Biochemistry - Lesson 4: Protein Structure, Folding, and Allostery (Hemoglobin Case Study)
Proteins adopt diverse structural architectures, ranging from simple monomers to large multisubunit complexes, enabling them to carry out specialized biological functions. This lesson explores protein structure at multiple hierarchical levels, considers the role of chaperones in folding, and highlights allosteric regulation using hemoglobin as a prime example of quaternary structure and cooperative ligand binding.
Hierarchical Levels of Protein Structure
- Primary Structure: The linear amino acid sequence that dictates all higher-level conformations.
- Secondary Structure: Local conformations stabilized by hydrogen bonds, typically α-helices and β-sheets, plus turns/loops connecting them.
- Tertiary Structure: The 3D folding of a single polypeptide chain, driven by side chain interactions (hydrophobic, ionic, hydrogen bonds, disulfide bridges).
- Quaternary Structure: The arrangement of multiple polypeptide subunits into a functional complex (e.g., hemoglobin’s four subunits).
Secondary Structure Elements
α-Helices form when a section of the polypeptide backbone twists into a right-handed coil, with hydrogen bonds between the carbonyl oxygen (residue i) and the amide hydrogen (residue i+4). Side chains radiate outward from the helical axis.
β-Sheets feature extended polypeptide strands aligned side by side, linked by backbone hydrogen bonds. Strands can be parallel or antiparallel, and the sheet is “pleated” due to the zigzag backbone. Short turns and larger loops provide directional changes, connecting α-helices and β-strands in tertiary folds.
Tertiary Structure and Chaperones
Tertiary structure results from packing secondary elements into a stable three-dimensional fold. Key stabilizing forces:
- Hydrophobic effect: Nonpolar residues cluster away from water in the protein core.
- Electrostatic interactions: Ionic bonds between positively and negatively charged side chains.
- Hydrogen bonds: Between side chains or between side chains and backbone groups.
- Disulfide bridges: Covalent bonds formed between cysteine residues in oxidizing environments.
Molecular chaperones ensure correct folding in vivo by preventing improper contacts and aggregation. Examples include Hsp70, which binds transiently to nascent polypeptides, and the GroEL/GroES complex (in bacteria), which provides a protected environment for folding.
Quaternary Structure: Hemoglobin Case Study
Quaternary structure emerges when multiple polypeptide chains interact to form a functional assembly. Hemoglobin illustrates how distinct subunits can cooperate:
- Composed of four subunits (2 α and 2 β chains), each harboring a heme cofactor for oxygen binding.
- Cooperative effects: Oxygen binding to one subunit elevates oxygen affinity in the others (transition from T-state to R-state).
In contrast, myoglobin is a single-chain protein in muscle, displaying no quaternary interactions and exhibiting a hyperbolic O2-binding curve.
Comparison: Myoglobin vs. Hemoglobin
| Property | Myoglobin | Hemoglobin |
|---|---|---|
| Subunit Arrangement | Single polypeptide chain | Tetramer (α2β2) |
| Primary Role | Oxygen storage in muscle | Oxygen transport in blood |
| Oxygen-Binding Curve | Hyperbolic | Sigmoidal (cooperative) |
| Allosteric Regulation | Not allosteric | Bohr effect, 2,3-BPG modulation |
Allosteric Behavior and the Bohr Effect
Hemoglobin’s cooperative binding exemplifies allostery, where ligand binding at one site influences binding affinity at another. Key factors include:
- Cooperativity: O2 binding shifts hemoglobin from a low-affinity T-state to a high-affinity R-state.
- Bohr Effect: Reduced pH or elevated CO2 levels decrease hemoglobin’s O2 affinity, promoting oxygen release in tissues.
- 2,3-BPG: Binds to hemoglobin, stabilizing the T-state and lowering O2 affinity—key in physiological adaptation (e.g., high altitudes).
The chart compares hemoglobin’s sigmoidal oxygen-binding curve (red) to myoglobin’s hyperbolic curve (green). Hemoglobin’s cooperativity underlies its enhanced sensitivity to changes in oxygen partial pressure, essential for transport from lungs to tissues.
Summary
Protein structure spans from local secondary motifs to complex quaternary assemblies. Folding is influenced by side chain chemistry, chaperones, and intermolecular forces, ensuring proper conformation for function. Hemoglobin’s multi-subunit structure highlights how quaternary interactions enable allosteric regulation, underscoring the intricate connection between protein architecture and biological activity.
Suggested Reading:
Lehninger Principles of Biochemistry (chapters on protein folding, structure, and allostery)
Biochemistry by Berg, Tymoczko, and Stryer (sections detailing hemoglobin, myoglobin, and cooperative binding mechanisms)
