Advanced Topics

Quaternary Structure and Allostery

Summary

Quaternary structure describes the assembly of multiple polypeptide subunits into functional oligomers, while allostery refers to the regulation of protein activity through conformational changes propagated between distant sites.

Key Points

  • 1Quaternary structure involves assembly of multiple subunits through hydrophobic and complementary interfaces
  • 2MWC (concerted) model: all subunits switch states together; KNF (sequential): individual subunit changes propagate
  • 3Hemoglobin exemplifies cooperative oxygen binding regulated by allosteric effectors (BPG, pH, CO₂)
  • 4Modern ensemble models integrate conformational selection and induced fit mechanisms
  • 5Allosteric sites are valuable drug targets due to higher selectivity and tunable modulation

# Quaternary Structure and Allostery

Quaternary structure—the organization of multiple polypeptide chains into functional assemblies—and allostery—the coupling of distant sites through conformational change—are central concepts in understanding how proteins function as molecular machines and regulatory switches.

Quaternary Structure

Subunit Organization

- Oligomers: Complexes of 2+ polypeptide chains (protomers)

- Homo-oligomers: Identical subunits (e.g., hemoglobin α₂β₂ is heteromeric; TIM is homodimeric)

- Hetero-oligomers: Different subunits

- Symmetry: Most oligomers exhibit rotational symmetry (cyclic, dihedral)

Interface Types

| Interface | Characteristics | Example |

|-----------|-----------------|---------|

| Isologous | Same surface from each subunit | Many homodimers |

| Heterologous | Different surfaces from each subunit | Hemoglobin αβ interface |

| Domain swapping | Exchange of structural elements | RNase A, prion fibrils |

Energetics of Assembly

  • Driven by burial of hydrophobic surface area
  • Complementary shape and charge
  • Release of interfacial water molecules (entropy gain)
  • Typical interface: 1000-3000 Ų buried surface per subunit

    • Classical Allosteric Models

    Monod-Wyman-Changeux (MWC) Model

    The "concerted" or "symmetry" model:

  • Protein exists in two states: T (tense, low affinity) and R (relaxed, high affinity)
  • All subunits switch states simultaneously (symmetry preserved)
  • Ligands shift equilibrium toward R state
  • Explains cooperative binding without sequential conformational changes

    • Koshland-Némethy-Filmer (KNF) Model

    The "sequential" model:

  • Ligand binding induces conformational change in individual subunits
  • Changes propagate to neighboring subunits
  • Symmetry can be broken
  • Allows for negative cooperativity

    • Ensemble Allosteric Model

    Modern framework integrating both:

  • Proteins sample multiple conformations
  • Ligands redistribute ensemble populations
  • No strict requirement for symmetric or sequential changes
  • Explains "dynamic" allostery with no structural change

    • Hemoglobin: The Paradigm

    Structural Basis of Cooperativity

  • T-state: Low O₂ affinity, salt bridges constrain structure
  • R-state: High O₂ affinity, broken salt bridges, rotated αβ dimers
  • O₂ binding to heme iron pulls iron into porphyrin plane
  • Triggers helix F movement, propagates to interfaces

    • Allosteric Effectors

    - 2,3-BPG: Stabilizes T-state, reduces O₂ affinity at high altitude

    - H⁺ (Bohr effect): Low pH favors T-state, promotes O₂ release in tissues

    - CO₂: Carbamate formation stabilizes T-state

    Hill Coefficient

  • Measure of cooperativity: n = 1 (non-cooperative), n > 1 (positive), n < 1 (negative)
  • Hemoglobin: n ≈ 2.8 (maximum theoretical = 4 for tetramer)

    • Allosteric Mechanisms

    Conformational Selection

  • Pre-existing equilibrium between conformational states
  • Ligand binds preferentially to one state
  • Shifts population toward that state
  • Binding kinetics can distinguish from induced fit

    • Induced Fit

  • Ligand binding triggers conformational change
  • Conformation not significantly populated in absence of ligand
  • Active site molds around substrate
  • Originally proposed by Koshland

    • Dynamic Allostery

  • Regulation through changes in conformational dynamics
  • No net structural change between states
  • Entropy-driven: binding at one site alters flexibility elsewhere
  • Detected by NMR relaxation experiments

    • Allosteric Communication

    Structural Pathways

  • Networks of residues connecting allosteric and active sites
  • Identified through evolutionary coupling analysis
  • Correlated motions in molecular dynamics simulations

    • Community Analysis

  • Residues grouped by coordinated motion
  • Information flows between communities
  • Mutations at pathway residues disrupt allostery

    • Allostery in Drug Discovery

    Allosteric Modulators

    Advantages over orthosteric drugs:

  • Greater selectivity (unique allosteric sites)
  • Tunable effects (partial agonism/antagonism)
  • Preserve physiological regulation patterns

    • Examples

    - Maraviroc: Allosteric inhibitor of CCR5 (HIV entry)

    - Benzodiazepines: Positive allosteric modulators of GABA_A receptors

    - Cinacalcet: Calcium-sensing receptor allosteric modulator

    Cryptic Allosteric Sites

  • Sites revealed only by conformational sampling
  • Discovered through fragment screening, MD simulations
  • Expand druggable proteome

    • Engineering Allostery

    Protein Switches

  • Design of ligand-responsive proteins
  • Insertion of ligand-binding domains into enzymes
  • Applications in biosensors and metabolic engineering

    • Synthetic Allostery

  • Computational design of allosteric communication
  • Directed evolution of allosteric responses
  • Creation of novel regulatory proteins
    • Previous
    The Molten Globule State
    Next
    Solid-Phase Peptide Synthesis (SPPS)