A denatured protein has had its secondary, tertiary, and quaternary structure disrupted by heat, extreme pH, organic solvents, heavy metal ions, or chemicals like urea. These agents break hydrogen bonds, hydrophobic interactions, and sometimes disulfide bridges, the bonds that normally hold the folded chain in place.
The primary structure, the sequence of amino acids linked by peptide bonds, remains untouched. Losing the folded shape usually destroys biological activity, since enzymes and other functional proteins depend on a precise three-dimensional structure to bind their targets. Denatured proteins often become insoluble and clump together, which is why cooked or curdled proteins look different from their raw form.
The term also applies outside the kitchen. Hand sanitizers and disinfectants often work partly by denaturing the proteins in bacteria and viruses, disabling them. Some small, simply folded proteins can renature and regain function if the denaturing condition is removed gradually and gently.
Sterilizing surgical instruments with high heat works on the same principle, denaturing the proteins of any bacteria or viruses present. Formaldehyde is another common denaturing agent, historically used to preserve biological tissue samples by locking proteins in a fixed, denatured state.