Quaternary Structure
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You should understand how tertiary structure is stabilised by hydrophobic interactions, hydrogen bonds, and disulfide bridges, and be familiar with the concept of a polypeptide chain folding into a specific three-dimensional shape.
Why this matters
Many essential biological functions, from oxygen transport to enzyme catalysis, depend on proteins that are not single chains but assemblies of multiple subunits.
While a single polypeptide chain can fold into a functional tertiary structure, many proteins require the association of two or more polypeptide chains to achieve their biological activity. This level of organisation, known as quaternary structure, allows for complex regulatory mechanisms, such as allosteric control, and creates active sites that are too large or chemically diverse for a single chain to provide. Understanding how subunits interact is crucial for explaining how haemoglobin transports oxygen efficiently and how enzymes like aspartate transcarbamoylase are regulated by metabolic feedback.
The Concept of Subunit Assembly
Imagine a protein not as a single tangled string, but as a machine built from several distinct parts that snap together. In quaternary structure, these parts are called subunits. Each subunit is a complete polypeptide chain that has already folded into its own tertiary structure. The quaternary structure is the specific three-dimensional arrangement of these subunits relative to one another. The forces holding them together are the same non-covalent interactions that stabilise tertiary structure: hydrophobic interactions, hydrogen bonds, ionic bonds, and van der Waals forces. Unlike tertiary structure, however, quaternary structure involves the interface between separate chains. This assembly is often reversible; subunits can dissociate and reassociate in response to environmental changes, which is a key mechanism for regulating protein activity. For example, a protein might be inactive as a monomer but become active when it dimerises, or it might change its shape and affinity for a substrate when a ligand binds to one subunit, triggering a conformational change in the others.
Haemoglobin: A Classic Example
The most famous example of quaternary structure is haemoglobin, the oxygen-carrying protein in red blood cells. Haemoglobin is a tetramer, meaning it consists of four subunits. In adult humans, this is typically two alpha (\(\alpha\)) subunits and two beta (\(\beta\)) subunits, denoted as \(\alpha_2\beta_2\). Each subunit contains a haem group, which is a porphyrin ring with an iron atom at its centre. This iron atom is the site where oxygen binds. The quaternary structure of haemoglobin is critical for its function because it allows for cooperative binding. When oxygen binds to one haem group, it induces a conformational change in that subunit. This change is transmitted to the other subunits, making it easier for them to bind oxygen. This results in a sigmoidal (S-shaped) binding curve, which is highly efficient for loading oxygen in the lungs (high partial pressure) and unloading it in the tissues (low partial pressure). Myoglobin, by contrast, is a monomer with a single haem group and exhibits hyperbolic binding, making it suitable for oxygen storage in muscle tissue rather than transport.
Allosteric Regulation and Symmetry
Quaternary structure provides the structural basis for allosteric regulation, where the binding of a molecule at one site affects the activity at another site. This is possible because the subunits are physically connected and can transmit conformational changes. Many enzymes with quaternary structure exhibit symmetry, such as cyclic dimeric or tetrameric symmetry. This symmetry simplifies the description of the protein's states. For instance, a tetramer can exist in a "tense" (T) state with low affinity for substrate and a "relaxed" (R) state with high affinity. The transition between these states is concerted, meaning all subunits change shape simultaneously. This model, known as the Monod-Wyman-Changeux (MWC) model, explains how small molecules, called allosteric effectors, can shift the equilibrium between T and R states. An activator stabilises the R state, increasing catalytic activity, while an inhibitor stabilises the T state, decreasing activity. This allows cells to fine-tune metabolic pathways in response to the concentration of end-products or other signalling molecules.
Forces Stabilising Quaternary Structure
The stability of quaternary structure relies on the complementarity of the surfaces of the interacting subunits. These interfaces are often large and involve a significant number of non-covalent contacts. Hydrophobic interactions are particularly important, as non-polar residues from different subunits cluster together to exclude water, driving the association. Hydrogen bonds and ionic interactions (salt bridges) provide specificity, ensuring that the correct subunits assemble in the correct orientation. Disulfide bonds, while primarily stabilising tertiary structure within a single chain, can also form between chains in some extracellular proteins, adding covalent stability to the quaternary assembly. The dissociation of subunits is often sensitive to environmental conditions such as pH, ionic strength, and temperature. For example, increasing the ionic strength can disrupt salt bridges, leading to the dissociation of the complex. This reversibility is essential for the dynamic regulation of protein function in the cell.
The concept
The levels of protein structure
From amino-acid sequence to folded, functional shape.
A common misconception is that quaternary structure is always required for protein function. While many proteins are multimeric, many others, such as myoglobin or lysozyme, are functional monomers. The presence of quaternary structure is an evolutionary adaptation to specific functional needs, such as cooperativity or large active sites, rather than a universal requirement for protein stability or activity. C
Worked example
Problem. Adult haemoglobin (\(\alpha_2\beta_2\), molar mass about 64,500 g mol-1) binds one O2 at each of its four haem groups. How many millilitres of O2 (at 0 °C and 1 atm, 22.4 L mol-1) can 1.00 g of haemoglobin carry when fully saturated? How much O2 can one litre of blood carry with 150 g of haemoglobin per litre?
- Moles of Hb in 1.00 g: \(1.00/64\,500 = 1.550\times10^{-5}\ \mathrm{mol}\).
- Moles of O2: \(4 \times 1.550\times10^{-5} = 6.20\times10^{-5}\ \mathrm{mol}\).
- Volume: \(6.20\times10^{-5} \times 22\,400\ \mathrm{mL\,mol^{-1}} = 1.39\ \mathrm{mL}\) per gram.
- Per litre of blood: \(150 \times 1.39 = 208\ \mathrm{mL}\) of O2, far more than the roughly 3 mL that dissolves in a litre of plasma.
Units check. g ÷ g mol-1 gives mol, and mol × mL mol-1 gives mL. Clinical texts quote 1.34–1.39 mL g-1; the lower figure allows for haemoglobin that cannot bind oxygen.
Three quick tests
- What is the primary difference between tertiary and quaternary structure?
Answer
Tertiary structure involves the folding of a single polypeptide chain, while quaternary structure involves the assembly of multiple polypeptide chains (subunits). - Why is haemoglobin's quaternary structure important for oxygen transport?
Answer
It allows for cooperative binding of oxygen, resulting in a sigmoidal binding curve that is efficient for loading oxygen in the lungs and unloading it in the tissues. - What type of interactions primarily stabilise quaternary structure?
Answer
Non-covalent interactions, including hydrophobic interactions, hydrogen bonds, ionic bonds, and van der Waals forces.
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Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.