Levels of Protein Structure

Overview of Protein Structure

Proteins are complex macromolecules that fold into diverse three-dimensional shapes, which are essential for their biological functions. The structure of proteins is organized into four hierarchical levels: primary, secondary, tertiary, and quaternary.

• Primary structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as α-helices and β-sheets stabilized by hydrogen bonds.

• Tertiary structure: The overall three-dimensional arrangement of a single polypeptide chain, stabilized by interactions between side chains.

• Quaternary structure: The spatial arrangement of multiple polypeptide subunits in a protein complex.

Certain structural elements are found in many proteins, and general rules guide how proteins obtain their final form.

Peptide Bonds and Primary Structure

Formation of Peptide Bonds

Peptide bonds are covalent bonds that link amino acids together in a polypeptide chain. They are formed through a condensation reaction between the carboxyl group of one amino acid and the amino group of another, releasing water:

• Condensation reaction:

• Peptide bond: (amide bond)

• N-terminus: The end of the peptide with a free amino group.

• C-terminus: The end of the peptide with a free carboxyl group.

Peptide Bond Properties

• Partial double-bond character: Peptide bonds exhibit resonance, resulting in restricted rotation around the C-N bond.

• Planarity: Peptide bonds are rigid and planar, contributing to the overall structure of the protein backbone.

• Polarity: Peptide bonds are polar and can participate in hydrogen bonding.

Peptide Nomenclature

• Dipeptide: Two amino acids joined by one peptide bond.

• Tripeptide: Three amino acids.

• Tetrapeptide: Four amino acids.

• Pentapeptide: Five amino acids.

• Peptides/oligopeptides: Short chains, often synthetic, typically < 40 residues.

• Polypeptide: Long chain, usually produced naturally; may be described as a protein.

• Protein: Polypeptide (or >1 polypeptide) with a biological function.

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, written from the N-terminus to the C-terminus. This sequence determines the protein's final structure and function.

• Covalent peptide bonds: Join each amino acid to the next.

• Sequence specificity: Every protein has a unique amino acid sequence.

• Example:

Protein Backbone and Conformation

The Protein Backbone

The backbone of a polypeptide is the repeating sequence of atoms: N–Cα–C–N–Cα–C–...

• Side chains: Project from the backbone and determine the chemical properties of the protein.

Backbone Conformation

Backbone conformation is described by two torsion angles:

• Phi (φ): N–Cα

• Psi (ψ): Cα–C(=O)

• Steric constraints: Limit the range of these angles, influencing protein folding.

• Ramachandran plot: Visualizes allowed regions of φ and ψ angles for different amino acids.

Secondary Structure

Regular Secondary Structures

Secondary structures are local folding patterns stabilized by hydrogen bonds:

• α-helix: Right-handed helix, 3.6 residues per turn, stabilized by H-bonds between C=O of residue (i) and NH of residue (i+4). Typical angles:

• β-sheet: Formed from β-strands arranged side by side, joined by loops. Can be parallel (strands run in same direction) or antiparallel (strands run in opposite directions). Typical angles:

  • Parallel:

  • Antiparallel:

Irregular Secondary Structures

• β-turns: Short regions that reverse the direction of the polypeptide chain.

• Coils and loops: Non-repeating φ, ψ angles, often found on the protein surface.

Stabilizing Forces

• α-helices: Stabilized by H-bonds within the same helix.

• β-sheets: Stabilized by H-bonds between neighbouring strands.

Tertiary Structure

Three-Dimensional Arrangement

Tertiary structure refers to the overall 3D shape of a single polypeptide chain, determined by interactions between side chains and the arrangement of secondary structures.

• Stabilized by:

  • Hydrophobic interactions

  • Van der Waals forces

  • Hydrogen bonds

  • Ionic interactions

  • Disulfide bonds (covalent bonds between cysteine residues)

• Protein classes:

  • Fibrous proteins: Elongated, structural roles, insoluble in water.

  • Globular proteins: Compact, functional roles, soluble in water.

Quaternary Structure

Multi-Subunit Proteins

Quaternary structure describes the spatial arrangement of multiple polypeptide chains (subunits) in a protein complex. Subunits associate through the same forces as tertiary structure.

• Example: Hemoglobin is a heterotetramer with two α and two β subunits, each containing a heme group.

• Monomer: A protein without quaternary structure.

Table: Secondary Structure Torsion Angles

Key Concepts and Applications

• Directionality: Amino acid sequences are always written and read from N-terminus to C-terminus.

• Charge retention: Only the terminal α-amino and α-carboxyl groups retain their charge in a peptide; internal groups are neutralized by peptide bond formation.

• Ramachandran plot: Used to visualize allowed conformations of φ and ψ angles for different amino acids; glycine and proline have unique patterns due to their side chain properties.

• Protein function: Determined by structure; even single amino acid substitutions can have significant effects (e.g., sickle cell anemia).

Additional info: The notes cover foundational concepts in protein structure, including peptide bond chemistry, nomenclature, backbone conformation, secondary structure types, and the forces stabilizing protein architecture. These are essential for understanding protein folding, stability, and function in biochemistry.