Structure-Function Relationships

Protein Architecture - Building Blocks Basics

• Primary (1°): Linear amino acid sequence; peptide bonds. Dictates higher structures.
• Secondary (2°): Local folding.
  • α-helix: Intra-chain H-bonds.
  • β-pleated sheet: Inter/intra-chain H-bonds.
  • Ramachandran plot: Defines allowed $ \phi $ and $ \psi $ angles.
• Tertiary (3°): Single polypeptide 3D shape. Stabilized by disulfide bonds, hydrophobic interactions, H-bonds, ionic bonds. Forms functional domains.
• Quaternary (4°): Assembly of >1 polypeptide subunits (e.g., Hemoglobin $ \alpha_2\beta_2 $).

⭐ Proline, with its rigid cyclic structure, acts as a 'helix breaker' in α-helices and is often found in β-turns.

Folding & Friends - Shaping Up Proteins

• Anfinsen's dogma: Primary structure (amino acid sequence) dictates 3D conformation.
• Driving forces for folding:
  • Hydrophobic effect (major): Nonpolar residues buried.
  • Hydrogen bonds, ionic interactions, van der Waals forces.
• Molecular chaperones (e.g., Hsp60, Hsp70): Assist correct folding, prevent aggregation; some require ATP.
• Denaturation: Loss of 2°, 3°, 4° structure & function. Causes: heat, extreme pH, organic solvents, urea.

⭐ Christian Anfinsen's ribonuclease A experiment: Primary structure dictates tertiary structure.

Functional Forms - Proteins At Work

• Hemoglobin (Hb) & Myoglobin (Mb):
  • Mb: Monomer, O2 storage, hyperbolic O2 binding curve.
  • Hb: Tetramer (adult HbA is $\alpha_2\beta_2$), O2 transport, sigmoidal O2 binding curve (cooperative binding).
  • 
• Allosteric Regulation of Hb:
  • O2: Positive homotropic effector.
  • Heterotropic effectors (↓O2 affinity, stabilize T-state):
    • H+ (Bohr effect: ↓pH → ↓O2 affinity).
    • CO2 (forms carbaminohemoglobin, contributes to Bohr).
    • 2,3-BPG (stabilizes T-state).
  • 📌 Mnemonic: 'CADET, face Right' for factors shifting O2-Hb curve right (↑CO2, ↑Acid/↓pH, ↑2,3-BPG, ↑Exercise, ↑Temperature).

• ⭐ Fetal hemoglobin (HbF, with alpha-2 gamma-2 chains) has higher O2 affinity than adult hemoglobin (HbA, with alpha-2 beta-2 chains) as it binds 2,3-BPG less avidly.

• Enzymes:
  • Active site (binding & catalytic sites) dictates substrate specificity (Lock-and-key vs. Induced fit) and catalytic efficiency by lowering activation energy ($E_a$).
• Collagen:
  • Unique triple helix (Gly-X-Y repeats; X=Pro, Y=Hyp/Hyl).
  • Post-translational modifications (hydroxylation requiring Vit C, glycosylation) & cross-linking are vital for tensile strength.
  • Defects: Scurvy (Vit C deficiency), Osteogenesis Imperfecta.
• Immunoglobulins (Antibodies):
  • Modular: Variable regions (Fab, antigen binding) & Constant regions (Fc, effector functions).
  • Composed of heavy & light chains linked by disulfide bonds.
  • 

Misfits & Maladies - When Proteins Go Rogue

• Protein misfolding: Can cause loss of normal function or gain of toxic function, often leading to aggregation (e.g., amyloid fibrils) and deposition.
• Key Proteinopathies & Associated Proteins:
  • Alzheimer's Disease: Extracellular plaques of Amyloid-β (Aβ) (from APP); intracellular neurofibrillary tangles of hyperphosphorylated Tau protein.
  • Parkinson's Disease: Intraneuronal aggregates of α-synuclein (Lewy bodies).
  • Prion Diseases (e.g., CJD, Kuru): Conformational change of PrPC to pathogenic PrPSc; infectious, induces misfolding.
• Cellular defense: Chaperones attempt refolding; Ubiquitin-Proteasome System degrades misfolded proteins.

⭐ Prion diseases are unique as they can be infectious (e.g., Kuru), sporadic (e.g., sporadic CJD), or inherited (e.g., familial CJD), all involving PrPC → PrPSc conversion.

High‑Yield Points - ⚡ Biggest Takeaways

• Primary structure (amino acid sequence) dictates 3D conformation and function.
• Secondary structures (e.g., α-helix, β-sheet) are stabilized by backbone hydrogen bonds.
• Tertiary structure forms the active site; stabilized by disulfide bonds, hydrophobic interactions, and ionic bonds.
• Quaternary structure (e.g., Hemoglobin) involves multiple interacting polypeptide subunits.
• Protein misfolding leads to diseases like Alzheimer's disease, Parkinson's disease, and prion diseases.
• Chaperones (e.g., HSPs) assist in correct protein folding and prevent aggregation.