What Happens During Protein Denaturation? Causes, Effects & Examples
Denaturation meaning can be given as, when the solution of a protein is boiled frequently, the protein becomes insoluble. That means it is denatured and remains insoluble even when the solution gets cooled. The denaturation of the proteins of egg white by the heat process, as when boiling an egg is given as an example of irreversible denaturation. The denatured protein contains a primary structure as the native or original protein. At high temperatures, the weak forces between charged groups and the weaker forces of nonpolar groups' mutual attraction are disturbed. However, resultantly, the tertiary structure of the protein is lost. This is the protein denaturation definition.
About Denaturation
Denaturation is brought about in multiple ways. Proteins are denatured by acid or alkaline treatment, reducing or oxidizing agents, and certain organic solvents. Attractive among the denaturing agents are the ones that affect both secondary and tertiary structures without the effect on the primary structure. Most frequently, the agents used for this purpose are guanidinium chloride and urea. These molecules break the hydrogen bonds and salt bridges between the positive and negative side chains, removing the peptide chain's tertiary structure.
When the denaturing agents are removed from a protein solution, the native protein reforms in several cases, denaturation is also accomplished by reducing the disulfide bonds of cystine. It means the disulfide bond conversion (―S―S―) to the two sulfhydryl groups (―SH). This produces two cysteines, and the reoxidation of cysteines by exposure to air can sometimes regenerate the native protein. However, in the other cases, the wrong cysteines become bound to each other by resulting in a variable protein. Ultimately, denaturation is also accomplished by exposing the proteins to organic solvents such as acetone or ethanol. Organic solvents are also thought to interfere with the nonpolar group's mutual attraction.
Conformation of Proteins in Interfaces
Similar to several other substances with both hydrophobic and hydrophilic groups, the soluble proteins tend to migrate into the interface between water and oil and water and air; here, the term oil means a hydrophobic liquid such as xylene or benzene. Proteins spread within the interface form thin films. Measurements of the interfacial tension or surface tension of such films represent that the tension can be reduced by the protein film. Proteins form a monomolecular layer when forming an interfacial film.
That is a layer one molecule in terms of height. Although once it was thought that globular protein molecules unfold completely in the interface, now, it has been established that several proteins can be recovered from native state films. The lateral pressure application of protein denaturation film causes it to increase in thickness and ultimately to form a layer with a height corresponding to the native protein molecule’s diameter.
In an interface, the protein molecules, because of Brownian motions (which are called molecular vibrations), occupy more space than perform those in the film after the application of protein denaturation pressure. This Brownian motion of compressed molecules given as limited to the two dimensions of the interface since the protein molecules cannot move either upward or downward.
Classification of Proteins
Classification by Solubility
Franz Hofmeister and Emil Fischer, after two German chemists, independently stated in 1902 that proteins are importantly polypeptides consisting of several amino acids; an attempt was made to classify the proteins based on their physical and chemical properties because the biological function of the proteins had not yet been established.
Primarily, proteins were classified based on their solubility in a solvent count. However, this particular classification is no longer satisfactory because proteins having quite a different function and structure at times have the same solubilities; conversely, proteins of similar structure and function at times have variable solubilities. However, still, the terms associated with the old classification are widely used. They are defined as follows:
Classification by Biological Functions
Because the old classification is in such a bad state, it is much more preferable to classify proteins according to their biological function. However, such a type of classification is far from the ideal situation because one protein can contain more than one function. For example, the contractile protein myosin also acts as an ATPase (otherwise adenosine triphosphatase), which is a denatured enzyme that hydrolyzes adenosine triphosphate (that removes a phosphate group from the ATP by introducing the water molecule).
The other problem with the functional classification is that the protein’s definite function frequently is unknown. A protein is not called an enzyme as long as its substrate (it means the specific compound upon which it acts) is unknown. Even it cannot be tested for its enzymatic action when its substrate is unknown.
Function and Special Structure of Proteins
Despite the limitations of proteins, a functional classification can be used to explain the relationship between a protein's function and structure whenever possible. Because their structure is simpler than that of globular proteins, and their function, the maintenance of either a flexible or rigid structure, is more clearly related to their function, structural and fibrous proteins are discussed first.
FAQs on Protein Denaturation Explained: Structure and Function
1. What is meant by protein denaturation as per the CBSE syllabus?
Protein denaturation is a process where a protein loses its native secondary, tertiary, and quaternary structures due to external stress or chemical agents. This disrupts the protein's unique three-dimensional shape, causing it to lose its biological activity. Importantly, the primary structure—the sequence of amino acids held by peptide bonds—remains intact during this process.
2. What are the common agents or factors that cause protein denaturation?
Several physical and chemical agents can cause a protein to denature. The most common examples include:
- Heat: High temperatures disrupt the weak hydrogen bonds and hydrophobic interactions.
- pH Changes: Extreme acidic or alkaline conditions alter the ionic charges on amino acid side chains, breaking the salt bridges and hydrogen bonds.
- Organic Solvents: Substances like ethanol or acetone interfere with the internal hydrophobic interactions.
- Heavy Metal Ions: Ions like lead (Pb²⁺) or mercury (Hg²⁺) disrupt disulfide bonds.
- Mechanical Agitation: Vigorous shaking or stirring can physically break the delicate forces maintaining the protein's shape.
3. How does cooking an egg provide a real-world example of protein denaturation?
Cooking an egg is a classic example of heat-induced protein denaturation. The egg white is primarily composed of a protein called albumin, which is globular and soluble in its native state. When you apply heat, the energy breaks the weak bonds holding the albumin in its folded shape. The protein chains unfold and then tangle with each other, forming a solid, opaque white mass. This change from a clear liquid to a white solid is a direct result of the irreversible denaturation and subsequent coagulation of albumin.
4. What is the difference between protein denaturation and coagulation?
While related, denaturation and coagulation are two distinct steps. Denaturation is the initial unfolding of the protein's three-dimensional structure, making it lose its function. Coagulation is the subsequent process where these unfolded protein molecules aggregate or clump together to form a larger, insoluble mass. Therefore, denaturation is the cause, and coagulation is often the visible effect, like the hardening of an egg white or the curdling of milk.
5. Why is protein denaturation often an irreversible process?
Denaturation is often irreversible because the complex, specific folding of a large protein is a highly organised process. Once the delicate secondary and tertiary structures are completely disrupted, the unfolded polypeptide chains tend to get tangled and form random, stable aggregates. While some small, simple proteins can refold correctly (a process called renaturation) if the denaturing agent is gently removed, for most complex proteins, the original intricate structure is too complex to be restored spontaneously, leading to a permanent loss of function.
6. Is protein denaturation always a negative or harmful phenomenon?
No, protein denaturation is not always harmful; in fact, it is essential in some biological and practical contexts. For example, during digestion, the acidic environment of the stomach denatures the proteins in our food. This unfolding makes it easier for digestive enzymes like pepsin to access and break the peptide bonds. Similarly, sterilisation using heat works by denaturing the essential proteins of bacteria and viruses, thereby killing them.
7. How does a change in pH specifically cause a protein to denature?
A protein's tertiary structure is stabilised by specific interactions, including ionic bonds (salt bridges) between the positively charged R-groups of basic amino acids (like lysine) and negatively charged R-groups of acidic amino acids (like aspartic acid). A drastic change in pH alters the protonation state of these groups. In a highly acidic solution, the acidic groups become neutral (COOH), and in a highly alkaline solution, the basic groups become neutral (NH₂). This neutralisation disrupts the ionic bonds, causing the protein to unfold and lose its native shape.
