Question

In fibrous proteins, polypeptide chains are not held together by (A) van der Waals forces (B) Disulphide linkage (C) Hydrogen bonds (D) Both (B) and (C)

Short answer

In fibrous proteins, polypeptide chains are not held together by \( \boxed{\text{(B) Disulphide linkage}}. \)

Step-by-step solution

Option A: van der Waals forces

In fibrous proteins, polypeptide chains are held together by van der Waals forces, which are weak interactions between molecules due to fluctuation in their electric dipoles. Therefore, this option is incorrect.

Option B: Disulphide linkage

Disulphide linkages refer to covalent bonds formed between the sulfur atoms of two cysteine residues, holding protein molecules together. Although these covalent bonds are present in some proteins, they typically play a role in stabilizing the overall tertiary and quaternary structure. However, fibrous proteins specifically derive their structural stability mainly from non-covalent interactions. Therefore, disulphide linkages do not significantly contribute to holding polypeptide chains together in fibrous proteins.

Option C: Hydrogen bonds

In fibrous proteins, polypeptide chains are held together by hydrogen bonds. These are weak non-covalent interactions that form between polar groups, such as the amide and carbonyl groups in the protein backbone. They play a key role in stabilizing secondary structures like alpha-helix and beta-sheet found in fibrous proteins. Therefore, this option is incorrect.

Option D: Both (B) and (C)

Since disulphide linkages do not significantly contribute in holding polypeptide chains together in fibrous proteins, and hydrogen bonds do, this option is incorrect. Based on the analysis, the correct answer is: \( \boxed{\text{(B) Disulphide linkage}}. \)

Key concepts

Understanding Polypeptide Chains

Polypeptide chains are the fundamental building blocks of proteins. They are long chains of amino acids linked together by peptide bonds, which are a type of covalent bond. These chains fold and organize into specific three-dimensional structures necessary for protein function.
A polypeptide chain begins with a unique sequence of amino acids. Each amino acid contributes distinct chemical properties to the chain, influencing the way it folds and functions.

• The sequence of amino acids determines the primary structure of a protein.
• This sequence is crucial, as even a small change can affect the protein's overall shape and, consequently, its function.

In fibrous proteins, such as collagen and keratin, these polypeptide chains are often elongated and form extended structures. These provide tremendous strength and structural support to tissue. Understanding the composition and arrangement of polypeptide chains is essential to grasp how fibrous proteins achieve their specific functions.

The Role of van der Waals Forces

Van der Waals forces are weak, non-covalent interactions that occur between molecules. These forces arise from temporary dipoles created when the electrons in molecules shift momentarily. In the context of proteins, van der Waals forces can help to stabilize structures by providing subtle attractions between the polypeptide chains.

• Although weak individually, collectively they can exert a significant stabilizing effect.
• Their primary role is found in the stabilization of the tertiary and quaternary structures of proteins through the close packing of atoms.

While they play a role in the overall structural integrity of proteins, in fibrous proteins, more specific forces, such as hydrogen bonds, are primarily responsible for stabilizing the intricate arrangements of polypeptide chains.

Exploring Disulphide Linkage

Disulphide linkages refer to strong covalent bonds formed between the sulfur atoms of two cysteine residues in proteins. These bonds serve as lock-like structures that can greatly enhance the stability of a protein's tertiary and quaternary structure.

• Disulphide linkages are predominantly found in globular proteins rather than fibrous proteins.
• They play a crucial role in maintaining the shape and function of proteins under adverse conditions.

While disulphide linkages are crucial in stabilizing the overall three-dimensional structure of some proteins, they are not a prominent feature in the structural integrity of fibrous proteins like collagen or keratin. These proteins rely more on non-covalent interactions for their structural formation.

The Importance of Hydrogen Bonds

Hydrogen bonds are significant forces in the stability and integrity of fibrous proteins. These are non-covalent interactions occurring when a hydrogen atom, covalently bonded to a more electronegative atom like oxygen or nitrogen, forms a weak attraction to another electronegative atom.
In fibrous proteins, hydrogen bonds play an essential role in maintaining their secondary structure.

• In structures such as the alpha-helix and beta-sheet, hydrogen bonds form between the backbone amide and carbonyl groups.
• These bonds are crucial for providing the strength and elasticity needed in fibrous structures.

Hydrogen bonds ensure that fibrous proteins maintain their required shape, allowing them to effectively fulfill their roles in organisms, such as providing structural support and facilitating movement.