Tertiary Structure and Peptide Bonds: What Holds a Fold Together

By What Peptides Editorial Team · Updated 2026-09-14 · Part of What Peptides Are: Structure, Bonds and How Chains Are Built

Tertiary structure is not held together by peptide bonds. Peptide bonds hold the chain together in the correct order, which is the primary structure, but the three-dimensional fold is produced by interactions between side chains and between side chains and the backbone. This is one of the most persistent mix-ups in introductory biochemistry, and clearing it up makes the rest of the structural hierarchy much easier to hold in mind.

The peptide bond does have a role in folding, just not the one usually assumed. Because resonance locks it planar, it removes two degrees of freedom per residue and acts as a rigid spacer rather than a flexible joint, which sharply limits the shapes a chain can adopt. This page sets out what actually stabilises a fold, where the peptide bond fits, and how the four levels of structure relate. The wider context is in our peptide chemistry and structure reference.

What Tertiary Structure Is

Tertiary structure is the complete three-dimensional arrangement of all the atoms in one polypeptide chain, including the positions of the side chains. It is distinguished from secondary structure by range: secondary structure describes local repeating motifs a few residues long, while tertiary structure describes how those motifs pack against each other and how the regions between them are arranged. Two chains can share the same helix content and still have completely different folds.

For most single-chain structures the dominant driver is hydrophobic packing. Nonpolar side chains such as leucine, isoleucine, valine, phenylalanine and tryptophan are excluded from contact with water, so they cluster into an interior core while polar and charged residues remain on the surface. The gain is mostly entropic: water molecules that would otherwise be forced into ordered shells around exposed nonpolar groups are released. Individual contacts are weak, but a core containing dozens of them is collectively very stabilising.

Polar interactions add specificity on top of that bulk effect. Hydrogen bonds between side chains, or between side chains and backbone groups, fix particular geometries. Salt bridges between a lysine or arginine and an aspartate or glutamate contribute where they are buried from water. Aromatic side chains stack with each other at spacings of roughly 3.5 angstroms. Disulfide bridges are the one covalent exception, formed by oxidation of pairs of cysteine thiols, and they are not broken by heat alone. They also form only where the environment is oxidising, which is why they are common in secreted and extracellular structures and rare in those that fold in the reducing cytosol.

What Actually Holds the Fold Together

The table below lists the interactions in rough order of importance for a typical globular fold, with the caveat that the balance differs from one structure to another. The final row is the point of this page: the peptide bond is strong, covalent and permanent under ordinary conditions, which is exactly why it cannot be the interaction that folding switches on and off.

The strength column also explains why denaturation works the way it does. Individually weak interactions can be disrupted in bulk by heat, by extremes of pH, by chaotropic agents such as urea, or by detergents and organic solvents. Denaturation unfolds a chain without breaking a single peptide bond, which is why a denatured sample still has the same molecular mass and the same sequence as the native one.

Disulfides are the one entry that resists ordinary denaturation, and that is exactly why they are handled separately. They are covalent, they survive heating that unfolds everything else, and they are cleaved only by reducing agents such as dithiothreitol or beta-mercaptoethanol, which convert the bridge back to two free thiols. A sample treated with both a denaturant and a reducing agent is therefore fully unfolded and fully reduced, which is the condition used when a chain has to be separated purely by length.

The Hierarchy, and Where the Peptide Bond Sits

Four levels are conventionally listed. Primary structure is the covalent sequence of residues, and it is the only level held together by peptide bonds, along with any disulfide bridges. Secondary structure is the local backbone arrangement described by hydrogen bond patterns. Tertiary structure is the full three-dimensional fold of one chain. Quaternary structure is the assembly of two or more folded chains, which not every structure has.

Each level depends on the one below it, but they are stabilised by different forces, and that is the source of the confusion. Changing the sequence can destroy a fold because a core residue has been replaced. Heating a sample destroys the fold without touching the sequence. Adding a protease or strong acid destroys both, because hydrolysis cleaves the covalent backbone itself and the fragments no longer have a chain to fold.

So the accurate one-line answer is that the peptide bond defines the fold's search space rather than the fold. It fixes the order of residues, sets the geometry available to each of them by restricting rotation, and stays intact through everything short of hydrolysis. the geometry of helices, sheets and turns covers the level above primary, which statements about peptide bonds are true deals with the bond's own properties, and what denaturation does and does not break extends the argument. The practical consequence for handling is set out in common cold-chain and storage conventions.

This page is part of the What Peptides Are: Structure, Bonds and How Chains Are Built guide.