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Tertiary & Quaternary
Structure

Explore how proteins fold into three-dimensional shapes, assemble into multi-subunit complexes, and organize into functional domains and motifs.

Biochemistry

The Four Levels of Protein Structure

Proteins are organized in a hierarchy. Primary structure is the amino acid sequence. Secondary structure includes local folding patterns like alpha-helices and beta-sheets. Today we focus on the higher levels: tertiary and quaternary structure.

1° Primary Sequence 2° Secondary Helix / Sheet 3° Tertiary 3D Fold 4° Quaternary Multi-subunit

What Is Tertiary Structure?

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It results from interactions between amino acid R-groups (side chains) that are far apart in the primary sequence but brought close together by folding.

Hydrophobic Interactions
Nonpolar R-groups cluster in the protein interior, away from water
Disulfide Bonds
Covalent S-S bridges between cysteine residues lock shape
Ionic Bonds
Electrostatic attraction between charged R-groups (e.g., Lys & Asp)
H-Bonds
Between polar side chains; weaker individually but abundant

Forces That Stabilize Tertiary Structure

Folded polypeptide Hydrophobic S-S Bond + - Ionic Bond Hydrogen Bond Strongest Weakest Disulfide Ionic H-Bond Hydrophobic

What Is Quaternary Structure?

Quaternary structure describes the arrangement of two or more polypeptide subunits into a functional multi-subunit complex. Each individual subunit has its own tertiary structure; quaternary structure is how those subunits fit together.

\(\alpha_1\)
 subunit
\(\beta_1\)
 subunit
\(\beta_2\)
 subunit
\(\alpha_2\)
 subunit
Hemoglobin: \(\alpha_2\beta_2\) tetramer

Tertiary vs Quaternary

Tertiary (3°) Quaternary (4°)
Scope Single polypeptide chain Multiple subunits together
Key forces Hydrophobic, disulfide, ionic, H-bonds Same forces, but at subunit interfaces
Required? All proteins Only multi-subunit proteins
Classic example Myoglobin (1 chain) Hemoglobin (4 chains)
Disrupted by Denaturing agents, heat, pH Same + dissociation of subunits

Structural Motifs (Supersecondary Structure)

A motif (also called a supersecondary structure or fold) is a recognizable combination of secondary-structure elements that appears in many different proteins. Motifs are smaller than domains and do not fold independently.

Helix-Turn-Helix
Two alpha-helices connected by a short turn; common in DNA-binding proteins
Beta-Hairpin
Two antiparallel beta-strands connected by a tight turn (2-5 residues)
Greek Key
Four antiparallel beta-strands folded in a pattern resembling a Greek key design

Visualizing Common Motifs

Helix-Turn-Helix turn Beta-Hairpin H-bonds
\(\beta\)-\(\alpha\)-\(\beta\) Motif
\(\alpha\)-helix
 Greek Key crossover

Protein Domains

A domain is a compact, independently folding region of a polypeptide that often has its own distinct function. Large proteins may contain multiple domains connected by flexible linkers. A single domain typically contains 40-350 amino acid residues.

Domain 1 Binding site linker Domain 2 Catalytic site N C ~150 residues ~200 residues

Motifs vs Domains

Motif Domain
Size Small (2-3 secondary elements) Large (40-350+ residues)
Folds independently? No Yes
Has own function? Not usually Often yes (binding, catalysis)
Examples Helix-turn-helix, beta-hairpin SH2 domain, kinase domain
Relationship A domain may contain several motifs

Important Domain Examples

SH2 Domain
Recognizes and binds phosphotyrosine residues; critical in cell signaling pathways (e.g., Src kinase)
Zinc Finger
Small structural motif stabilized by zinc ion coordination; found in many transcription factors that bind DNA
Immunoglobulin
Beta-sandwich fold found in antibodies and cell-surface receptors; each antibody contains multiple Ig domains

How Tertiary Structure Forms

1
Translation produces the polypeptide. The ribosome synthesizes the amino acid chain based on mRNA instructions.
2
Local secondary structures form. Alpha-helices and beta-sheets fold quickly as the chain emerges.
3
Hydrophobic collapse. Nonpolar side chains cluster in the protein core, away from the aqueous environment.
4
Fine-tuning by noncovalent interactions. Ionic bonds, hydrogen bonds, and van der Waals forces refine the shape.
5
Chaperone assistance (often). Molecular chaperones (e.g., Hsp70) prevent misfolding and aggregation.

Example: Myoglobin (Tertiary Only)

A B E F G H Heme
\(\text{Fe}^{2+}\)
 ✓ Single polypeptide chain ✓ 8 alpha-helices (A-H) ✓ No quaternary structure

Example: Hemoglobin (Quaternary)

Hemoglobin consists of four subunits: two alpha chains (141 residues each) and two beta chains (146 residues each). Each subunit has its own tertiary structure resembling myoglobin, but together they form a quaternary complex with cooperative oxygen binding.

\(\alpha\) subunits
141 amino acids each, contain 7 alpha-helices and one heme group per subunit
\(\beta\) subunits
146 amino acids each, contain 8 alpha-helices and one heme group per subunit

Example: Src Kinase (Multi-Domain)

SH3 Proline-rich binding SH2 Phosphotyrosine binding Kinase Domain Catalytic activity: phosphorylates substrates N C 3 domains in 1 polypeptide chain

Common Mistakes

✕ Confusing motif and domain
A motif is a small structural pattern; a domain is a large, independently folding unit. Motifs do not fold on their own.
✕ All proteins have 4° structure
Quaternary structure only applies to proteins with multiple polypeptide subunits. Monomeric proteins (like myoglobin) stop at tertiary.
✕ H-bonds only in 2° structure
Hydrogen bonds stabilize all levels: backbone H-bonds in 2°, side-chain H-bonds in 3°, and intersubunit H-bonds in 4°.

Summary Table

Feature Tertiary Quaternary Domain Motif
Scope Whole chain 3D Multi-chain assembly Independent region Local pattern
Folds alone? Yes Each subunit does Yes No
Size Entire polypeptide 2+ polypeptides 40-350 residues 2-3 elements
Example Myoglobin Hemoglobin SH2 \(\beta\)-hairpin

Question 1 of 5

Which type of bond is the only covalent interaction that stabilizes tertiary structure?

Question 2 of 5

Myoglobin is a monomeric protein. Which of the following correctly describes its structural organization?

Question 3 of 5

What is the key difference between a motif and a domain?

Question 4 of 5

Hemoglobin exhibits cooperative oxygen binding. This cooperativity is possible because hemoglobin has:

Question 5 of 5

The SH2 domain is found in many different signaling proteins across species. This observation best supports the concept of: