Proteins

We explain what proteins are and the types of proteins that exist. What they are for, their structural levels and food.

What is Protein?

The proteins They are macromolecules made up of structural units called amino acids. They always contain carbon, oxygen, nitrogen, hydrogen and often also sulfur in their structure.

Amino acids are organic molecules composed of an amino functional group (-NH₂) at one end and a carboxyl functional group (-COOH) at the other end. There are twenty fundamental amino acids which in different combinations, constitute the basis of proteins. Two examples of amino acids are alanine and cysteine:

To form proteins, amino acids are joined together by peptide bonds that is, the union of the end with the amino functional group (-NH₂) of one amino acid, with the end containing the carboxyl functional group (-COOH) of another amino acid. Thus, the amino acids are linked in different combinations and as many times as necessary, until each specific protein is formed. An example of how the peptide bond is formed can be seen in the following figure, where alanine is represented in pink, cysteine ​​in red, and the peptide bond in blue:

See also: Enzymes

Types of proteins

Proteins are very important for the body, since they participate in all the processes it carries out. They can be classified according to:

• Its chemical composition:
  • Simple proteins Also known as holoproteins, they are made up only of amino acids or their derivatives.
  • Conjugated proteins Also known as heteroproteins, their structure is made up, in addition to amino acids, of other substances such as metals, ions, among others.
• Its three-dimensional shape (distribution in space of its structure):
  • Fibrous proteins Their structure is in the form of long fibers and they are insoluble in water.
  • Globular proteins Their structure is coiled and compact, almost spherical in shape and they are usually soluble in water.

What are proteins used for?

Proteins are essential for the human body and its growth. Some of its functions are:

• Structural Many proteins are responsible for giving shape, elasticity and support to cells and, therefore, tissues. For example: collagen, elastin and tubulin.
• Immunological Antibodies are proteins that act as a defense against external agents or infections that affect the human and animal organism.
• Motorboat Myosin and actin are proteins that allow movement. In addition, myosin is part of the contractile ring in cell division, allowing cytokinesis (separation of cells by strangulation).
• Enzymatic Some proteins accelerate certain metabolic processes. Some examples of enzymatic proteins are pepsin and sucrase.
• Homeostatic Homeostasis is the maintenance of internal balance in organisms. Proteins with homeostatic function, together with other regulatory systems, maintain the pH regulation of these organisms.
• Booking Many proteins are a source of energy and carbon for many organisms. For example: casein and ovalbumin.

Structural levels of proteins

The structure of a protein can be classified into various levels of organization and distribution of the units that compose it, according to:

• Primary structure It is the sequence of amino acids that make up a protein (it refers only to the types of amino acids that make up its structure and the order in which they are linked).
• Secondary structure Describes the local orientation of the different segments that make up a protein. In general, although there are other types, the main ones are: Alpha helix (it is a segment with a spiral-shaped structure on itself) and Folded beta sheet (it is a segment with a stretched and folded shape, similar to an accordion). The shapes of both segments are generated and stabilized mainly by hydrogen bond interactions.
• Tertiary structure It consists of the arrangement in space of the secondary structure, which can be molded to form globular or fibrous proteins. The tertiary structure is stabilized by Van der Waals interactions, by disulfide bridges between sulfur-containing amino acids, by hydrophobic forces and by interactions between amino acid radicals.
• Quaternary structure It is formed by the union of several peptide segments, that is, it is composed of the union of several proteins. Proteins with a quaternary structure are also called oligomeric proteins and do not constitute the majority of proteins. This structure is stabilized by the same type of interactions that stabilize the tertiary structure.

When proteins are subjected to high temperatures, drastic changes in pH, and the action of some organic solvents, among other factors, they become denatured. Denaturation is the loss of secondary, tertiary and quaternary structures, which leaves the polypeptide chain without any fixed three-dimensional structure, one could say, it is reduced to its primary structure. If the protein recovers these structures (returns to its original form) then it is renatured. The following image represents the different structures of a protein:

Protein-rich foods

Foods rich in protein are recommended for a healthy diet with high amounts of protein. Smoothies provide a large part of the daily protein source that is recommended to be ingested.

There are two types of foods rich in protein, those of plant origin and those of animal origin High-protein animal foods are eggs, fish, dairy products, and red and white meat. Nuts, soy, cereals and legumes are foods high in plant-based proteins.

References

• «Motor proteins of the cytoskeleton» by Jose Manuel Andreu (Special Lasker Award). (November 2012). http://dx.doi.org/
• “Biological chemistry” by Antonio Blanco. Eighth edition, El Ateneo (2006). ISBN: 9500204223.