PROTEIN DENATURATION: FACTORS AND CONSEQUENCES - BIOLOGY - 2025

The denaturation of proteins consists of the loss of the three-dimensional structure due to different environmental factors, such as temperature, pH or certain chemical agents. The loss of the structure results in the loss of the biological function associated with that protein, be it enzymatic, structural, transporter, among others.

The structure of the protein is highly sensitive to changes. The destabilization of a single essential hydrogen bond can denature the protein. In the same way, there are interactions that are not strictly essential to fulfill protein function, and, if destabilized, have no effect on function.

Structure of proteins

In order to understand the processes of protein denaturation, we must know how proteins are organized. These present primary, secondary, tertiary and quaternary structure.

Primary structure

It is the sequence of amino acids that make up said protein. Amino acids are the fundamental building blocks that make up these biomolecules and there are 20 different types, each with particular physical and chemical properties. They are linked together by means of a peptide bond.

Secondary structure

In this structure this linear chain of amino acids begins to fold through hydrogen bonds. There are two basic secondary structures: the α helix, spiral-shaped; and the folded sheet β, when two linear chains are aligned in parallel.

Tertiary structure

It involves other types of forces that result in specific folds of the three-dimensional shape.

The R chains of the amino acid residues that make up the protein structure can form disulfide bridges, and the hydrophobic parts of the proteins clump together on the inside, while the hydrophilic ones face water. The van der Waals forces act as stabilizers for the interactions described.

Quaternary structure

It consists of aggregates of protein units.

When a protein is denatured, it loses its quaternary, tertiary, and secondary structure, while the primary remains intact. Proteins that are rich in disulfide bonds (tertiary structure) provide greater resistance to denaturation.

Factors causing denaturation

Any factor that destabilizes the non-covalent bonds responsible for maintaining the native structure of the protein can cause its denaturation. Among the most important we can mention:

pH

At very extreme pH values, whether acidic or basic, the protein can lose its three-dimensional configuration. The excess of H ⁺ and OH ^- ions in the medium destabilizes the protein interactions.

This change in ionic pattern causes denaturation. Denaturation by pH can be reversible in some cases, and in others irreversible.

Temperature

Thermal denaturation occurs with increasing temperature. In organisms living in average environmental conditions, proteins begin to destabilize at temperatures above 40 ° C. Clearly, the proteins of thermophilic organisms can withstand these temperature ranges.

Temperature increases translate into increased molecular movements that affect hydrogen bonds and other non-covalent bonds, resulting in the loss of the tertiary structure.

These increases in temperature lead to a decrease in the reaction rate, if we are talking about enzymes.

Chemical substances

Polar substances - such as urea - in high concentrations affect hydrogen bonds. Also, nonpolar substances can have similar consequences.

Detergents can also destabilize the protein structure; however, it is not an aggressive process and they are mostly reversible.

Reducing agents

Β-Mercaptoethanol (HOCH2CH2SH) is a chemical agent often used in the laboratory to denature proteins. It is responsible for reducing disulfide bridges between amino acid residues. It can destabilize the tertiary or quaternary structure of the protein.

Another reducing agent with similar functions is dithiothreitol (DTT). Furthermore, other factors that contribute to the loss of native structure in proteins are heavy metals in high concentrations and ultraviolet radiation.

Consequences

When denaturation occurs, the protein loses its function. Proteins function optimally when in their native state.

Loss of function is not always associated with a denaturation process. It may be that a small change in protein structure leads to loss of function without destabilizing the entire three-dimensional structure.

The process may or may not be irreversible. In the laboratory, if the conditions are reversed, the protein may return to its initial configuration.

Renaturation

One of the most famous and conclusive experiments on renaturation was evidenced in ribonuclease A.

When the researchers added denaturing agents such as urea or β-mercaptoethanol, the protein was denatured. If these agents were removed, the protein returned to its native conformation and could perform its function with 100% efficiency.

One of the most important conclusions of this research was to demonstrate experimentally that the three-dimensional conformation of the protein is given by its primary structure.

In some cases, the denaturation process is totally irreversible. For example, when we cook an egg we are applying heat to the proteins (the main one is albumin) that constitute it, the white takes on a solid and whitish appearance. Intuitively we can conclude that, even if we cool it, it will not return to its initial form.

In most cases, the denaturation process is accompanied by loss of solubility. It also reduces the viscosity, the speed of diffusion and crystallizes more easily.

Chaperone proteins

The chaperones or chaperonins proteins are in charge of preventing the denaturation of other proteins. They also repress certain interactions that are not suitable between proteins to ensure correct folding of the same.

When the temperature of the medium increases, these proteins increase their concentration and act to prevent the denaturation of other proteins. This is why they are also called "heat shock proteins" or HSPs (Heat Shock Proteins).

Chaperonins are analogous to a cage or a barrel that protects the protein of interest inside.

These proteins that respond to situations of cellular stress have been reported in various groups of living organisms and are highly conserved. There are different classes of chaperonins and they are classified according to their molecular weight.

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