Quaternary Structure of Proteins

Overview

• Quaternary structure refers to the arrangement of more than one polypeptide chain in a protein.
• Similar interactions to tertiary structure (hydrogen bonds, London dispersion forces) contribute to quaternary structure.

Types of Quaternary Structures

1. Conjugated Proteins

• Composed of polypeptides and one or more non-polypeptide subunits.
• Example: Hemoglobin
  • Oxygen-carrying protein with 4 polypeptides.
  • Each subunit has one iron atom (prosthetic group) and can carry one oxygen molecule.
  • More on Hemoglobin

2. Non-Conjugated Proteins

• Made solely of polypeptides.
• Interactions are similar to those in tertiary structure.
• Formed by linking multiple polypeptide chains.
• Examples: Insulin, Collagen

Form and Function in Protein Types

Globular & Fibrous Proteins

• Function is highly dependent on protein form.
• Important to understand how structure relates to function.

Comparison of Globular vs. Fibrous Proteins

Shape

• Fibrous Proteins: Long, narrow, often only alpha helices.
• Globular Proteins: Rounded/spherical, contain alpha helices and beta-pleated sheets.

Role

• Fibrous Proteins: Structural (strength and support).
• Globular Proteins: Functional (catalytic, transport, etc.).

Solubility

• Fibrous Proteins: Generally insoluble in water.
• Globular Proteins: Generally soluble in water.

Stability

• Fibrous Proteins: Repetitive amino acid sequence, less sensitive to pH and temperature changes.
• Globular Proteins: Irregular amino acid sequence, more sensitive to pH and temperature changes.

Examples

• Fibrous Proteins: Collagen, myosin, fibrin, actin, keratin, elastin.
• Globular Proteins: Catalase, hemoglobin, insulin, immunoglobulins.