Question

Which one of the following is a highly insoluble protein?

• A. Albumin
• B. Casein
• C. Lactoglobulin
• D. Keratin

Hint:

Protein Classifications:
Insoluble Scleroproteins = Keratin, Collagen, Elastin.
Water Soluble = Albumins.
Salt Soluble = Globulins.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Fibrous scleroproteins: $lpha$- and $eta$-keratins contain dense intra- and intermolecular cystine disulfide crosslinks (-S-S-) conferring extreme chemical insolubility and proteolytic resistance.
Key Formula or Approach:
\[ \text{Keratin} = \text{High Cysteine/Cystine Content } (10 - 15\%) \implies \text{Covalent Disulfide Crosslinks } (-S-S-) \implies \text{Insoluble} \]

Step 2: Detailed Explanation:
Protein solubility classifications:
1. Fibrous Scleroproteins Albuminoids (Keratin): Structural proteins constituting wool, hair, hooves, horns, feathers, and claws. High content of cystine forms extensive covalent disulfide cross-linkages, making keratin completely insoluble in water, dilute acids, alkalies, and resistant to standard digestive proteases (pepsin, trypsin). Hydrolyzed only by autoclaving under pressure (hydrolyzed feather meal).
2. Globular Soluble Proteins: Albumin (water-soluble), Casein (phosphoprotein soluble in alkali/micellar suspension), Lactoglobulin (soluble in dilute salt solutions).

Step 3: Final Answer:
Hence, Keratin is a highly insoluble protein, matching option (D).