The denaturation of proteins occurs when high temperatures or chemical interactions destroy both their secondary and tertiary structures. Denaturation processes are not actually strong enough to break the peptide bonds within proteins and cause their chemical formulas to change.
Proteins are large, complex molecules with unique properties that depend on more than just the sequence of elements in their primary structures. Proteins are large enough to have different properties in different regions of the molecules, such that one part might be polar while another is not, and one part reacts strongly with certain chemicals while another reacts strongly with others. The folding of the protein, caused by structures and attractions between different parts of the protein, exposes or hides its different regions, determining how it reacts to its environment. Many biological functions actually trigger proteins to change their conformation, exposing a reactive subgroup when necessary.
Denaturation breaks down these secondary and tertiary structures, exposing the entire protein molecule. One common example is the cooking of an egg. The proteins of the egg, which are folded to reduce their binding to one another and remain liquid, are disrupted, so they can bind to each other and become solid. Their chemical composition does not change, but their chemical properties do.