Describe tertiary and quaternary structures

Tertiary Structure

Why Does It Form?

The linear amino acid sequence (primary structure) contains all the information needed to fold. But WHY does it fold into one specific shape?

Free Energy Minimization: Proteins fold to reach their lowest free energy state (most thermodynamically stable). The driving forces:

1. Hydrophobic Effect (STRONGEST driver)

   • Nonpolar amino acids (Phe, Leu, Val, Ile, Met, Trp) hate water
   • They cluster in the protein core to minimize contact with aqueous cytoplasm
   • This is entropy-driven: water molecules gain freedom when hydrophobic residues hide inside

2. Hydrogen Bonds

   • Between side chains (Ser-OH···O=C-Asp)
   • Between backbone and side chains
   • Stabilize the folded conformation (~5-20 kJ/mol each)

3. Ionic Interactions (Salt Bridges)

   • Between charged residues: Lys⁺···⁻OC-Glu
   • Stronger in hydrophobic interior (~20 kJ/mol) than on surface

4. Disulfide Bonds (Covalent)

   • Cys-S-S-Cys cross-links
   • Permanent covalent bonds (~200 kJ/mol)
   • Common in extracellular proteins (oxidizing environment)
   • Rare inside cells (reducing environment breaks S-S bonds)

5. Van der Waals Forces

   • Weak attractions between all atoms at close range
   • Individually weak (~1 kJ/mol) but sumed over thousands of atom pairs

WHY THIS FORM?

• $\Delta H$ (enthalpy): Energy change from forming/breaking bonds. Hydrogen bonds and van der Waals give negative $\Delta H$ (favorable).
• $T\Delta S$ (entropy): Disorder change. The chain loses conformational freedom (negative $\Delta S$, unfavorable), but water gains freedom when hydrophobic residues bury (positive $\Delta S$, favorable).
• For spontaneous folding: $\Delta G_{\text{fold}} < 0$

The hydrophobic effect dominates because releasing ordered water from hydrophobic surfaces provides a large positive entropy gain that overcomes the chain's entropy loss.

Domains and Motifs

WHY DOMAINS? Large proteins (>200 amino acids) often fold as multiple domains because:

1. Each domain can evolve independently
2. Domains can be mixed and matched (modular evolution)
3. Folding kinetics: smaller units fold faster and avoid misfolding

Structure:

• Core: Hydrophobic residues (Ile, Leu, Phe) packed tightly
• Surface: Hydrophilic residues (Lys, Arg, Asp, Glu) face water
• Four disulfide bonds (8 cysteines): Cys6-Cys127, Cys30-Cys115, Cys64-Cys80, Cys76-Cys94
• Active site cleft: Between two domains, lined with Glu35 and Asp52

WHY THIS ARRANGEMENT?

• The cleft needs to accommodate the bacterial polysaccharide substrate
• Glu35 (protonated, acts as acid) and Asp52 (ionized, stabilizes cation) must be positioned 6 Å apart
• Disulfide bonds lock the cleft in the correct geometry for catalysis
• Without S-S bonds, the protein denatures and loses activity

Diagram: See below for tertiary structure visualization.

Structure:

• Eight α-helices labeled A-H
• Helices pack to form a hydrophobic pocket for the heme group (iron-porphyrin)
• Heme iron coordinates with His93 (proximal histidine)
• His64 (distal histidine) stabilizes bound O₂

WHY THIS FOLD?

• The heme must be buried to prevent iron oxidation (Fe²⁺ → Fe³⁺ makes it unable to bind O₂)
• But there must be a channel for O₂ to reach the iron
• The distal His prevents CO from binding too tightly (CO competes with O₂)

Step-by-step:

1. Chain synthesized with random coil
2. Helices form (secondary structure) within microseconds
3. Helices collapse around heme (hydrophobic effect) in milliseconds
4. Final side-chain adjustments (tertiary structure) complete

Quaternary Structure

NOT ALL PROTEINS HAVE QUATERNARY STRUCTURE. Only those with ≥2 polypeptide chains.

Why Assemble Multiple Subunits?

1. Cooperative Binding (Alostery)

   • Subunits can communicate: binding at one site affects others
   • Example: Hemoglobin's sigmoidal O₂ binding curve

2. Larger Active Sites

   • Some reactions require a cavity bigger than one polypeptide can form
   • Subunits contribute residues to a shared active site

3. Stability

   • Burying more hydrophobic surface at subunit interfaces
   • More interactions = more stable complex

4. Regulation

   • Subunits can dissociate/associate in response to signals
   • Mix different subunit types for specialized functions

5. Error Correction

   • Symmetric assemblies reduce genetic load (one gene makes multiple copies)
   • Mistakes in one subunit don't necessarily destroy function

WHY THIS EQUATION?

• $[P]$ is the concentration of free monomer subunits
• $[P_n]$ is the concentration of assembled n-mer
• Large $K_{\text{assoc}}$ means subunits strongly prefer the assembled state
• This depends on the interface energy: more favorable interactions (hydrogen bonds, hydrophobic contacts) → larger $K_{\text{assoc}}$

Structure:

• Each subunit resembles myoglobin (α-helical, binds one heme)
• α₁β₁ and α₂β₂ form tight dimers
• The two dimers associate more loosely (can rotate relative to each other)

WHY FOUR SUBUNITS? Cooperativity! The protein switches between two states:

• T state (Tense): low O₂ affinity, stabilized by salt bridges between subunits
• R state (Relaxed): high O₂ affinity, salt bridges broken

Mechanism (step-by-step):

1. No O₂ bound: Hemoglobin in T state. Salt bridges between His146β-Asp94β, other ionic contacts hold it tight.
2. First O₂ binds to one heme: Heme iron pulls into the porphyrin plane, tugs on His93 (proximal His).
3. His93 moves: Pulls the F helix, strains the T state.
4. Strain propagates through α₁β₁ interface to α₂β₂.
5. T → R transition: All four subunits shift. Salt bridges break. Other heme sites now have higher affinity (easier for O₂ to bind).
6. Subsequent O₂ molecules bind faster (positive cooperativity).

WHY THIS STEP? Each O₂ binding event destabilizes T state incrementally. Once enough strain accumulates, the entire tetramer flips to R state. This gives the sigmoidal (S-shaped) binding curve: $Y = \frac{(pO_2)^n}{K_d + (pO_2)^n}$ where $n > 1$ (Hill coefficient, measures cooperativity). For hemoglobin, $n \approx 2.8$.

AT TISSUES (low pO₂): Hemoglobin in T state, releases O₂ efficiently. AT LUNGS (high pO₂): Hemoglobin in R state, loads O₂ efficiently.

WHY THIS STRUCTURE?

• Two binding sites: can cross-link antigens (aglutination)
• Hinge region (flexible): allows Fab arms to adjust angle for antigen spacing
• Fc region: recognized by immune cells for phagocytosis or complement activation
• Disulfide bonds: stabilize the structure in extracellular (oxidizing) environment

Types of Subunit Symmetry

• Point Symmetry: Rotational symmetry around a central point (e.g., hemoglobin: 2-fold axis)
• Helical Symmetry: Subunits spiral (e.g., actin filaments, tobacco mosaic virus)
• Cubic Symmetry: Icosahedral (20 faces), used by many virus capsids for efficient enclosure

The fix: Many proteins are monomeric (single polypeptide chain). Examples:

• Myoglobin (O₂ storage)
• Ribonuclease (RNA digestion)
• Lysozyme (bacterial cell wall cleavage)

Test: Does the protein have >1 polypeptide chain? If no → no quaternary structure.

The fix: Primary structure is only the peptide backbone sequence. Disulfide bonds form after folding (during or after tertiary structure formation), so they are part of tertiary (if within one chain) or quaternary (if between chains) structure.

Example: Insulin has two chains (A and B) linked by disulfide bonds. The A-B disulfide bonds are quaternary structure features.

The fix: Quaternary structure is not just the sum of parts. The interface between subunits is critical:

• New interactions form (interface hydrogen bonds, hydrophobic contacts)
• Subunits can allosterically regulate each other (conformational changes propagate)
• The complex has emergent properties (e.g., hemoglobin cooperativity doesn't exist in isolated subunits)

Test: Can the isolated subunit perform the function? Often no—quaternary structure is required for activity.

Connections

• 1.4.04-Explain-secondary-structure-alpha-helix-beta-sheet - Tertiary structure is built from secondary structure elements
• 1.4.03-Describe-primary-structure-peptide-bond - Primary sequence determines all higher levels
• 1.4.07-Protein-folding-and-denaturation - How tertiary structure forms and can be lost
• 1.4.08-Protein-function-and-enzyme-catalysis - 3D structure determines function
• 2.3.05-Hemoglobin-oxygen-transport-cooperativity - Quaternary structure enables cooperativity
• 3.1.02-Enzyme-active-sitesand-induced-fit - Tertiary structure creates active sites

But a LEGO model isn't just spirals lying flat. You have to fold ALL the spirals and sheets together into a 3D shape—like folding a paper airplane. That's tertiary structure. The model gets its final shape because:

1. Some bricks are "hydrophobic" (scared of water)—they hide in the middle.
2. Some bricks stick together with weak magnets (hydrogen bonds, ionic bonds).

Now, some toys need multiple models connected together. Like Voltron—five robot lions combine into one giant robot. That's quaternary structure: multiple folded chains (each with its own 3D shape) joining to work as a team. Hemoglobin is like Voltron: four chains team up to carry oxygen, and when one catches an oxygen, it signals the others to get ready (cooperativity).

Without the right3D shape, the protein is like a pile of unfolded LEGO—useless. The shape IS the function.

For quaternary: "Subunits Cooperate Reliably Every Afternoon"

• Stability
• Cooperativity (alostery)
• Regulation
• Error correction
• Active site assembly

Summary Table

#flashcards/biology

What is tertiary structure? :: The complete 3D arrangement of a single polypeptide chain, including how all secondary structure elements pack together in space.

Concept Map

Hinglish (regional understanding)