Keratin helical structure

The homy layer consists of about 10% extracellular components such as lipids, proteins, and mucopolysaccharides. Around 5% of the protein and lipids form the cell wall. The majority of the remainder is present in the highly organized cell contents, predominantly as keratin fibers, which are generally assigned an a-helical structure. They are embedded in a sulphur-rich amorphous matrix, enclosed by lipids that probably he perpendicular to the protein axis. Since the stratum comeum is able to take up considerably more water than the amount that corresponds to its volume, it is assumed that this absorbed fluid volume is mainly located in the region of these keratin structures. 

Secondary bonding is involved in forming the helical structures allowing the various bundles of alpha-keratin to be connected by weak secondary interactions, which in turn allow them to readily slide past one another. This sliding or slippage along with the... 

Macromolecules of importance to the biomaterials scientist having some portion composed of a helical structure include hemoglobin, myosin, actin, fibrinogen, and keratin. The a helix is a rather condensed structure because the rise per residue is 1.5 A and as such is quite different from that of the collagen triple helix and the p structure of silk. The rise per residue in the two latter structures is about twice that found in the a helical structure. For this reason the extensibility of the a helix is greater than that of the collagen triple helix and the p structure and in the case of keratin, tensile deformation of the a helix leads to formation of a p structure. 

Our last example of the mechanical properties of a protein is that of keratin found in the top layer of skin. The stratum corneum in skin is almost exclusively made up of different keratins that have an a-helical structure. The helices do not run continuously along the molecule so the structure is not ideal. However, the stress-strain characteristics are shown in Figure 6.4 and demonstrate that at low moisture content the stress-strain curve for keratins in skin is approximately linear with a UTS of about 1.8 GPa and a modulus of about 120 MPa. These values are between the values reported for elastin and silk, which is consistent with the axial rise per amino acid being 0.15 nm for the a helix. Thus the a helix with an intermediate value of the axial rise per amino acid residue has an intermediate value of the... 

Hair keratin has historically been associated with the a-helical structure. For that reason, it is termed a-keratin. And indeed the basic keratin polypeptides are a-helical except for their N- and C-terminal domains. These are believed to be involved in head-to-tail condensation to form keratin polymers. When hair keratin is stretched, the resulting secondary structure is the parallel pleated sheet (see Chapter 4). Stretched keratin is referred to as /3-keratin to emphasize its secondary structure. 

Biological macromolecules are often distinguished by their helical structures to which one-dimensional space-group symmetries are applicable. Figure 8-15a shows Linus Pauling s sketch of a polypeptide chain, which he drew while he was looking for the structure of alpha-keratin. When he decided to fold the paper, he arrived at the alpha-helix. The solution may have come in a sudden moment,... 

Keratin in the cortex comprises 85% or more of the mass of the hair shaft. Cortical keratin is composed of two types of structural proteins, matrix proteins and fibrous proteins. 2-1 Matrix proteins have a high sulfur content and contain polypeptides with a molecular weight of approximately 10 to 28 kDa. Fibrous proteins are embedded in matrix proteins and are characterized by a low sulfur content. They have a molecular weight of approximately 40 to 58 kDa. Also, matrix proteins have a nonhelical structure and are readily soluble at pH 4.5 in 0.5 M KCl, whereas fibrous proteins exhibit a helical structure and are insoluble in this same solution. 

Fig. 10.28. Formation of a cytokeratin filament. The central rod of the keratin monomer is principally a-helical structure. A specific acidic keratin monomer combines with a specific basic keratin monomer to form a heterodimer coil (a coiled coil structure). Two dimers combine in antiparallel fashion to form a tetramer, and the tetramers combine head-to-tail to form pro to filaments. Approximately eight protofilaments combine to form a filament. The filament is thicker than actin filaments (called thin filaments or micro filaments) and thinner than microtubules (thick tubes) and is therefore called an intermediate filament.

The principal component of wool is the protein keratin, which is a classic example of a-helical structure. The principal component of silk is the protein fibroin, which is a classic example of p-pleated sheet structure. The statement is somewhat of an oversimplification, but it is fundamentally valid. 

Keratin is the major component of hair (and nails). It s a protein that makes up the outer layer of your skin, your hair, fingernails, and toenails. Actually, keratin is everywhere in the animal world the hooves, claws, and horns of most mammals, the scales of reptiles, the shells of turtles, and the feathers, beaks, and claws of birds. Many copies of this protein molecule assemble into a large helical structure that provides the struc-... 

They are insoluble because the peptide chains are linked by disulfide bonds. Many keratins contain coils of a-helixes. Some keratines, however, were found to consist of complicated j -helical structures. This apparently has not been fully explained. Wool keratin was shown to range in molecular weight from 8,000-80,000. The extensibility of a-keratins is believed to be due to the helical structures. The extent of keratin hardness (in claws, horns, and nails) is believed to be due to the amount of sulfur links. 

The a-helical structure is found in many proteins it is the predominant structure of the polypeptide chains of fibrous proteins such as myosin, the protein of muscle, and of a-keratin, the protein of hair, unstretched wool, and nails. 

Hair is composed of a protein called keratin. The secondary structure of keratin is a-helix throughout, meaning that the protein has a wound-up helical structure. As we learned earlier, this structure is maintained by hydrogen bonding. 

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