Which structure of protein intact after coagulation of egg white on boiling

The correct option is Primary.

Let's understand the process of protein denaturation during the boiling/coagulation of egg white step-by-step:

1. Understanding Protein Structure Levels:
Proteins have four levels of structural organization:
- Primary Structure: The linear sequence of amino acids linked together by strong, covalent peptide bonds.
- Secondary Structure: Local folding patterns such as $\alpha$-helices and $\beta$-pleated sheets, stabilized mainly by hydrogen bonds.
- Tertiary Structure: The overall three-dimensional folding of a single polypeptide chain, stabilized by hydrogen bonds, disulfide links, hydrophobic interactions, and ionic attractions.
- Quaternary Structure: The spatial arrangement and association of multiple polypeptide subunits.

2. Effect of Boiling (Denaturation):
When an egg is boiled, the thermal energy (heat) disrupts the weak, non-covalent interactions (such as hydrogen bonds, hydrophobic interactions, and ionic bonds) that maintain the secondary, tertiary, and quaternary structures of the egg white proteins (primarily ovalbumin).
As these stabilizing bonds break, the protein chains unfold and lose their specific three-dimensional shapes. This process is called denaturation.

3. Why the Primary Structure Remains Intact:
Unlike hydrogen bonds and other weak interactions, the primary structure is held together by covalent peptide bonds (-CO-NH-). These covalent bonds are highly stable and require much higher energy (such as strong chemical hydrolyzing agents or specific proteolytic enzymes) to break.
Therefore, the heat from boiling is not sufficient to disrupt the covalent peptide bonds. As a result, while the higher-order structures (secondary, tertiary, and quaternary) are destroyed during coagulation, the primary structure (the basic sequence of amino acids) remains completely intact.