Chapter 6: The Three-Dimensional Structure of Proteins – Study Notes

Three-Dimensional Structure of Proteins

Four Levels of Protein Structure

Proteins exhibit a hierarchical organization, with four distinct levels of structure that determine their shape and function.

• Primary Structure: The linear sequence of amino acids in a polypeptide chain, held together by peptide bonds. This sequence dictates all higher levels of structure. Example: The order of amino acids in insulin.

• Secondary Structure: Local regions of repeating main chain structure, stabilized by hydrogen bonds. The most common elements are α-helices and β-sheets. Example: The α-helix in myoglobin.

• Tertiary Structure: The overall three-dimensional arrangement of secondary structural elements within a single polypeptide chain. This level is stabilized by various interactions, including hydrophobic effects, ionic bonds, hydrogen bonds, and disulfide bridges. Example: The globular fold of lysozyme.

• Quaternary Structure: The spatial arrangement of multiple polypeptide chains (subunits) in a multisubunit complex. Example: Hemoglobin, which consists of four subunits.

Common Secondary Structure Elements

Secondary structure refers to regular, repeating patterns within the polypeptide backbone, primarily stabilized by hydrogen bonds.

• α-Helix: A right-handed coil where side chains radiate outward from the helix axis. Hydrogen bonds are nearly parallel to the helix axis, and the helix often has distinct hydrophilic and hydrophobic faces. Formula: $n = 3.6$ residues per turn.

• β-Sheet: Composed of β-strands connected laterally by at least two or three backbone hydrogen bonds, forming a sheet-like structure. Side chains alternate above and below the plane of the sheet. Strands can be parallel (same direction) or antiparallel (opposite direction). Example: Silk fibroin is rich in β-sheets.

Table: Comparison of α-Helix and β-Sheet

Feature	α-Helix	β-Sheet
Hydrogen Bonding	Within chain, parallel to axis	Between strands, perpendicular to axis
Side Chain Orientation	Radiate outward from helix	Alternate above/below sheet
Common Location	Globular proteins, fibrous proteins	Fibrous proteins, globular proteins

Fibrous Proteins: Structural Materials of Cells and Tissues

Fibrous proteins are elongated molecules with extensive secondary structure, providing structural support in cells and tissues.

• Keratin: Found in hair, nails, feathers, scales, and intermediate filaments. Characterized by a coiled-coil structure, with large hydrophobic residues repeating every four positions, forming a stable interface between helices.

• Fibroin: The main protein in silk cocoons, composed of close-packed β-sheets interrupted by compact folded regions, imparting elasticity.

• Collagen: The most abundant connective tissue protein, forming a triple helix. Collagen provides tensile strength and serves as a matrix for bone mineralization.

Globular Proteins: Tertiary Structure and Functional Diversity

Globular proteins have compact, complex tertiary structures and perform diverse biological functions.

• Domains: Larger proteins often contain two or more distinct domains, each folding independently and often associated with specific functions (e.g., DNA binding, oligomerization, cofactor binding).

• Examples: Myoglobin (oxygen storage), neuraminidase (enzyme), triosephosphate isomerase (metabolic enzyme).

Factors Determining Secondary and Tertiary Structure

Protein folding and stability are governed by thermodynamic principles and various molecular interactions.

• Favorable Intramolecular Enthalpic Interactions:

  • Charge-charge interactions (ionic bonds)

  • Intramolecular hydrogen bonds

  • Van der Waals interactions (dense packing)

• Unfavorable Loss of Conformational Entropy:

  • Unfolded state: high entropy (many conformations)

  • Folded state: low entropy (few conformations)

• Favorable Gain of Solvent Entropy (Hydrophobic Effect):

  • Burying hydrophobic side chains releases ordered water molecules, increasing entropy.

• Disulfide Bonds: Covalent bonds between cysteine residues greatly increase protein stability. Example: Disulfide bonds in bovine pancreatic trypsin inhibitor (BPTI).

• Binding of Ions or Prosthetic Groups: Association with metal ions or cofactors can stabilize protein structure. Example: Heme group in hemoglobin.

Quaternary Structures—Symmetries

Quaternary structure describes the arrangement and symmetry of multiple polypeptide subunits in a protein complex.

• Helical Symmetry: Subunits arranged in a helix, as seen in actin filaments and tobacco mosaic virus. These structures can grow indefinitely in length.

• Point-Group Symmetry:

  • C2 Symmetry: One 2-fold axis (e.g., transthyretin dimer forms a β-sandwich).

  • C3 Symmetry: One 3-fold axis.

  • D2 Symmetry: Three 2-fold axes (e.g., tetrameric phosphofructokinase).

  • D4 Symmetry: One 4-fold axis and two 2-fold axes.

Table: Examples of Quaternary Symmetry

Symmetry Type	Example Protein	Features
Helical	Actin, Tobacco Mosaic Virus	Indefinite length, helical arrangement
C2	Transthyretin	β-sandwich, twofold axis
D2	Phosphofructokinase	Tetramer, three 2-fold axes

Key Equations and Concepts

• Residues per turn in α-helix: $n = 3.6$

• Hydrophobic effect: $\Delta S_{solvent} > 0$ when hydrophobic groups are buried

• Disulfide bond formation: $2 \text{Cys} \rightarrow \text{Cys-S-S-Cys}$

Summary Table: Protein Structure Levels

Level	Description	Stabilizing Forces
Primary	Amino acid sequence	Peptide bonds
Secondary	Local folding (α-helix, β-sheet)	Hydrogen bonds
Tertiary	3D arrangement of secondary elements	Hydrophobic effect, ionic bonds, H-bonds, disulfide bonds
Quaternary	Assembly of multiple polypeptides	Non-covalent interactions, sometimes covalent bonds

Additional info: Protein folding is a spontaneous process driven by the balance of enthalpic and entropic contributions, and the final structure is the most thermodynamically stable conformation under physiological conditions.