Which structure of proteins gives rise to fibrous and globular proteins?

Tertiary Structure

The correct answer is Option (3) → Tertiary Structure

Option 1: Primary Structure: Primary structure is the linear sequence of amino acids in a polypeptide chain. It only tells us which amino acids are present and in what order. There is no information about folding, bending, or shape here. So this level alone cannot decide whether a protein will be fibrous or globular.

Option 2: Secondary Structure: Secondary structure involves local folding of the polypeptide backbone into $\alpha$-helix or $\beta$-pleated sheet due to hydrogen bonding between $-\text{CO}$ and $-\text{NH}$ groups. These structures form segments within the protein, not the overall shape. Both fibrous and globular proteins may contain $\alpha$-helices or $\beta$-sheets, so this level does not determine the final form.

Option 3: Tertiary Structure: Tertiary structure refers to the complete 3D folding of a single polypeptide chain. This folding occurs due to interactions between side chains (R-groups): hydrogen bonds, ionic bonds, disulfide bonds, hydrophobic interactions.

If folding is tight and compact → globular protein.

If chains remain long and aligned → fibrous protein.

So, this level decides the overall shape and function.

Option 4: Quaternary Structure: Quaternary structure exists only in proteins made of more than one polypeptide chain (e.g., hemoglobin). It describes how different subunits assemble together.

However, even before subunits combine, each chain already has a tertiary structure that defines its shape. Hence, this level is not responsible for distinguishing fibrous vs globular nature.