essential for one species and natural for another.
A spatial view of protein
A protein molecule is roughly shaped like a bead necklace. The fundamental strand of the protein, formed as an amino acid sequence (whose sequence is genetically determined), constitutes the so-called primary structure of the protein.
However, the biological role of most proteins depends on a much more elaborate spatial form. Thus, the fundamental yarn is able to curl about itself, resulting in a coiled filament leading to the secondary structure, held stable by bonds that arise between amino acids.
New folds of the spiral lead to a new, globular form, which is kept stable thanks to new bonds that occur between amino acids. This globose form represents the tertiary structure.
In certain proteins, polypeptide chains in tertiary globular structures unite, resulting in a very complex spatial form, which determines the biochemical role of the protein. This new form constitutes the quaternary structure of these proteins.
The figure below shows the four hemoglobin structures together. Hemoglobin is present within the red blood cells and its biological role is to bind to oxygen molecules, transporting them to our tissues.