Protein tertiary structure is held together by five main types of interactions: hydrophobic effects, hydrogen bonds, ionic bonds (salt bridges), van der Waals forces, and disulfide bonds. The first four are non-covalent, meaning they don’t involve shared electrons. Disulfide bonds are the exception, providing a strong covalent link. Together, these forces fold a protein’s chain into its final three-dimensional shape and keep it there.
The Hydrophobic Effect
The hydrophobic effect is the single most important driver of protein folding. In an aqueous environment like the inside of a cell, nonpolar (water-avoiding) amino acid side chains are thermodynamically unstable when exposed to water. They cluster together in the protein’s interior, away from the surrounding water, forming a tightly packed hydrophobic core. This isn’t so much an “attraction” between the nonpolar groups themselves as it is the water pushing them together. When nonpolar side chains are exposed individually, each one forces the surrounding water molecules into rigid, ordered shells. Burying those side chains in the core releases those constrained water molecules, which increases the overall disorder of the system and lowers its free energy.
The process is more nuanced than a simple collapse. As two nonpolar groups approach each other in water, their individual water shells merge into a shared layer, then eventually squeeze out the intervening water entirely. Each step involves small energy barriers and attractions as the surrounding water reorganizes. The net result is a strong thermodynamic push to bury nonpolar residues, and this is what gives globular proteins their characteristic shape: a greasy interior with a polar, water-friendly surface.
Hydrogen Bonds Between Side Chains
Hydrogen bonds in the tertiary structure form between the polar side chains (R-groups) of amino acids. This is distinct from the hydrogen bonds in secondary structure, which occur along the protein backbone and create alpha helices and beta sheets. In tertiary structure, a hydrogen atom bonded to an electronegative atom like oxygen or nitrogen on one side chain is attracted to a lone pair of electrons on another nearby side chain. Amino acids like serine, threonine, asparagine, glutamine, tyrosine, and others with polar groups are common participants.
Each individual hydrogen bond is relatively weak, contributing roughly 1 to 3 kcal/mol of stabilization energy. But proteins contain dozens or even hundreds of these bonds, and their cumulative effect is substantial. They also play a critical role in specificity: hydrogen bonds have strict geometric requirements, so they help ensure the protein folds into one particular shape rather than a random compact glob.
Ionic Bonds and Salt Bridges
Ionic interactions, often called salt bridges, form between amino acid side chains that carry opposite charges at physiological pH. The acidic side chains of aspartate and glutamate carry a negative charge, while the basic side chains of lysine, arginine, and histidine carry a positive charge. When a positively charged group sits close to a negatively charged group, typically within about 2.5 to 4 angstroms, the electrostatic attraction stabilizes that particular region of the fold.
Salt bridges are sensitive to pH. The acidic side chains of aspartate and glutamate have pKa values around 3.5 to 4.3. If the surrounding pH drops near or below those values, these groups pick up a proton, lose their charge, and the salt bridge breaks. This is one reason extreme pH denatures proteins. Ionic strength matters too: high salt concentrations can screen the charges and weaken these interactions. In structural studies, researchers have observed that disrupting even a single salt bridge can shift a protein from its folded form to an alternative conformation.
Van der Waals Forces
Van der Waals forces are the weakest individual interactions in the tertiary structure, each contributing only a fraction of a kcal/mol. They arise from momentary fluctuations in electron density around atoms, creating temporary tiny dipoles that attract neighboring atoms. Individually trivial, they become significant because the hydrophobic core of a protein is densely packed. Hundreds or thousands of atoms sit in close contact, and all those tiny attractions add up.
This tight packing is key. The protein interior has a packing density comparable to a crystal of small organic molecules. The enhanced London dispersion forces that result from this dense arrangement are actually a major contributor to the overall stability gained by burying nonpolar side chains, working hand in hand with the hydrophobic effect.
Disulfide Bonds
Disulfide bonds are the only common covalent interaction in tertiary structure, making them by far the strongest individual stabilizing link. They form between two cysteine residues whose sulfur-containing side chains come close enough in space for their sulfur atoms to undergo oxidation, losing two electrons and creating a sulfur-to-sulfur (S-S) bond. The two cysteines involved don’t need to be near each other in the amino acid sequence; they just need to be brought into proximity by the protein’s fold.
These bonds are especially important for proteins that function outside the cell, like antibodies, digestive enzymes, and hormones such as insulin. The extracellular environment is harsher than the cell interior, with more mechanical stress and chemical variability. Disulfide bonds provide the permanent, robust cross-links these proteins need to maintain their shape under those conditions. Inside cells, the chemical environment is more reducing, so disulfide bonds are less common in intracellular proteins.
Metal Ion Coordination
Beyond the five classic interactions, metal ions also stabilize tertiary structure in many proteins. Zinc, magnesium, iron, and other metal ions coordinate with specific amino acid side chains to lock portions of the structure in place. Zinc, for instance, commonly coordinates with the side chains of histidine, cysteine, aspartate, and glutamate, typically binding four groups in a tetrahedral arrangement. The zinc finger motif in DNA-binding proteins is a well-known example: without the zinc ion, the small protein domain cannot hold its shape.
Magnesium tends to coordinate with five or six groups, showing a strong preference for oxygen-containing side chains from aspartate and glutamate. These metal-binding sites can serve a purely structural role, acting as stabilizing anchors, or they can be part of the protein’s active site where catalysis occurs. In either case, removing the metal ion often causes the local structure to collapse.
How These Interactions Work Together
No single interaction type is responsible for a protein’s tertiary structure. The hydrophobic effect provides the main thermodynamic driving force, collapsing the chain into a compact shape. Hydrogen bonds and salt bridges then fine-tune the fold, selecting one specific arrangement over the many possible compact forms. Van der Waals forces stabilize the tightly packed core. Disulfide bonds, where present, act as permanent clamps. The net stability of a folded protein is surprisingly small, often only 5 to 15 kcal/mol, which is the difference between many large stabilizing forces and many large destabilizing ones.
This marginal stability is actually functional. It means proteins can unfold and refold in response to cellular signals, and it allows the flexibility needed for enzymes to shift shape during catalysis. But it also means the tertiary structure is vulnerable to disruption.
What Disrupts Tertiary Structure
Because the folded state depends on a delicate balance of forces, changing the environment can tip that balance and denature the protein. High temperatures increase molecular motion enough to overcome the weak non-covalent interactions holding the core together, and the enthalpy change during thermal unfolding reflects the exposure of previously buried hydrophobic groups to water. Chemical denaturants like urea and guanidinium chloride compete directly with intramolecular hydrogen bonds: they form their own hydrogen bonds with the backbone and side chains, effectively prying the structure apart. Extreme pH disrupts salt bridges by changing the charge state of acidic and basic side chains. High ionic strength screens electrostatic interactions. Even changes in pressure can force water into the hydrophobic core.
Disulfide bonds, being covalent, are not broken by heat or pH changes under normal conditions. Breaking them requires a chemical reducing agent that donates electrons back to the sulfur atoms, converting the S-S bond back into two free S-H groups. This is why reducing agents are used alongside denaturants in laboratory protocols that need to fully unfold a protein.