Identify Incorrect Statement About Tertiary Structure of Proteins

To identify the incorrect statement about the tertiary structure of proteins, let us analyze each option based on the principles of biochemistry:

1. Option A: The tertiary structure of proteins represents the overall three-dimensional arrangement of a polypeptide chain, which gives rise to two major spatial shapes: fibrous (long, thread-like structures) and globular (spherical structures). Thus, statement A is correct.

2. Option B: The $3^\circ$ (tertiary) structure of proteins is stabilized by various non-covalent interactions and covalent bonds between the side chains ($R$-groups) of amino acids. These major forces include:

   • Hydrogen bonding
   • Disulfide linkages ($-\text{S}-\text{S}-$ bonds)
   • van der Waals (hydrophobic) forces
   • Electrostatic forces of attraction (ionic/salt bridges)

   Thus, statement B is correct.

3. Option C: When a native protein is subjected to changes in temperature or pH, the hydrogen bonds and ionic interactions stabilizing the secondary and tertiary structures are disrupted. This causes the protein to lose its specific $3^\circ$ structure and biological activity, a process called denaturation. Consequently, the tertiary structure does not remain intact during pH changes. Thus, statement C is incorrect.

4. Option D: Primary protein structure (a linear polypeptide chain) folds into secondary structures ($\alpha$-helix and $\beta$-pleated sheet), which further fold upon themselves to generate the complex tertiary structure. Thus, statement D is correct.

Therefore, the incorrect statement is option C.