Question

proteins are found to have two different types of secondary structures, viz, \(\alpha\)-helix and \(\beta\)-pleated sheet structure. \(\alpha\)-helix structure of protein is stabilised by (A) Peptide bonds (B) van der Waals forces (C) Hydrogen bonds (D) Dipole-dipole interactions

Short answer

The α-helix structure of proteins is primarily stabilized by hydrogen bonds, formed between N-H groups and C=O groups of amino acids located three or four residues earlier along the protein sequence. Therefore, the correct answer is (C) Hydrogen bonds.

Step-by-step solution

Understanding the α-helix structure of proteins

The α-helix is one of the most common secondary structures found in proteins. It is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence.

Identify the forces responsible for protein stability

Proteins are held together by different types of forces and interactions, such as covalent peptide bonds, hydrogen bonds, van der Waals forces, and electrostatic (dipole-dipole) interactions. In the α-helix structure, the characteristic spiral shape is primarily formed and stabilized by specific interactions between the amino acids in the protein chain. Now, let's examine each option provided in the exercise: (A) Peptide bonds: Covalent peptide bonds join the amino acids together in the primary structure of a protein. However, they are not responsible for the stability of the α-helix structure. (B) van der Waals forces: These weak forces, resulting from the transient attraction between neighboring atoms, contribute to the overall stability of proteins but are not the primary force stabilizing the α-helix structure. (C) Hydrogen bonds: In the α-helix structure, the N-H group of each amino acid residue forms a hydrogen bond with the C=O group of another amino acid located three or four residues earlier in the sequence. This pattern of hydrogen bonding is the primary force that stabilizes the α-helix structure. (D) Dipole-dipole interactions: These interactions occur between charged or polar molecules and contribute to the overall stability of a protein but are not the primary force contributing to the α-helix structure.

Determine the correct answer

Based on the analysis and understanding of the forces responsible for the stability of the α-helix structure of proteins, the correct answer is (C) Hydrogen bonds.

Key concepts

Understanding \(\alpha\)-Helix Structure

The \(\alpha\)-helix structure is a fundamental component of protein architecture, resembling a right-handed spiral or coil. This configuration allows proteins to form a compact and stable structure, which is vital for their biological function. Each turn of the helix typically contains 3.6 amino acid residues, with the backbone N-H group of one residue forming a hydrogen bond with the C=O group of the amino acid four residues ahead. This regular pattern of hydrogen bonding contributes significantly to the helix's stability.

Interestingly, the particular spatial arrangement of the \(\alpha\)-helix also affects its physical properties; for example, it enables the formation of a dipole moment along the axis of the helix, contributing to interaction with other molecules and ions. The \(\alpha\)-helix is not rigid but exhibits a certain degree of flexibility which is essential for the dynamic function of proteins within the cells.

Significance of Hydrogen Bonds in Proteins

Hydrogen bonds are one of the strongest non-covalent interactions that contribute to the stabilization of protein secondary structures such as the \(\alpha\)-helix. These bonds occur when a hydrogen atom, covalently attached to a strongly electronegative atom like nitrogen or oxygen, experiences attraction to another electronegative atom with a lone pair of electrons.

In the case of the \(\alpha\)-helix, a hydrogen bond forms between the amide hydrogen (N-H group) and the carbonyl oxygen (C=O group) of the peptide backbone. The pattern is specific: each N-H group hydrogen-bonds to the C=O group of the amino acid located exactly four residues earlier on the chain. These bonds provide an internal scaffolding that gives the \(\alpha\)-helix its characteristic shape and resilience, allowing proteins to withstand various environmental conditions without losing their functional conformation.

Protein Stability

Protein stability refers to the ability of proteins to maintain their structure and functional state under varying physiological conditions. It's a fine balance between the intrinsic tendency of proteins to fold into their native structures and the disruptive forces that challenge their integrity, such as changes in temperature, pH levels, and the presence of denaturing agents.

A combination of bonds and interactions, including hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces, work collectively to stabilize the protein's structure. In the context of the \(\alpha\)-helix, hydrogen bonds play a pivotal role, but it's essential not to overlook the contribution of other forces which add robustness to the protein's architecture. By preserving the stability of secondary structures like the \(\alpha\)-helix, proteins are able to perform a vast array of biological functions that are critical for the survival of living organisms.