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4.4.2. Protein properties


In general, fibrous proteins are insoluble in water, while globular proteins are water soluble.

The globular proteins have a high molecular mass, so, to the dissolved, result in colloidal dispersions. In them, many of the apolar amino acids are located inside the protein, and in the polar ones, the free radicals (-R) of the polar amino acids are linked by hydrogen bonds with the water molecules that remain on the outside. In this way, the protein is covered by a layer of water molecules (solvation layer) that prevents it from binding to other proteins.

If this solvation layer disappears, there are interactions between different parts of the protein that will make it insoluble and will precipitate.

Protein denaturation

The denaturation of a protein occurs when the bonds holding the spatial configuration of the protein are broken, missing the secondary structurestertiary (mainly) and quaternary. As a consequence of this, it loses its properties and cannot perform its function.

If unfavorable environmental changes occur in a colloidal protein dispersion, such as increased temperature, pH variations, alterations in the saline concentration of the medium, molecular agitation, etc., the bonds (hydrogen bonds, van der Waals forces, hydrophobic interactions, etc.) that maintain the globular conformation can be disrupted and the protein adopts the filamentous conformation. The protein will precipitate, but also, when the active center is altered, its properties will disappear and they will cease to be functional.

The denaturation does not affect the peptide bonds, but are affected disulfide bonds, the hydrogen bonds and weak interactions.

Sometimes, renaturation occurs, when upon returning to normal conditions, the protein regains its original conformation.

The denaturation can be:

  • Irreversible. If the denatured protein cannot regain its native conformation and functionality.
  • Reversibly. Renaturation occurs, recovering the protein its native conformation and functionality.

Some examples of denaturation in everyday life are when the milk is cut due to the denaturation of casein, the precipitation of egg white when the ovalbumin is denatured by the effect of heat, the "permanent" or fixing of a hairstyle of the hair by effect of the heat on the keratins of the hair, and so on.

De Alejandro Porto - Derivada de File:Protein folding.png de Emw, CC BY-SA 3.0, Enlace

Protein specificity

The function of a protein is determined by the primary structure, which in turn determines the higher structural levels.

In some proteins, some amino acids can be replaced by others while maintaining its function. This has meant that, during the evolutionary process, a great variability of proteins has been given rise, which allows each species to have its specific proteins and that, even, differences appear between individuals of the same species. The differences between homologous proteins, that is, with the same function, are large between evolutionarily distant species and scarce between related species.

Two types of specificity are distinguished:

  • Function specificity. Each protein is specialized in a certain function. The amino acid sequence determines the quaternary structure of the protein, which is ultimately responsible for its characteristic function. A small variation in the amino acid sequence can cause the protein to lose functionality.
  • Species specificity. Each species has unique proteins, but proteins that perform the same function in different species often have a similar composition and structure. These proteins are called homologous proteins, such as insulin, exclusive to vertebrates, but whose A chain is identical in humans, pigs, dogs, rabbits, and sperm whales.

Buffer capacity

Proteins, like the amino acids that compose them, have an amphoteric behavior. They tend to buffer the pH variations of the medium, since they can behave like an acid (releasing protons) or a base (capturing protons).

Fundamental insights on the properties of proteins

The properties of proteins depend on the amino acids that form them, whose protruding free radicals react with other molecules. The  active center  of the protein is formed by the amino acids of a protein whose radicals have the ability to bind to and react with other molecules.

  • Protein solubility.
  • Denaturation and renaturation of proteins.
    • In the denaturation of a protein, the bonds that maintain the spatial configuration of the protein are broken, losing the secondarytertiary  (mainly) and quaternary structures. Peptide bonds are not affected, but disulfide bonds, hydrogen bonds, and weak interactions are. As a consequence of this, it loses its properties and cannot perform its function .      
    • Denaturation can be:
      • Irreversible. If the denatured protein cannot regain its native conformation and functionality.
      • Reversibly. Renaturation occurs, recovering the protein its native conformation and functionality.
  • Protein specificity
    • The primary structure determines the function of a protein, which in turn, determines the upper structural levels. In some proteins, if an amino acid is substituted, its function does not change. This has meant that by evolution specific proteins have emerged for each species and that  evolutionarily distant species  have very different proteins although they have the same function.  
    • Types of specificity:
      • Function specificity . 
      • Species specificity.
  • Protein buffering capacity .