Quaternary Structure: When Multiple Subunits Combine
Definition
Quartenary structure
The quaternary structure of a protein refers to the specific three-dimensional arrangement of multiple polypeptide chains (subunits) in a single functional protein.
Some proteins consist of two or more polypeptide chains, called subunits, that assemble into a single functional unit.
The way these subunits fit together is the protein's quaternary structure.
The same interactions that hold tertiary structure together also hold the subunits together: hydrogen bonds, ionic bonds, hydrophobic interactions, and sometimes disulfide bonds.
Quaternary proteins fall into two categories.
Non-conjugated proteins contain only polypeptide chains.
Conjugated proteins also include a non-polypeptide component called a prosthetic group.
Non-Conjugated Proteins: Polypeptide Chains Only
Insulin: Two Disulfide-Linked Chains
Insulin consists of two polypeptide chains, an A chain and a B chain, linked by disulfide bonds.
This compact quaternary structure gives insulin the precise shape it needs to bind insulin receptors on target cells and trigger glucose uptake.
Collagen: A Triple-Helix Structural Protein
Collagen is made of three polypeptide chains twisted together into a triple helix.
This rope-like quaternary structure gives collagen enormous tensile strength, making it the main structural protein in skin, tendons, ligaments, and cartilage.
Conjugated Proteins: Polypeptides Plus a Prosthetic Group
Haemoglobin: Four Subunits Plus Heme
Haemoglobin has four polypeptide subunits: two alpha chains and two beta chains.
Each subunit carries a heme group containing an iron ion (Fe²⁺).
The heme group is a prosthetic group, a non-polypeptide component permanently bound to the protein and essential for its function.
Each iron ion binds one molecule of oxygen, so a single haemoglobin molecule can carry up to four oxygen molecules.
When oxygen binds to one subunit, it causes a conformational change that increases the affinity of the remaining subunits for oxygen.
This makes haemoglobin efficient at loading oxygen in the lungs, where oxygen concentration is high, and releasing it in respiring tissues, where oxygen concentration is low.
Common Mistake
The heme group is not a polypeptide chain.
Haemoglobin has four polypeptide subunits and four heme prosthetic groups.
Do not count the heme groups as subunits when describing quaternary structure.
Active recall
What is the difference between a non-conjugated and a conjugated protein?
Name the prosthetic group in haemoglobin and state which ion it contains.
How many polypeptide subunits does haemoglobin have, and what are they?
Explain cooperative binding in haemoglobin in one or two sentences.
How does collagen's triple-helix structure relate to its function in tendons?
