Keratin composition and disulfide bonding in hair curvature

Mechanisms of Hair Curvature

Curly Keratin

Hair composition is composed primarily of structural proteins, which make up 60-95% of its composition (Barthelemy et al. 2012). These proteins include keratin intermediate filaments (KIFs) and keratin-associated proteins (KAPs) that together form a matrix held together through disulfide, hydrogen, electrostatic, and amide bonds. The nature of these bonds renders hair more or less vulnerable to environmental stressors, such as sunshine, pollutants, and hair treatments.

Direct comparisons of the three main types of keratin, hair, epidermal, and simple type, revealed that hair is comparatively cysteine-rich (Strnad et al. 2011). These cysteines in turn play an important role in determining hair structure. For example, 97-98% of the cysteines in sheep wool have been shown to be involved in forming disulphide bonds. The proline content of hair KIFs is also relatively high and is believed to induce kinks in the filament, thereby facilitating disulphide bond formation.

The head and tail region of inner root sheath KIFs have a relatively low cysteine content, which has been suggested to give a greater degree of flexibility for forming alternate molecular interactions between KIFs and the likewise cysteine-rich KAP proteins (Yu et al., 2009). Together with the suggested role that the inner root sheath plays determining the shape of the developing hair strand, the cysteine composition of KIFs and KAPs would therefore play a significant role in helping to determining hair curvature.

The relative ratio of KIF to KAP composition also varies between the inner and outer sides of hair fibers, depending on an individual's ethnicity (Kajiura et al., 2006). For example, a greater contribution of KAPs to the inner portion of African-American hair explains its curly shape. This was also found to be the case for naturally curly sheep wool. By comparison, the relatively equal prevalence of KAPs between the inner and outer hair shaft of Asian hair samples helps to explain its relative lack of curvature.