Keratin's shape is complex and hierarchical, depending on its type and location within the body. At the most basic level, a single keratin chain forms a spring-like alpha-helix shape.
Alpha-Keratin's Structure:
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Alpha-helix: Individual keratin chains twist into a right-handed alpha-helical structure due to hydrogen bonds within the chain itself. This is a fundamental component of many keratin types. (Source: "Three distinct regions can be identified in the molecular structure of α-keratin: the crystalline fibrils , the matrix , and the terminal domains of the filaments (Fig. 6a) (51). The hydrogen bond inside the α-helix chain causes the chain to twist and exhibit a helical shape.")
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Coiled-coil dimer: Two alpha-helices then twist around each other to form a coiled-coil dimer, a more stable structure. (Source: "A single chain of keratin is arranged into a spring-like structure known as alpha-helix. Two of these chains twist together to form a structure...")
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Protofilaments and filaments: These dimers assemble into protofilaments, and then protofilaments aggregate to create intermediate filaments, which are larger, rope-like structures. These filaments are highly organized in some keratin-rich tissues.
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Overall structure: The final shape and organization of keratin varies depending on the tissue and its function. For example, in hair, the filaments are organized in a very specific way to create the overall shape and texture of the hair. In skin, the structure is different, providing strength and protection.
Variations in Keratin Shape:
While the alpha-helix is a key building block, keratin's overall shape is not solely helical. The higher-order structures—the way these helices combine—determine the final form. Factors influencing overall shape include:
- Type of keratin: Different types of keratin exist (e.g., alpha-keratins found in hair and skin versus beta-keratins in reptiles and birds). This variation affects the higher-order structure.
- Processing and environmental factors: External treatments (like chemical straightening or keratin treatments for hair) can significantly alter the keratin structure and, therefore, the final shape.
The references highlight keratin's diverse arrangements from helical protein chains to the complex, fiber-like structures found in hair, nails, and skin. While the alpha-helix is a crucial component, the overall shape is a consequence of the intricate assembly of numerous protein chains.
