Each protein in the biological system has a unique three-dimensional structure and has specific biological activity.
When a protein in its native form is subject to heat, the hydrogen bonds are broken down.
Due to cleavage of hydrogen bonds, unfolding of protein molecule occurs and the protein loses it biological activity.
This loss of biological activity is called denaturation.
As a result of denaturation, globular proteins are converted into fibrous proteins.
Denaturation leads to coagulation. That is why coagulated proteins are also called as denatured proteins.
On boiling, the globular proteins present in the egg undergo coagulation to form fibrous proteins.