Proteins with specific three dimensional structure and biological activity, found in biological systems are called native (natural) proteins. When native proteins are subjected to physical or chemical changes like change in pH or temperature the arrangement of their hydrogen bonds gets disturbed. Because of this, globules of the protein unfold and helix gets uncoiled and protein loses its biological activity. This is called denaturation of protein. The coagulation of egg white on boiling is an example of denaturation of protein. During denaturation, its primary structures remains unaffected but secondary and tertiary structure are destroyed.
Example : Curding of milk which is caused due to the formation of lactic acid by the bacteria present in milk. Denaturation of protein can be of two types-reversible denaturation and irreversible denaturation.