Is Keratin Hard or Soft?
Keratin is both hard and soft, depending on its type and the amount of cystine it contains.
Keratin is a fibrous structural protein, a key component of skin, hair, and nails. It exists in two primary forms:
-
Hard Keratin: This type is found in hair and nails. It's characterized by a high cystine content (approximately 14%), making it strong, rigid, and resistant to degradation. Hard keratin's high cystine levels contribute to its robust, protective qualities. As noted by several sources, such as ScrippsLabs.com, and ScienceDirect Topics, the high cystine content is the key differentiator.
-
Soft Keratin: Located in the skin, this form possesses a significantly lower cystine content (around 2%). Consequently, soft keratin is more pliable and less resistant to damage compared to its hard counterpart. The difference in cystine levels directly impacts its physical properties, as explained in the ScienceDirect Topics and ScrippsLabs.com articles. This Quora answer even highlights that variations in composition contribute to differences in hardness between nails and hair.
The differing levels of cystine lead to substantial variations in the properties of keratin. A paper on the biomechanics of mammalian keratins indicates that this difference even impacts hydration resistance. Soft keratins swell and soften significantly in water, while hard keratins remain more resistant.
The presence of both hard and soft keratin in various parts of the body demonstrates the protein's versatility in providing a range of protective functions, from the toughness of hair and nails to the flexibility of skin. The structure, as described in Wikipedia's entry on Keratin, also points to the strength of the protein due to its bundled intermediate filaments.
