(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 and the helix gets uncoiled,causing the protein to lose its biological activity. This process is called denaturation of proteins.
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.