Protein Stability & Kinetics

🧬 Protein Dynamics & Enzyme Kinetics

Comprehensive Notes for CSIR-NET | GATE | DBT-BET

1. Protein Folding & Stability

Protein folding is the physical process by which a linear polypeptide folds into its characteristic and functional 3D structure (native state). Anfinsen’s Experiment proved that the primary amino acid sequence dictates the 3D folded structure.

• Hydrophobic Collapse: The primary driving force for folding. Non-polar side chains pack into the interior to avoid water, maximizing the entropy of the surrounding water molecules.
• Levinthal's Paradox: States that if a protein tried every possible conformation, it would take longer than the age of the universe to fold. Thus, folding is not random; it follows highly directed folding pathways.
• Molecular Chaperones: Proteins like Hsp70 and Chaperonins (GroEL-GroES) assist in the folding of other proteins and prevent misfolding/aggregation by binding to exposed hydrophobic regions and using ATP.

Forces Stabilizing Proteins

1. Hydrophobic Interactions: Major contributor to native state stability.
2. Hydrogen Bonds: Dictate secondary structure (α-helices, β-sheets).
3. Ionic Interactions (Salt Bridges): Between oppositely charged R-groups (e.g., Aspartate and Lysine).
4. Disulfide Bonds: Covalent bonds between Cysteine residues; highly stabilizing in extracellular proteins.

2. Solubility of Proteins

Protein solubility depends on the distribution of hydrophilic and hydrophobic residues on its surface, as well as the surrounding pH and salt concentration.

• Minimum Solubility at pI: A protein is least soluble at its isoelectric point (pI). Because the net charge is zero, electrostatic repulsion between protein molecules is lost, causing them to aggregate and precipitate out of solution (Isoelectric Precipitation).
• Salting In: At low salt concentrations, added ions shield protein molecules from each other, increasing solubility.
• Salting Out: At very high salt concentrations (e.g., Ammonium Sulfate), water molecules are stripped away from the protein to hydrate the salt ions. Exposed hydrophobic patches cause the protein to precipitate.

3. Chemical Kinetics & Order of Reaction

Chemical kinetics is the study of reaction rates. The Order of Reaction defines how the reaction rate depends on the concentration of the reactants.

Zero Order Reaction

The rate is entirely independent of reactant concentration.

Rate = k[A]⁰ = k

Half-life (t_1/2): Directly proportional to initial concentration [A]₀ / 2k.
(e.g., Enzyme catalysis at Vmax).

First Order Reaction

The rate is directly proportional to the concentration of one reactant.

Rate = k[A]¹

Half-life (t_1/2): Independent of initial concentration (0.693 / k).
(e.g., Radioactive decay).

4. What are Biocatalysts?

Biocatalysts are biological molecules that speed up biochemical reactions without being consumed. The vast majority are Enzymes (proteins), but some are Ribozymes (catalytic RNA).

Critical Concept: Enzymes increase reaction rates by lowering the Activation Energy (ΔG^‡) required to reach the transition state. They DO NOT change the overall free energy of the reaction (ΔG) or the reaction equilibrium constant (K_eq).

5. Factors Affecting Enzyme Activity

• 1. Temperature: Reaction rates double for every 10°C rise (Q10 rule) up to an optimum temperature. Beyond the optimum, the enzyme undergoes thermal denaturation, and activity rapidly drops to zero.
• 2. pH: Enzymes have an optimum pH where their catalytic residues are perfectly ionized (e.g., Pepsin at pH 2.0, Trypsin at pH 8.0). Plotting Activity vs. pH yields a bell-shaped curve. Extreme pH causes denaturation.
• 3. Substrate Concentration [S]: At low [S], activity increases linearly (First Order). As [S] becomes very high, all active sites become saturated, and the reaction hits a maximum velocity (V_max), becoming Zero Order with respect to the substrate. This creates the classic Michaelis-Menten hyperbolic curve.
• 4. Inhibitors & Activators: Molecules that bind to the enzyme and either decrease (Competitive, Non-competitive inhibitors) or increase (Allosteric activators) the catalytic rate.