Protein Structure
Protein structure refers to the hierarchical three-dimensional arrangement of a protein, described at four levels: primary (sequence of amino acids), secondary (local folding into alpha helices and beta sheets), tertiary (overall 3D fold of a single polypeptide), and quaternary (assembly of multiple polypeptide subunits). The structure of a protein is intimately linked to its function, and alterations in structure — through mutation, denaturation, or misfolding — can lead to loss of function or disease. Determining and predicting protein structure is one of the central challenges of biochemistry and structural biology.
Four Levels of Protein Structure
| Level | Description | Stabilising Forces | Example | Analytical Technique |
|---|---|---|---|---|
| Primary (1°) | Linear sequence of amino acids | Peptide bonds (covalent) | Haemoglobin: Val-His-Leu-... | Edman degradation, MS/MS |
| Secondary (2°) | Local regular structure (α-helix, β-sheet) | Hydrogen bonds | α-helix in myoglobin | Circular dichroism (CD) |
| Tertiary (3°) | Full 3D fold of one polypeptide chain | H-bonds, van der Waals, disulphide, hydrophobic | Lysozyme globular fold | X-ray crystallography, NMR |
| Quaternary (4°) | Multi-subunit assembly | Same as tertiary + subunit interfaces | Haemoglobin (2α + 2β) | Cryo-EM, SAXS |
| Denaturation | Unfolding of 2°/3°/4° structure | Disruption of non-covalent bonds | Egg white cooked by heat | Fluorescence, calorimetry |
Interactive Tools
Khan Academy — Protein Structure
Four levels of protein structure explained with clear diagrams.
Open ToolNCBI Protein Data Bank (PDB)
Search and visualise 3D protein structures from the Protein Data Bank.
Open ToolBrilliant.org — Protein Folding
In-depth wiki on how proteins fold and the forces that stabilise structure.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Peptide Bond
A peptide bond is a covalent amide linkage formed between the carboxyl group (–COOH) of one amino acid and the amino group (–NH2) of another, with the elimination of a water molecule in a condensation reaction. The resulting –CO–NH– bond has partial double bond character due to resonance delocalisation of the nitrogen lone pair into the carbonyl, making it planar and restricting rotation around the C–N bond. Peptide bonds are the primary covalent linkages in all protein chains.
Amino Acid
An amino acid is an organic molecule that contains both an amino group (–NH2) and a carboxyl group (–COOH) attached to the same central (alpha) carbon, along with a variable side chain (R group) that determines the identity and properties of each amino acid. There are 20 standard amino acids encoded by the genetic code that serve as the building blocks of proteins. Amino acids differ in polarity, charge, size, and chemical reactivity, which directly determines protein structure and function.
Polymer
A polymer is a large macromolecule composed of many repeating structural units called monomers, linked together by covalent bonds through a process called polymerisation. Polymers can be natural (e.g., cellulose, proteins, DNA) or synthetic (e.g., polyethylene, nylon, PVC), and their physical properties are governed by chain length, branching, cross-linking, and monomer identity. They are indispensable in modern industry, biology, and materials science.
From Greek 'proteios' (of the first rank, primary), coined by Swedish chemist Jöns Jacob Berzelius in a letter to Gerardus Johannes Mulder in 1838, who first described the macromolecule. The hierarchical description of protein structure (primary through quaternary) was formalised by Linderstrøm-Lang in 1952 and extended by others throughout the 20th century.