Tensile strength keratin

The four protein conformations that provide mechanical stability to cells, tissues, and organs include the random coil or amorphous structure that characterizes a part of the structure of elastin, the a helix, which is represented by the keratin molecule, the collagen triple helix, and the p structure of silk. In humans the P structure is found only in short sequences connecting parts of other structures such as the a helix, but serves as an example of the relationship between protein structure and properties. The ultimate tensile strength and modulus of each structure differs as discussed below. 

Mechanical structure and support. Collagen provides the tensile strength in skin, teeth, and bone keratin is the major component of hair and fin wA fibroin is one of two proteins that comprise spider silk. 

Pleated sheet structures ifi structures) have little stretchability, but high tensile strength. In pleated sheets the peptide chains lie in a plane, either parallel to each other as in j -keratin of bird feathers or antiparallel as in the more highly crystalline silks. 

The section ends with some comments on fracture of biological fibres. These fibres are characterised by a hierarchical structural design with length scales ranging from molecular to macroscopic. Clearly, detailed quantitative models for prediction of tensile strength of biological fibres are far from being available. However, some trends in connection with cellulose and keratin fibres are briefly discussed. 

The first known structure of a fibrous protein was that of keratin. It comes in two forms, a- and -keratin, and their structures are what their designations imply. a-Keratin is the major protein of hair, nails, feathers, and skin, and is entirely a-helical, but it also has a higher order structure, with three strands of a-helix twisted around each other like strands of spaghetti. This results in a filament of increased stiffness and tensile strength. Leather is practically all a-keratin. /3-Keratin, on the other hand, is largely made up of antiparallel /3-pleated sheets. It is the principal protein of birds beaks and claws and of silk. 

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