Quaternary Structure and Protein Function

Many proteins consist of more than one polypeptide chain. When two or more polypeptides associate to form a functional unit, the arrangement is called the quaternary structure. Proteins can also be classified by their overall shape and role into globular and fibrous proteins, each optimised for very different biological tasks. This lesson covers OCR specification point 2.1.2 (d)(iii)-(iv): quaternary structure and the distinction between globular and fibrous proteins with examples.

1. Quaternary Structure

Quaternary structure refers to the arrangement of multiple polypeptide subunits in a functional protein complex. A protein with quaternary structure has more than one polypeptide chain, and each chain has its own primary, secondary and tertiary structure.

The subunits are held together by the same types of interactions found in tertiary structure — hydrogen bonds, ionic bonds, disulfide bridges (sometimes), hydrophobic interactions, van der Waals forces.
Each subunit may be identical (homomeric) or different (heteromeric).
A quaternary protein may contain non-protein components called prosthetic groups (e.g., haem in haemoglobin, Zn²⁺ in carbonic anhydrase).

Key Definition — Prosthetic group: A non-protein component that is permanently attached to a protein and essential for its function. Examples: haem group in haemoglobin; FAD in succinate dehydrogenase.

1.1 Haemoglobin — a Classic Example

Haemoglobin is the oxygen-carrying pigment of vertebrate red blood cells. It consists of:

Four polypeptide subunits: two α-globin chains and two β-globin chains.
Four haem prosthetic groups, one per subunit.
Each haem contains a central Fe²⁺ ion that reversibly binds one O₂ molecule.

Therefore, one molecule of haemoglobin can carry four oxygen molecules.

graph TD
  H[Haemoglobin] --> A1[α-chain 1 + haem + Fe²⁺]
  H --> A2[α-chain 2 + haem + Fe²⁺]
  H --> B1[β-chain 1 + haem + Fe²⁺]
  H --> B2[β-chain 2 + haem + Fe²⁺]

The four subunits interact cooperatively: when one subunit binds O₂, a conformational change makes the others bind O₂ more readily. This gives haemoglobin its characteristic sigmoidal (S-shaped) oxygen dissociation curve.

1.2 Other Examples of Quaternary Structure

Protein	Subunits	Function
Haemoglobin	2α + 2β + 4 haem	Oxygen transport
Insulin	2 chains (A and B) linked by disulfide bridges	Blood glucose regulation
Collagen	3 identical α-chains wound in a triple helix	Structural connective tissue
Antibodies (IgG)	2 heavy + 2 light chains linked by disulfide bridges	Immune recognition
DNA polymerase	Several different subunits	DNA replication
ATP synthase	Multiple subunits forming a rotary motor	ATP synthesis

2. Globular vs Fibrous Proteins

Proteins are divided into two broad structural classes — globular and fibrous — which correspond to very different biological roles.

2.1 Globular Proteins

Globular proteins have compact, roughly spherical, water-soluble structures. They typically:

Fold so that hydrophobic R groups are buried inside and hydrophilic R groups are on the surface — this makes them soluble in water.
Have complex, often irregular tertiary structures.
Perform dynamic roles that require interaction with other molecules in aqueous environments.
Tend to be sensitive to temperature and pH (can denature).

Examples of globular proteins:

Haemoglobin — oxygen transport in blood.
Myoglobin — oxygen storage in muscle.
Insulin — hormone regulating blood glucose.
Enzymes — most enzymes (e.g., catalase, amylase, DNA polymerase).
Antibodies (immunoglobulins) — immune defence.
Albumin — transport of fatty acids, bilirubin, drugs in blood plasma.

2.2 Fibrous Proteins

Fibrous proteins are long, thin, insoluble molecules typically formed from repeated parallel polypeptide chains. They typically:

Have regular, repetitive amino acid sequences.
Contain large regions of secondary structure (often α-helix or β-pleated sheet) that extend the length of the molecule.
Are held together by many hydrogen bonds and often disulfide bridges.
Are insoluble in water due to predominantly hydrophobic surfaces.
Perform structural or mechanical roles.
Are stable over a wide range of temperatures and pH.

Examples of fibrous proteins:

Collagen — connective tissue (tendons, skin, bone matrix, cartilage).
Keratin — hair, wool, nails, claws, feathers, outer layer of skin.
Elastin — elastic fibres of skin, lungs, blood vessel walls.
Silk fibroin — silk from silkworm cocoons and spider webs.
Fibrin — blood clots.

Lesson 10: Quaternary Structure and Protein Function