Quaternary structure: non-conjugated and conjugated proteins

Molecules (Form and function) · Proteins · note 7 of 8

Spec B1.2.11

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Quaternary structure: non-conjugated and conjugated proteinsSpec B1.2.11

In short

Quaternary structure is the linking of two or more polypeptides to form a single protein. Non-conjugated proteins consist only of polypeptides, for example insulin (two chains) and collagen (three chains). Conjugated proteins also contain a non-polypeptide component, for example haemoglobin, which has four polypeptides each with a haem group containing iron that binds oxygen.

Many proteins consist of a single polypeptide. Others are made of two or more polypeptides linked together; this arrangement is quaternary structure. The chains are held together by the same types of interaction as tertiary structure.

Non-conjugated protein: A protein made only of polypeptides.
Conjugated protein: A protein with one or more non-polypeptide components (prosthetic groups) as well as polypeptides.

Examples of quaternary structure
Protein	Polypeptides	Non-polypeptide part	Type
Insulin	Two: an A chain (21 amino acids) and a B chain (30 amino acids) linked by two disulfide bonds	None	Non-conjugated
Collagen	Three, wound together into a triple helix	None	Non-conjugated
Haemoglobin	Four: two alpha and two beta chains	Four haem groups, each containing an iron ion that binds one O₂	Conjugated

In haemoglobin each polypeptide holds one haem group, so one molecule carries up to four oxygen molecules. The haem is not made of amino acids, which is what makes haemoglobin conjugated. Insulin and collagen are made only of polypeptide chains.

Insulin is first made as one long polypeptide, proinsulin. A middle section is then cut out, leaving two chains that are held together by disulfide bonds. The IB guide uses insulin's two linked chains as an example of quaternary structure.

Insulin: an A chain of 21 amino acids and a B chain of 30, joined by two disulfide bridges between the chains, with a third disulfide bridge within the A chain (cysteines highlighted). Collagen: three polypeptides wound into a triple helix. Haemoglobin: four folded subunits, two alpha and two beta, each holding a haem group with Fe²⁺. Insulin and collagen are labelled non-conjugated, haemoglobin conjugated. — Quaternary structure: insulin (2 chains) and collagen (3 chains) are non-conjugated; haemoglobin has 4 chains plus haem groups, so it is conjugated.

Exam tip:

Nature of science: technology lets us see what the unaided senses cannot. Cryogenic electron microscopy rapidly freezes protein samples and combines many images of single molecules into 3D structures, showing proteins and their interactions with other molecules.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between essential and non-essential amino acids?

Essential amino acids cannot be synthesised by the body, so they must be obtained from food. Non-essential amino acids can be made in the body from other amino acids. Some plant proteins are low in certain essential amino acids, so vegans eat a variety of plant protein sources to get all of them.

Why do proteins denature at high temperatures?

Proteins denature at high temperatures because heat makes the molecule vibrate more until hydrogen bonds and other weak interactions holding its three-dimensional shape break. The protein unfolds and loses its function. Peptide bonds are not broken, so the amino acid sequence is unchanged, but the change in shape is usually permanent.

How does pH affect protein structure?

A change in pH changes the charges on R-groups, because amine and carboxyl groups gain or lose hydrogen ions. Ionic bonds between oppositely charged R-groups break, and hydrogen bonding changes, so the protein's three-dimensional shape alters. Outside its stable pH range a protein denatures and stops functioning, for example an enzyme losing its active site shape.

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