Protofibril, keratin

FIGURE 4-11 Structure of hair, (a) Hair a-keratin is an elongated a helix with somewhat thicker elements near the amino and carboxyl termini. Pairs of these helices are interwound in a left-handed sense to form two-chain coiled coils. These then combine in higher-order structures called protofilaments and protofibrils. About four protofibrils—32 strands of a-keratin altogether—combine to form an intermediate filament. The individual two-chain coiled coils in the various substructures also appear to be interwound, but the handedness of the interwinding and other structural details are unknown, (b) A hair is an array of many a-keratin filaments, made up of the substructures shown in (a). 

The assembly of hair a-keratin from one a helix to a protofibril, to a microfibril, and finally, to a single hair. (Illustration copyright by Irving Geis. Reprinted by permission.)... 

Figure 2.25. Structure of keratin protofibrils. The diagram illustrates the structure of keratin in intermediate filaments containing a helical sequences. Two coils are wound around each other and then packed into protofilaments. Eight protofilaments are packed into a filament.

Wool, which is the hair of sheep and goats, contains more than 200 different compounds, of which approximately 80% are keratins (fibrous, helical proteins). Two or three helices are chemically interconnected or cross linked by, for example, S-S bonds to form a protofibril. Eleven... 

Human hair is made of protofibrils, which are twisted bundles of the coiled protein alpha-keratin. Alpha-keratin is so strong and flexible that a human hair can be tied in a knot without breaking. 

A plausible explanation for these results is that inside the keratin structure water is molecularly dispersed and forms monomolecular layers around the various protein structural units, i.e., the micro-or protofibrils of the keratin. The low values obtained for Vw can be explained by assuming that when water molecules penetrate the hair structure, they fill, at least in part, pre-existing voids. The results also suggest that the cortex of the hair structure is more porous than the cuticle, since removal of the cuticle descaling of the hair) reduces the value of... 

The majority part of the interior of the fiber mass is the cortex, which, from the point of view of mechanical properties, is also the most important component. The cortex consists of elongated, spindle-shaped cells aligned in the direction of the fiber axis. Within these cells resides the major part of the keratinized protein in the form of macrofibrils, which in turn are formed by lower levels of organization, i.e., microfibrils and finally protofibrils. The latter two are low-sulfur proteins and more or less crystalline in nature with their a-helical parts as crystalline lattice components. They are embedded in a noncrystalline, nonfibrillar matrix of disulfide cross-linked, globular proteins. 

There are some 20 amino acids in nature, see Figure 14.29. These are organized into an a-helix in the fibrous proteins, which in turn are combined to form protofibrils as shown. In addition to being crystalline, the fibrous proteins are cross-linked though disulfide bonds contained in the cystine amino acid mer, which is especially high in keratin. Animal tendons, composed of collagen, another fibrous protein, have also been shown to have a surprisingly complex hierarchical structure (166). 

Keratins are products of epithelial cells. They occur in the outer skin layer (epidermis), and in some skin structures (fur, horns and hooves). The main group are the so-called a-keratins, based on a right-handed polypeptide with an a-helical structure and molecular weight of 10-50 kDa, stabilised by disulfide bridges and hydrogen bonds. Three polypeptides form left-handed helices called protofibrils and 11 protofibrils (two inside and nine outside) create microfibrils. Several hundred microfibrils create macrofibrils that finally form the keratin fibre (e.g. hairs and wool fibres). 

Mechanically, the most important component of wool fibers is the cortex. Figure 5.15 shows the stmcture of the cortex in a wool fiber. The protein in wool fibers is called keratin. Three keratin chains in wool form helices with helix angles ranging from 30° to 35°. Three helix chains twist compactly together to form a protofibril with a diameter of aroimd 2 run. Eleven protofibrils assemble to form a microfibril with a diameter of 7-8 nm. Microfibrils also are called intermediate filaments and they helically wind together into a macrofibril with a diameter of... 

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