Protein Structure and Folding: Key Concepts in Biochemistry

Protein Structure: Levels and Features

Primary Structure

The primary structure of a protein refers to the linear sequence of amino acids joined by peptide bonds. This sequence determines the protein's unique characteristics and function.

• Amino acid residue: An individual amino acid within a polypeptide chain, after water is removed during peptide bond formation.

• Peptide bond: The covalent bond formed between the carboxyl group of one amino acid and the amino group of another.

• Resonance structures: Peptide bonds exhibit resonance, giving partial double-bond character and restricting rotation.

Example: The sequence Ala-Gly-Ser represents a primary structure.

Secondary Structure

The secondary structure of proteins refers to regular, repeating local conformations stabilized by hydrogen bonds. The most common types are α-helices and β-sheets.

• α-Helix: A right-handed coil stabilized by hydrogen bonds between the backbone N-H and C=O groups of amino acids i and i+4.

• β-Sheet: Formed by hydrogen bonds between backbone atoms in adjacent polypeptide chains or segments. Can be parallel or anti-parallel.

• Ramachandran plot: A graphical representation of the allowed angles of rotation (φ and ψ) for the backbone of a polypeptide.

• Key features of α-helix:

  • H-bonds stabilize the helix

  • 3.6 amino acids per turn

  • Pitch (rise per turn): 5.4 Å

  • Side chains point outward from the helix axis

  • Length of α-helix: $\text{Length} = \text{number of residues} \times 1.5\ \text{Å}$

• Key features of β-sheet:

  • Strands are 3.4 Å apart

  • Side chains alternate above and below the plane

  • Parallel and anti-parallel arrangements

Example: Silk fibroin is rich in β-sheets.

Tertiary Structure

The tertiary structure describes the overall 3D shape of a single polypeptide chain, stabilized by interactions among side chains.

• Interactions: Hydrophobic interactions, hydrogen bonds, ionic bonds, van der Waals forces, and disulfide bridges.

• Disulfide bonds: Covalent bonds formed between two cysteine residues, stabilizing the folded structure.

• Residue orientation: Hydrophobic residues are usually buried inside; hydrophilic residues are exposed to solvent.

Example: Myoglobin is a globular protein with a compact tertiary structure.

Quaternary Structure

The quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a protein complex.

• Subunit interactions: Non-covalent interactions and sometimes covalent bonds (e.g., disulfide bridges).

• Example: Hemoglobin consists of four subunits (two α and two β chains).

Protein Folding and Stability

Thermodynamic Factors

Protein folding is driven by thermodynamic principles, favoring the most stable conformation.

• Favorable interactions: Intramolecular hydrogen bonds, ionic interactions, van der Waals forces.

• Loss of conformational entropy: Folding reduces the number of possible conformations (lower entropy).

• Hydrophobic effect: Hydrophobic side chains cluster in the protein interior, releasing ordered water molecules and increasing entropy.

Protein Denaturation and Refolding

Denaturation disrupts the native structure of proteins, often reversible under suitable conditions.

• Denaturing agents: Heat, pH changes, chemicals (e.g., urea, guanidinium chloride).

• Anfinsen experiment: Demonstrated that the primary structure contains all information needed for proper folding.

Interactions in Biological Molecules

Covalent and Non-covalent Interactions

Proteins are stabilized by a variety of interactions:

• Covalent bonds: Peptide, phosphodiester, disulfide

• Non-covalent interactions: Hydrogen bonds, electrostatic/ionic, hydrophobic, van der Waals, metal coordination, π-stacking

Special Protein Types and Examples

Fibrous Proteins

• α-Keratins: Found in hair, nails, and skin; coiled-coil structure stabilized by hydrophobic interactions.

• Collagen: Triple helix structure; requires vitamin C for proper cross-linking.

• Fibroin: β-sheet protein in silk; high proportion of glycine and alanine.

Globular Proteins

• Examples: Myoglobin, hemoglobin, enzymes

• Properties: Compact, soluble, diverse functions

Tables

Length of α-Helices and β-Sheets

Structure Type	Residue Interval (Å)	Pitch (Å)	Residues per Turn
α-Helix	1.5	5.4	3.6
β-Strand	3.4	n/a	n/a

Key Equations and Concepts

• Peptide bond resonance: $\ce{O=C-NH}$

• α-Helix length: $\text{Length} = \text{number of residues} \times 1.5\ \text{Å}$

• Ramachandran angles: $\phi$ (N–Cα bond), $\psi$ (Cα–C bond)

Additional info:

• Protein folding is a spontaneous process driven by the decrease in free energy ($\Delta G$).

• Misfolded proteins can lead to diseases such as Alzheimer's, Parkinson's, and prion diseases.

• Disulfide bonds are crucial for extracellular protein stability.