The loss of the secondary, tertiary, and quaternary structures of a protein by a chemical or physical agent that leaves the primary structure intact.
Protein conformations are stabilized in their native states by secondary and tertiary structures and through the aggregation of subunits in quaternary structure. Any physical or chemical agent that destroys these stabilizing structures changes the conformation of the protein. We call this process denaturation.
NOTE! Denaturation changes secondary, tertiary, and quaternary structures. It does NOT affect primary structures (that is, the sequence of amino acids that make up the chain).