Describe alpha helix and tertiary structure of protein. (IFS 2023, 10 Marks)

Introduction

Proteins are essential macromolecules in living organisms, performing a wide range of functions such as catalyzing biochemical reactions, providing structural support, and facilitating communication between cells. The structure of a protein is crucial for its function, and two important levels of protein structure are the alpha helix and tertiary structure.

Alpha Helix

• The alpha helix is a common structural motif found in proteins. It is a right-handed coiled structure stabilized by hydrogen bonds.
• Formation: It consists of a spiral of amino acids, where the carbonyl oxygen of one amino acid forms a hydrogen bond with the amide hydrogen of the fourth amino acid, creating a helical shape.
• Key Features:
  
  • Each turn of the helix contains approximately 3.6 amino acids.
  • The helix has 3.6 residues per turn, resulting in a periodic structure.
  • The side chains of the amino acids extend outward from the helix, while the backbone is in the interior.
• Stability: The alpha helix is stabilized by intramolecular hydrogen bonds, making it a stable and regular secondary structure in proteins.
• Occurrence: Common in globular proteins and membrane proteins.

Tertiary Structure of protein:

• The tertiary structure refers to the three-dimensional folding of a polypeptide chain, formed by interactions between the side chains (R groups) of the amino acids.
• Types of Interactions Involved:
  
  • Hydrogen Bonds: Between polar side chains.
  • Ionic Bonds: Between positively and negatively charged side chains.
  • Hydrophobic Interactions: Non-polar side chains tend to cluster in the interior of the protein, away from water.
  • Disulfide Bridges: Covalent bonds between sulfur atoms of cysteine residues, stabilizing the protein.
  • Van der Waals Forces: Weak, non-covalent interactions between atoms.
• Significance:
  
  • The tertiary structure is responsible for the unique function of each protein.
  • It determines the protein's active sites and interaction with other molecules.
  • It also governs the protein's stability and folding patterns.
• Globular vs. Fibrous Proteins:
  
  • Globular proteins have a compact, spherical tertiary structure.
  • Fibrous proteins, like collagen, have elongated, fibrous tertiary structures.

Conclusion

The alpha helix and tertiary structure of a protein are important aspects of its overall structure and function. The intricate folding and interactions that occur at the alpha helix and tertiary structure levels contribute to the diversity and complexity of proteins in living organisms.