Denaturation is a process of structural change in a certain macromolecule (protein, nucleic acid). During this process macromolecule loses its native state: its quaternary structure, tertiary structure, and secondary structure are degraded. This can occur in a living system, as a consequence of a stress, strong acid or base, alcohol, radiation, heat etc. When denaturation happens cell loses its activity and possibly cell death occurs.
One of the examples of protein is degradation is food cooking, boiling eggs denatures its proteins.