Denaturation of Proteins, Enzymes

A number of naturally occurring proteins have specific characteristics with respect to amino acid sequence and composition that give a unique 3-D shape to protein. Proteins are chains of long amino acids and have high molecular mass. Like carbohydrates and lipids, proteins cannot be categorized on the basis of similar structures. 

Proteins have primary, secondary, tertiary and quaternary structures.

The primary structure is a linear structure. The secondary, tertiary and quaternary structures are folded structures with disulphide bonds.

Primary Structure of the Protein

Analytical procedures revealed that the primary structure of the protein is the sequence of the peptide bonds. With respect to the arrangement of amino acids in the primary structure of the protein, amino acids are present in a sequence of long chains like a long thread or are irregularly coiled.

Secondary Structure of the Protein

The peptide bond in this structure of protein assumes a helical shape asymmetrically. Such structural features are the secondary structure of the protein.

The Tertiary Structure of a Protein

The tertiary structure of a protein is formed as a result of interaction between side chains (R-group) of amino acids. Tertiary structure is formed by further folding of the secondary structure of a protein. 

Quaternary Structure of a Protein

Two or more peptide chains are linked together and form a quaternary structure.

Denaturation of Proteins

Proteins are highly organized 3-D structures and these structures are delicate. Denaturation is a process by which a protein loses its native shape due to disruption in its chemical bonds and hence becomes inactive.

Any change in the 3-D structure of protein makes it incapable of functioning and hence the protein becomes denatured. Denaturation involves disruption of both the secondary and tertiary structures. Disruption breaks peptide bonds while the primary structure remains the same after denaturation. The alpha-helix and beta-sheets of protein are disrupted due to bonding disruptions. In tertiary structure, all four bonds including hydrogen bond, salt bridge, disulfide bond, and non-polar hydrophobic interactions may be disrupted due to denaturation.

A number of reagents and factors can cause denaturation. When a protein denatures the cell undergoes a series of changes, these are loosening and then tightening 

Causes of Denaturation of Protein

A. Physical agents-

1. Heat- Most proteins can easily be denatured by heat by affecting the weak interactions mainly hydrogen bonds.

2. Violent shaking- This can be easily seen when we constantly churn the egg whites.

3. Hydrostatic pressure- Pressure ranging from 5,000-10,000 atm destabilizes the hydrophobic interactions and hence protein gets denatured.

4. UV Radiation- Radiation supplies kinetic energy to molecules causing atoms to vibrate and resulting in the weakening of hydrogen bonds.

B. Chemical agents-

1. Acids and alkalis- This disrupts salt bridges by weakening ionic interactions. This reaction can be easily seen in the digestive system when gastric juice coagulates milk. Acetic acid, sulfosalicylic acid is an acidic protein denaturing chemical reagents whereas sodium bicarbonate acts as a base denaturing reagent.

2. Organic solvents- solvents like ether and alcohol disrupt hydrogen bonds in the molecule.

3. Salts of heavy metals- Heavy metals like Hg+2, Pb+2, Ag+1, Tl+1, Cd+2 disrupt salt bridges in proteins as well as disulfide bonds present in the molecule.

4. Chaotropic agents include urea, lithium perchlorate, and guanidinium chloride. These precipitate soluble proteins in the chemical mixture.

5. Detergents- Acts as amphiphilic molecules. Dissociates non-polar R group which leads to the unfolding of the protein. 

 6. Disulfide bond reducers- This includes 2-mercaptoethanol, and dithiothreitol.

Process of Denaturation of Protein

• The secondary, tertiary, and quaternary structures of a protein can easily be denatured by a process called denaturation

• This changes the native structure of a protein

• Heat, reaction with acids or bases, and even violent shaking can cause denaturation

• The albumin protein in egg white can easily be denatured by heating

• The same thing can be accomplished by violent shaking

• Heavy metal poisoning such as lead (Pb) and cadmium (Cd) causes a change in the structure of proteins by binding to functional groups on the protein surface

• Denaturation of proteins can be done by physical changes and reactions with chemicals

• Mainly denaturation processes are irreversible, but in some, it is reversed and called renaturation of protein

• Denaturation occurs due to a change in temperature

• The Curdling of milk is another example of the denaturation of proteins

Denaturation of Enzymes

Enzymes are molecules that act as catalysts and help in speeding up biological reactions. Most of the enzymes act as proteins. Without enzymes, the chemical reactions will not be happening at the same rate as now. Enzymes have a specific region called the active site where substrates bind for the catalysis and hence a complex is formed called enzyme-substrate complex. Enzymes are specific in their action. The lock and Key model and Induced Fit model explain the action of enzymes. In the lock and key model, the substrate fixes itself in the active site of the enzyme whereas, in an induced-fit model of enzyme action, the enzyme changes the shape so that the substrate can fix with the active site for further catalysis. Change in the active site of enzyme results in non-functioning of the enzyme or enzyme is denatured and hence as a result of which no end product is obtained.

Cause of Denaturation of Enzymes

Factors causing the denaturation of enzymes include:

1. Temperature- Enzyme denatures at high temperature. High temperature breaks disulfide bonds of the tertiary structure of protein and masks the active site of enzymes. 

2. pH- Extremes of pH denatures enzymes.

3. Co-factors- Cofactors alter the shape of enzymes making it unavailable for the substrate to attach itself. E.g.- Na+, K+ 

Conclusion 

Proteins are categorized into fibrous protein, insoluble in water, and globular protein, soluble in water. Protein has 4 levels of structure. The primary structure is the linear arrangement of amino acids. The secondary structure consists of an alpha-helix and beta-pleated sheet. Further folding of the secondary structure of protein results in the tertiary structure of protein giving helical structure. The quaternary structure is characterized by ionic bonds, hydrogen bonds, disulfide bonds, and dispersion forces. A number of physical factors and chemical reagents cause the denaturation of proteins and enzymes. All enzymes are proteins but all proteins are not enzymes. Enzymes help to speed up the chemical reaction happening in the living system. Enzymes lower the activation energy of the chemical reaction and hence the product is obtained easily. Enzymes act as only a catalyst and it is not used up in the reaction. Enzymes can be reused for the same chemical reaction.