The Three-Dimensional Structure of Proteins (Chapter 6 Study Notes)

Chapter 6: The Three-Dimensional Structure of Proteins

Four Levels of Protein Structure

Proteins exhibit a hierarchy of structural organization, each level contributing to the final shape and function of the molecule.

• Primary Structure: The linear sequence of amino acids in a polypeptide chain, determined by the genetic code. This sequence dictates all higher levels of structure.

• Secondary Structure: Local regions of regular structure stabilized by hydrogen bonds, such as α-helices and β-sheets.

• Tertiary Structure: The overall three-dimensional arrangement of all atoms in a single polypeptide chain, including the spatial arrangement of secondary structural elements.

• Quaternary Structure: The spatial arrangement of multiple polypeptide chains (subunits) in a multisubunit complex.

Example: Hemoglobin is a classic example of a protein with quaternary structure, consisting of four polypeptide subunits.

Common Secondary Structure Elements

Secondary structures are stabilized by hydrogen bonds and are fundamental to protein folding and function.

• 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.

• 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 (N→C direction the same) or antiparallel (N→C direction opposite).

Example: Silk fibroin is rich in β-sheets, contributing to its strength and flexibility.

Side Chain Positions in Secondary Structures

• In an α-helix, side chains project outward from the helical backbone, minimizing steric hindrance.

• In a β-sheet, neighboring side chains are located on opposite faces of the sheet, contributing to the sheet's stability and interactions with other molecules.

Fibrous Proteins: Structural Materials of Cells and Tissues

Fibrous proteins are elongated molecules with well-defined secondary structures, serving as structural materials in cells and tissues.

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

• Fibroin: The main protein in silk, composed of extensive close-packed β-sheets interrupted by compact folded regions, providing both strength and elasticity.

• Collagen: The most abundant connective tissue protein, forming a triple helix structure. Collagen fibers provide tensile strength to tissues and serve as a matrix for bone mineralization.

Example: Collagen's triple helix is stabilized by unusual amino acids such as hydroxyproline.

Globular Proteins: Tertiary Structure and Functional Diversity

Globular proteins fold into compact, roughly spherical shapes and perform a wide variety of functions.

• Contain varying amounts of α-helix, β-sheet, and loop regions.

• Larger proteins often consist of multiple domains, each folding independently and often associated with a specific function (e.g., DNA binding, enzymatic activity).

Example: Human ubiquitin is a small globular protein involved in protein degradation pathways.

Representations of 3D Structures

• Cartoon models highlight secondary structure elements.

• Stick models show atomic details and hydrogen bonds.

• Surface models illustrate the solvent-accessible surface and charge distribution.

Folding into Defined Structures with Diverse Functions

Protein folding is driven by the amino acid sequence and results in a unique, functional three-dimensional structure.

• A typical protein domain is about 200 amino acids and can fold independently.

• Domains often correspond to functional units within the protein, such as binding sites or catalytic centers.

Factors Determining Secondary and Tertiary Structure

The stability and folding of proteins are governed by a balance of enthalpic and entropic factors.

• Favorable intramolecular enthalpic interactions:

  • Charge-charge interactions (ionic bonds)

  • Intramolecular hydrogen bonds

  • Van der Waals interactions (dense packing of atoms)

• Unfavorable loss of conformational entropy: Folding reduces the number of possible conformations, decreasing entropy.

• Favorable gain of solvent entropy (hydrophobic effect): Burying hydrophobic side chains in the protein interior releases ordered water molecules, increasing overall entropy.

• Disulfide bonds: Covalent bonds between cysteine residues that greatly increase protein stability.

• Binding of ions or prosthetic groups: These interactions can further stabilize the folded protein structure.

Equation: $\Delta G = \Delta H - T\Delta S$ Where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy, $T$ is temperature, and $\Delta S$ is the change in entropy.

Quaternary Structures—Symmetries

Many proteins function as assemblies of multiple polypeptide chains, exhibiting various types of symmetry.

• Helical symmetry: Seen in structures like actin filaments and the tobacco mosaic virus, allowing for indefinite growth in length.

• Point-group symmetry: Includes various arrangements such as:

  • $C_2$: One 2-fold axis

  • $C_3$: One 3-fold axis

  • $D_2$: Three 2-fold axes

  • $D_4$: One 4-fold axis and two 2-fold axes

Symmetry Type	Example	Description
Helical	Actin, Tobacco Mosaic Virus	Capable of indefinite lengthening
$C_2$	Transthyretin dimer	Twofold symmetry about a perpendicular axis
$D_2$	Phosphofructokinase tetramer	Three 2-fold axes

Example: The tetrameric enzyme phosphofructokinase displays $D_2$ symmetry, important for its regulatory function.

Additional info: The notes above expand on the provided slides by including definitions, examples, and explanations of key concepts such as the hydrophobic effect, the role of domains, and the thermodynamic equation for protein folding. Table entries and some descriptions are inferred from standard biochemistry textbooks to ensure completeness and clarity.