8.4 Folding Thermodynamics, Haemoglobin Cooperativity & Denaturation
Key Takeaways
The thermodynamic driving force for tertiary folding is hydrophobic collapse, wherein nonpolar side chains coalesce in the protein core, releasing ordered clathrate water cages to increase solvent entropy (Delta S_solvent >> 0) and overcome the unfavorable loss of polypeptide conformational entropy (Delta S_chain < 0).
Denaturation disrupts secondary, tertiary, and quaternary structures without cleaving primary covalent peptide bonds; physical and chemical denaturants operate via distinct mechanisms: heat disrupts weak non-covalent bonds, urea and guanidinium disrupt hydrogen bonding, SDS disrupts hydrophobic cores and imparts uniform negative charge, extreme pH alters side-chain ionization and salt bridges, and reducing agents (beta-mercaptoethanol, DTT) cleave covalent disulfide bonds.
Quaternary cooperativity produces haemoglobin's sigmoidal oxygen-binding curve because oxygen binding shifts subunits from the low-affinity T state toward the high-affinity R state, whereas monomeric myoglobin has no subunit interface and therefore binds oxygen along a simple hyperbolic curve.
8.4 Folding Thermodynamics, Haemoglobin Cooperativity & Denaturation
Protein Folding Thermodynamics: The Hydrophobic Effect and Folding Funnel
Protein folding is a spontaneous thermodynamic process (ΔG_folding < 0) governed by the Gibbs free energy equation:
ΔG_folding = ΔH_folding - T ΔS_folding
The Entropic Paradox and Hydrophobic Collapse
At first glance, folding a disordered, highly flexible polypeptide chain into a single, highly ordered native conformation represents an immense loss of conformational entropy of the chain (ΔS_chain << 0), which strongly opposes folding.
- The Solvent Entropy Solution (Hydrophobic Effect): In an unfolded protein, nonpolar hydrophobic side chains are exposed to the surrounding aqueous solvent. Water molecules cannot form hydrogen bonds with nonpolar hydrocarbons. To maximize water-water hydrogen bonding, surrounding water molecules are forced to order themselves into rigid, ice-like cage structures known as clathrate cages surrounding each hydrophobic patch. This ordering represents an immense entropic penalty for the solvent.
- When nonpolar residues collapse into the interior core of the protein (hydrophobic collapse), the clathrate cages collapse, releasing hundreds of ordered water molecules into the bulk solution. This release produces a massive increase in solvent entropy (ΔS_solvent >> 0).
- Because ΔS_solvent far exceeds |ΔS_chain|, the net entropy change of the system is positive (ΔS_total > 0). When multiplied by temperature (-T ΔS < 0), this entropic gain drives folding spontaneously.
Enthalpic Contributions (ΔH)
Non-covalent bonds (salt bridges, hydrogen bonds, van der Waals contacts) formed within the folded protein release heat, yielding a negative enthalpy of folding (ΔH < 0). However, this is partially offset by the enthalpy required to break existing hydrogen bonds between the unfolded polypeptide and water.
The Folding Funnel Energy Landscape
Protein folding does not occur through random sampling of all possible conformations (which would take billions of years, a conundrum known as Levinthal's paradox). Instead, proteins fold along a rugged, convergent thermodynamic folding funnel:
- The wide top of the funnel represents high entropy and high free energy (unfolded conformational ensemble).
- Rapid hydrophobic collapse drives the protein into a partially structured molten globule intermediate containing secondary structure elements but lacking tight tertiary packing.
- The protein funnels downward through local energetic minima, assisted by molecular chaperones (e.g., Hsp70, GroEL/GroES) that prevent aggregation, until it reaches the deep global free energy minimum at the bottom of the funnel: the native conformation.
Quaternary Cooperativity: Hemoglobin vs. Myoglobin
The physiological distinction between the monomeric storage protein myoglobin and the tetrameric transport protein hemoglobin illustrates quaternary cooperativity.
| Characteristic | Myoglobin (Mb) | Hemoglobin (Hb) |
|---|---|---|
| Subunit Composition | Monomer (single polypeptide chain) | Heterotetramer (α2β2) |
| Heme Iron Groups | 1 heme (1 O2 molecule bound) | 4 hemes (4 O2 molecules bound) |
| Binding Curve Shape | Hyperbolic | Sigmoidal (S-shaped) |
| Cooperative Binding | Non-cooperative (nH = 1.0) | Positive cooperativity (nH ≈ 2.8–3.0) |
| Physiological Function | Oxygen storage in muscle tissue | Oxygen transport from lungs to tissues |
| Oxygen Affinity (P50) | Very high affinity (P50 ≈ 2.8 mmHg) | Moderate affinity (P50 ≈ 26 mmHg) |
| Allosteric Regulation | No allosteric effectors | Modulated by H+, CO2, 2,3-BPG, Temp |
The Hill Coefficient (nH)
Cooperativity is quantified by the Hill equation, yielding the Hill coefficient nH:
- nH = 1: Non-cooperative binding. Ligand binding at one site has zero influence on other sites (e.g., myoglobin).
- nH > 1: Positive cooperativity. Binding of the first ligand increases the binding affinity for subsequent ligands (e.g., hemoglobin, where nH ≈ 2.8).
- nH < 1: Negative cooperativity. Binding of the first ligand decreases affinity for subsequent ligands.
T-State (Tense) to R-State (Relaxed) Transition
- Tense (T) State: Low-affinity conformation; stabilized by an extensive network of inter-subunit salt bridges and hydrogen bonds; dominant in deoxygenated hemoglobin.
- Relaxed (R) State: High-affinity conformation; binding of O2 pulls the heme iron into the plane of the porphyrin ring, shifting the proximal histidine (His F8) and breaking salt bridges at the α1β2 interface, relaxing the tetramer.
Allosteric Effectors: The Bohr Effect and 2,3-BPG
Allosteric effectors shift the hemoglobin oxygen saturation curve:
- Right Shift (Decreased O2 Affinity, Increased O2 Unloading in Tissues):
- Increased [H+] (Decreased pH / Bohr Effect): Protons protonate His146, forming a salt bridge that stabilizes the T-state.
- Increased partial pressure of CO2 (PCO2): CO2 reacts with N-terminal amines to form negatively charged carbamino groups that stabilize the T-state.
- Increased 2,3-Bisphosphoglycerate (2,3-BPG): Highly negative molecule that binds specifically into the positively charged central cavity present only in the T-state tetramer. Fetal hemoglobin (HbF, α2γ2) replaces a basic histidine with a neutral serine, binding 2,3-BPG weakly and giving HbF higher O2 affinity than adult HbA to pull oxygen across the placenta.
- Increased Temperature: Weakens stabilizing bonds, favoring O2 release in active metabolic tissue.
- Mnemonic for Right Shift: CADET, face Right! (CO2, Acid/pH down, 2,3-DPG/BPG, Exercise, Temperature).
Mechanisms of Protein Denaturation
Denaturation is the loss of native secondary, tertiary, and quaternary structure resulting in the loss of biological activity. Denaturation never breaks covalent primary peptide bonds; it disrupts non-covalent interactions and, when reducing agents are present, covalent disulfide bonds.
Systematic Classification of Denaturing Agents
| Denaturing Agent | Specific Non-Covalent / Covalent Target | Molecular Mechanism of Action | Common Experimental / Clinical Context |
|---|---|---|---|
| Thermal Energy (Heat) | Hydrogen bonds, hydrophobic interactions | Increases kinetic energy and molecular vibrations, overcoming weak non-covalent stabilization. | PCR thermal denaturation (95°C); cooking egg albumin. |
| Urea (6–8 M) | Hydrogen bonds, backbone hydration | Competes directly for hydrogen-bonding sites on peptide backbone and polar side chains; alters water structure. | Anfinsen ribonuclease folding experiment; protein solubilization. |
| Guanidinium Chloride (6 M) | Hydrogen bonds, salt bridges | Powerful chaotropic cation that disrupts electrostatic networks and internal hydrogen bonding. | Macromolecular unfolding assays in structural biophysics. |
| Sodium Dodecyl Sulfate (SDS) | Hydrophobic core packing | Amphipathic detergent inserts hydrocarbon tails into hydrophobic interior; coats protein with uniform negative charge (1.4 g SDS / g protein). | SDS-PAGE gel electrophoresis (separates proteins strictly by molecular mass). |
| Reducing Agents (β-ME, DTT) | Covalent disulfide bridges (cystine) | Reduces covalent disulfide bonds (–S–S–) back to free sulfhydryl thiols (–SH HS–) via disulfide exchange. | SDS-PAGE sample buffer; Christian Anfinsen ribonuclease experiments. |
| Extreme pH (Strong Acid / Base) | Salt bridges, hydrogen-bond donors/acceptors | Protonates carboxylates (–COO– → –COOH at low pH) or deprotonates ammoniums (–NH3+ → –NH2 at high pH), destroying ionic salt bridges. | Gastric digestion by HCl; acid precipitation of milk casein. |
| Heavy Metal Ions (Pb2+, Hg2+, Ag+) | Salt bridges, sulfhydryl groups | Heavy cations bind tightly to negatively charged carboxylates or form coordinate covalent bonds with cysteine thiols. | Heavy metal poisoning; mercury toxicity disrupting active site cysteines. |
The Anfinsen Experiment
Christian Anfinsen incubated native bovine ribonuclease A (RNase A) with 8 M urea (disrupting hydrogen bonds) and β-mercaptoethanol (reducing all 4 disulfide bonds). RNase A became completely denatured and enzymatically inactive.
- When urea and β-mercaptoethanol were removed simultaneously by dialysis in the presence of oxygen, RNase A spontaneously refolded into its native conformation, recovering 100% enzymatic activity and forming the correct 4 disulfide pairings out of 105 possible combinations.
- This conclusively established that primary sequence alone contains all necessary structural information to specify the native tertiary fold.
Which of the following thermodynamic statements accurately accounts for the spontaneous folding of a globular protein into its native tertiary conformation in an aqueous physiological environment?
The burial of hydrophobic nonpolar side chains collapses ordered clathrate water cages, generating a large favorable increase in solvent entropy (Delta S_solvent > 0) that overcomes the unfavorable decrease in polypeptide conformational entropy (Delta S_chain < 0).
Protein folding is driven entirely by a massive increase in the conformational entropy of the polypeptide chain (Delta S_chain > 0) as it adopts a singular geometric native state.
The formation of covalent peptide bonds along the backbone releases immense heat, producing a negative enthalpy of folding (Delta H < 0) that completely accounts for the negative Delta G of folding.
Water molecules form rigid ice-like cage structures surrounding the folded protein core, maximizing solvent hydrogen bonding and causing a large positive enthalpy change (Delta H > 0).
A researcher repeats Christian Anfinsen's classic ribonuclease A experiment. After denaturing ribonuclease A in 8 M urea and reducing all disulfide bonds with beta-mercaptoethanol, the researcher removes the beta-mercaptoethanol via dialysis while maintaining 8 M urea, and then later removes the urea in a second dialysis step in the presence of oxygen. What is the expected experimental outcome and structural explanation?
The protein recovers 100% of its native enzymatic activity because primary structure spontaneously directs correct disulfide bond pairing even in the presence of 8 M urea.
The protein recovers approximately 50% activity because urea selectively disrupts antiparallel beta-sheets while leaving alpha-helices intact.
The protein forms scrambled, mismatched disulfide pairings with only ~1% of native enzymatic activity because urea maintains an unfolded state that permits random oxidation among the eight cysteine residues.
The protein undergoes irreversible covalent peptide bond cleavage because beta-mercaptoethanol acts as a protease in the presence of urea.
Sections you finish are checked off in the contents.