23. Following are statements on ß-turns:
A. All the 20 coded amino acids have equal propensity to form ß -turns.
B. Pro cannot occur in ß-turns.
C. Pro-Gly sequence strongly favours ß-turns.
D. In Asn-Gly ß-turns, Asn can have positive ϕ,Ψ values.
Choose the combination with all correct statements
(1) B, D (2) A, C
(3) A, D (4) C, D

Understanding Beta-Turns in Proteins: Structure, Characteristics, and Key Amino Acids

Introduction

Beta-turns (β-turns) are critical structural motifs in proteins that facilitate compact folding by reversing the direction of the polypeptide chain. They play a significant role in protein stability and function. This article explores the nature of β-turns, their preferred amino acids, and answers a key multiple-choice question regarding β-turn formation.

What are Beta-Turns?

β-turns are short, four-residue motifs that connect two antiparallel β-strands, allowing the polypeptide chain to form a loop-like structure. They are stabilized by hydrogen bonds between the carbonyl oxygen of the first residue and the amide hydrogen of the fourth residue in the turn.

Types of Beta-Turns

Beta-turns are classified into different types based on the dihedral angles (ϕ, Ψ) of the second and third residues:

• Type I: Most common, featuring a left-handed turn.
• Type II: Often contains Glycine at the third position due to steric constraints.
• Type III: Similar to 3₁₀ helices, with different backbone angles.
• Type IV: Unspecified dihedral angles, making them variable.

Amino Acid Propensities in Beta-Turns

Not all amino acids contribute equally to β-turn formation. Some residues have a higher propensity due to their size, flexibility, and ability to form stabilizing interactions.

Common Amino Acids in Beta-Turns

• Proline (Pro): Often found at the second position due to its rigid cyclic structure, which naturally induces a turn.
• Glycine (Gly): Frequently found in β-turns due to its small size and high flexibility.
• Asparagine (Asn) and Serine (Ser): Frequently present in β-turns due to their ability to form hydrogen bonds.

Analysis of the Given Statements on Beta-Turns

The question provides four statements about β-turns:

Statement A: All 20 amino acids have equal propensity to form β-turns.

• This is incorrect. Different amino acids have varying tendencies to be present in β-turns, with Proline and Glycine having a higher preference compared to bulky or hydrophobic residues like Valine or Isoleucine.

Statement B: Proline cannot occur in β-turns.

• This is incorrect. Proline is one of the most favored residues in β-turns, especially at the second position, as it naturally promotes a turn due to its rigid pyrrolidine ring.

Statement C: Pro-Gly sequence strongly favors β-turns.

• This is correct. The Pro-Gly combination is highly favorable for β-turn formation because Proline induces a turn, while Glycine provides the necessary flexibility to accommodate sharp bends.

Statement D: In Asn-Gly β-turns, Asn can have positive ϕ, Ψ values.

• This is correct. In certain β-turns, the backbone dihedral angles (ϕ, Ψ) of Asparagine can adopt positive values, making Asn-Gly a favorable sequence for β-turn formation.

Correct Answer: (4) C, D

Importance of Beta-Turns in Protein Function

Beta-turns are essential for:

• Protein folding and stability: They help in compacting the protein structure.
• Molecular recognition: Found in active sites of enzymes and binding motifs.
• Protein-protein interactions: Involved in antigen-antibody recognition and signaling pathways.

Experimental Methods to Identify Beta-Turns

1. X-ray Crystallography – Provides high-resolution structures of proteins to locate β-turns.
2. Nuclear Magnetic Resonance (NMR) Spectroscopy – Identifies β-turns in solution.
3. Circular Dichroism (CD) Spectroscopy – Detects the presence of secondary structures including β-turns.

Conclusion

Beta-turns are crucial for protein structure and function. Among the given statements, C and D are correct, emphasizing that the Pro-Gly sequence strongly favors β-turns and that Asn in Asn-Gly β-turns can exhibit positive dihedral angles. Understanding β-turn formation aids in protein engineering, drug design, and molecular biology research.