When a protein in its native form, is subjected to physical changes like change in temperature or chemical changes like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein.The denaturation causes change in secondary and tertiary structures but primary structures remains intact .Examples of denaturation of protein are coagulation of egg white on boiling, curdling of milk, formation of cheese when an acid is added to milk.
`alpha`-helix and `beta`-pleated structures of proteins are classified as

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A. primary structure
B. secondary structure
C. tertiary structure
D. quaternary structure.