Proteins

🧬 Protein Structure & Dynamics: Ultimate Notes

High-Yield Concepts for CSIR-NET | GATE | DBT-BET

1. The Ramachandran Plot

Developed by G.N. Ramachandran, this plot visualizes the energetically allowed regions for the backbone dihedral angles φ (Phi) and ψ (Psi) of amino acid residues in protein structure.

🔥 CSIR-NET / GATE Exam Points

• Glycine: Due to its lack of a bulky side chain (R=H), Glycine has the broadest range of allowed angles. It is frequently found outside the colored regions.
• Proline: The cyclic side chain restricts rotation around the N-Cα bond. Its φ angle is rigidly fixed at approximately -60°.
• Right-handed α-helix: Found in the 3rd quadrant (-φ, -ψ). β-sheets: Found in the 2nd quadrant (-φ, +ψ).

2. Secondary Structure of Proteins

Secondary structures are locally folded regions stabilized by hydrogen bonds between the backbone carbonyl oxygen (C=O) and amide nitrogen (N-H).

α-Helix

• Residues per turn: 3.6
• Pitch (rise per turn): 5.4 Å
• Hydrogen Bonding: Occurs between residue n and n+4. All H-bonds are parallel to the helix axis.
• Helix Breakers: Proline (causes a kink) and Glycine (too flexible).

β-Sheets

• Structure: Formed by laterally packed β-strands.
• Antiparallel Sheets: Strands run in opposite directions. H-bonds are linear and extremely stable.
• Parallel Sheets: Strands run in the same direction. H-bonds are bent and slightly weaker.

3. Super Secondary Structure (Motif)

A recognizable combination of specific secondary structures. Motifs are not independently stable if removed from the protein.

• β-α-β Motif: Connects two parallel beta strands.
• Zinc Finger Motif: Binds a Zinc ion; critical for DNA binding transcription factors.
• Helix-Turn-Helix: Another classic DNA binding motif.

4. Domain & Fold

• Domain: A distinct, compact structural unit of a protein that is independently stable and can fold by itself. Different domains usually carry out specific distinct functions (e.g., binding domain vs. catalytic domain).
• Fold: Refers to the overall 3D arrangement and topological connection of secondary structures in a domain (e.g., Rossmann fold, TIM barrel).

5. Myoglobin & Haemoglobin (Oxygen Dynamics)

Property	Myoglobin (Mb)	Haemoglobin (Hb)
Structure	Monomer (1 heme)	Tetramer (α₂β₂) (4 hemes)
Function	Oxygen Storage (Muscle)	Oxygen Transport (Blood)
Binding Curve	Hyperbolic	Sigmoidal (Allosteric)

🔥 Allosteric Modulators (The Bohr Effect)

High CO₂, High H⁺ (Low pH), and High 2,3-BPG all stabilize the T-state (Tense state) of Haemoglobin, effectively lowering its affinity for oxygen and forcing it to release O₂ into actively respiring tissues. This shifts the sigmoidal curve to the RIGHT.

6. Fibrous Proteins: Keratin & Collagen

α-Keratin

• Found in hair, nails, and outer skin.
• Structure consists of a coiled-coil of two right-handed α-helices wrapping around each other in a left-handed supertwist.
• Rich in hydrophobic amino acids.
• Strength is governed by the number of disulfide bonds (Cys-Cys linkages) between polypeptide chains.

Collagen

• The most abundant protein in mammals (tendons, cartilage, bone matrix).
• Structure: A Right-handed triple helix formed by three Left-handed individual helical chains.
• Primary sequence has a strict repeat: Gly-X-Y (where X is often Proline, and Y is often Hydroxyproline). Glycine is essential at every 3rd position due to limited space in the center of the triple helix.

⚠️ Exam Trap: Vitamin C (Ascorbate) is required as a cofactor for Prolyl Hydroxylase. Deficiency leads to under-hydroxylated collagen, causing Scurvy.