(N/A) Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein.
When a protein in its native form is subjected to physical changes like a change in temperature or chemical changes like a change in $pH$,the hydrogen bonds are disturbed.
Due to this,the globules unfold,the helix gets uncoiled,and the protein loses its biological activity. This is called denaturation of protein.
During the denaturation of $2^{\circ}$ and $3^{\circ}$ proteins,the structures are destroyed,but the $1^{\circ}$ structure remains intact.
The coagulation of egg white on boiling is a common example of denaturation.
Another example is the curdling of milk,which is caused by the formation of lactic acid by bacteria present in the milk.