Denaturation of Proteins and Its Causes

Introduction

Protein is one of the very important fundamental units of our body system which is supplied to our body through different food products such as milk, cheese, nuts, etc. Denaturation involves the breaking of many weak bonds in the structure of a protein. These weak bonds or linkages like hydrogen bonding provide an ordered structure to a protein molecule. The changes that occur during denaturation in the structure of protein generally result in alterations in properties like functional properties and chemical and physical properties. The process of denaturation of proteins occurs under different chemical conditions. Changes in pH, temperature and other chemical activity disturb the weak linkages present in the structure of the protein due to which the globular protein structure gets unfolded and there is uncoiling of the helix structure.

What is the denaturation of proteins?

Denaturation is a biochemical process that can be defined as any variation in the secondary structure, tertiary structure, or quaternary structure of a protein molecule that destroys covalent bonds. A folded protein consists of a balance between weak intramolecular interactions which is present in a protein i.e., electrostatic, hydrophobic, and Van Der Waals Interactions and interaction between protein and solvent within a protein molecule. When a protein structure is denatured, in general, only the secondary and tertiary protein structures are altered in contrast the peptide bonds formed between the amino acids present in the primary structure remain intact. When a normal protein is subjected to a temperature change, the protein structure unfolds which leads to a single strand structure of amino acids.

Causes of denaturation of proteins

A protein's denaturation is defined as a structural alteration that generally causes it to permanently lose its biological properties. Different physical and chemical conditions can cause the denaturation of proteins. Some of the main reasons are listed below −

• The destruction of the weak linkages such as hydrogen bonds − This weak interaction present in the structure of protein gets disturbed when we try to change the temperature, pH, or other chemical activities. This destroys the ordered structure of a protein.

• High levels of thermal energy supplied − This can disrupt the hydrogen bonding by coagulation of protein when heated to its isoelectric point.

• Change in the pH value − This will alter the charge value of the protein due to the unfolding of the structure, which again will alter protein solubility.

• Addition of organic solvents − Organic solvents like alcohol or acetone act like denaturing agents and are used to increase the dissolvability of the denatured protein.

• Effect of pressure − The hydrophobic interaction of the protein starts to weaken at a very high pressure which leads to denaturation of the protein.

Process of denaturation of the protein

The denatured state of a protein is very important to understanding the unfolding process of protein. Any chemical or physical change leads to the denaturation of protein. For example, if we boil an egg, the heat supplied here disrupts the protein molecular structure. As a result, the liquid substance present is converted to a semi-solid substance. Similarly, when we beat an egg, due to the change in kinetic energy, the protein’s molecular structure again gets disrupted. In general, the denaturation process cannot be reversed. However, the process can be reversed only in certain exceptional cases, this is the renaturation of proteins.

The structure of the protein

Protein’s structure is much more complex. They are macromolecules and have four different types of structure. They are- Primary, Secondary, Tertiary and Quaternary structures.

• Primary structure − An amino acid consists of an amino group and a carboxyl group. Since two functional groups are present in individual amino acids, they can make a long chain by forming peptide bonds. The peptide bond formed here is a bond between the -NH_2 group of one amino acid and the -COOH of another amino acid. If at the end of the peptide chain in a protein molecule we have a free carboxyl group, it is called C-terminus or carboxy-terminus protein and if we have a free amino group then it is N-terminus or amino-terminus protein. A total of 20 different α-amino acids are used for the construction of a protein structure. The name of an amino acid used to be written in abbreviation form which simplifies the writing sequence of a protein. Hence,

  • The primary structure of proteins reveals the precise arrangement of the amino acids that link together to form peptide chains.

  • Several polypeptide chains join to form protein structures. The amino acids are organized in these polypeptide chains in a specific sequence that is unique to the protein. A change in the sequence of amino acids in the chain changes the entire protein.

• Secondary structure − This secondary structure of a protein is not a simple chain of polypeptides, is of two types, the α-helix, and the ß-sheet structure.

  • The α-helix structure − In this coiled strand structure, the substituents present in the side chain of the amino acid groups are extended to the outside. In this case, the hydrogen bonds are formed when the peptide chain is twisted into a right-handed strand where the -NH group of an amino acid is forming a hydrogen bond with the -CO group of the adjacent part of the helix.

  • The ß-sheet structure − The polypeptide chains in this instance are spread out next to one another and then joined by hydrogen bonds. The hydrogen bond formed in a ß-sheet structure is between the two strands. It is an inter-strand linking, not an intra-strand. Here the sheet structure consists of two strands that are next to each other. This time, the amino group of the neighbouring strand and the carbonyl group of one strand establish hydrogen bonds.

• Tertiary structure − In this arrangement, to gain maximum stability there will be bending, and twisting of the structure of protein molecules. The hydrogen bonds formed here are between groups present in the sidechain. Like disulphide bridges, these hydrogen bonds can connect two chain segments that are apart in terms of sequencing. The ionic interactions between negatively and positively charged sites present on side chains of amino acid and the formation of disulphide bridge help us to stabilize a protein's tertiary structure.

• Quaternary structure − The protein structure is made up of several polypeptide chains called protein subunits. In a homodimer, the subunits part may be the same and in a heterodimer these parts are different. The quaternary structure deals with the interaction of protein subunits with each other and their arrangement to form a large aggregate and complex protein structure. Salt bridges, disulfide bridges, and hydrogen bonds all work together to maintain this complex structure of the protein.

Conclusion

In the native state, protein structures are folded into well-defined and possess three-dimensional structures. The changes that occur during denaturation in the structure of protein generally result in alterations in properties like functional properties and chemical, and physical properties. Only some proteins can have their original structure recovered by removing the denaturing agent and re-establishing conditions that are conducive to the protein's native state. There are four different levels of structure of proteins- primary, secondary, tertiary, and quaternary protein structure.

FAQs

1. How is the process of denaturation of proteins important to human beings?

In the process of digestion, the acid HCl in the stomach helps in breaking down the protein component of the food which helps the body to consume the nutrition easily. Without breaking the complex secondary and tertiary structure of the protein, digestion would have been very difficult.

2. What is the process of protein folding?

The process through which the protein structure takes on its useful shape is known as folding. A protein can only coil and fold into a specific shape to carry out its biological tasks.

3. Is DNA a protein?

DNA itself is not a protein but contains a protein molecule in the nucleus called Histone.

4. What are the types of proteins?

The seven types of proteins are hormonal proteins, structural proteins, contractile proteins, storage proteins, transport proteins, enzymes, and antibiotics.

5. Where can we find proteins?

In our daily life, we consume proteins in form of food such as Seeds, nuts, meat and fish, beans and lentils, dairy products, eggs, etc.