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Biomolecules - Denaturation of proteins

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Native Protein: A protein found in a biological system with a unique three-dimensional structure and specific biological activity is called a native protein.

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Denaturation Definition: When a native protein is subjected to physical change (such as change in temperature) or chemical change (such as change in pHpH), the hydrogen bonds are disturbed, leading to the unfolding of globules and uncoiling of helixes.

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Structural Impact: During denaturation, secondary (2∘2^{\circ}) and tertiary (3∘3^{\circ}) structures are destroyed, but the primary (1∘1^{\circ}) structure remains intact.

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Biological Activity: The most significant consequence of denaturation is the loss of biological activity of the protein (e.g., enzymes losing their catalytic power).

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Examples of Denaturation: Coagulation of egg white (albumin) on boiling and the curdling of milk caused by the action of bacteria forming lactic acid.

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Chemical Agents: Denaturation can be caused by salts of heavy metals, concentrated acids, bases, or organic solvents like ethanol and acetone.

📐Formulae

Native Protein→pH changeΔ (Heat)Denatured ProteinNative \ Protein \xrightarrow[pH \text{ change}]{\Delta \text{ (Heat)}} Denatured \ Protein

Primary Structure remains: −NH−CH(R)−CO−NH−CH(R′)−CO−\text{Primary Structure remains: } -NH-CH(R)-CO-NH-CH(R')-CO-

C3H6O3 (Lactic acid formation during curdling)C_3H_6O_3 \text{ (Lactic acid formation during curdling)}

💡Examples

Problem 1:

Why is the primary structure of a protein not affected during the process of denaturation?

Solution:

Denaturation involves the disruption of relatively weak bonds like hydrogen bonds, disulfide bridges, and hydrophobic interactions that maintain the 2∘2^{\circ} and 3∘3^{\circ} structures. The primary structure is held together by strong covalent peptide bonds (−CONH−-CONH-), which are not broken under the conditions of denaturation.

Explanation:

Covalent bonds require much higher energy (chemical hydrolysis) to break compared to the physical/chemical stresses that cause denaturation.

Problem 2:

Explain the coagulation of egg white in terms of protein denaturation.

Solution:

Egg white contains a globular protein called albumin. Upon heating, the kinetic energy increases, breaking the hydrogen bonds and S−SS-S linkages. The protein chains unfold and then randomly coagulate into a solid, insoluble fibrous mass.

Explanation:

This is an example of irreversible denaturation where the water-soluble globular protein transforms into an insoluble solid.

Problem 3:

What happens to the pHpH of milk during curdling and how does it affect the proteins?

Solution:

Bacteria present in milk convert lactose into lactic acid (CH3CH(OH)COOHCH_3CH(OH)COOH). This lowers the pHpH, causing the denaturation and precipitation of the milk protein, casein.

Explanation:

Change in pHpH alters the charge on the side chains of amino acids, disrupting ionic attractions and hydrogen bonding in the protein's native state.

Denaturation of proteins - Revision Notes & Key Formulas | ICSE Class 12 Chemistry