Feather keratins 3-sheet structure

The / -keratins, as their name implies, contain much more / -sheet structure. Indeed, they represented the second major structural class described by Pauling and co-workers. The / -keratins are found mostly in birds and reptiles, in structures like feathers and scales. 

Plan of the proposed structure for feather rachis keratin. The structure consists of pleated-sheet layers, between which there are double layers of 3.7-residue helixes. 

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. 

Ordered secondary structures like a-helixes and p-sheets occurring in ther-moplastically processed proteins are effectively forms of crystallinity. In analysis of extruded feather keratin, which used sodium sulfite to break disulfide bridges, a shift from low crystallinity to higher crystallinity was seen between DSC scans of samples with 4% sodium sulfite and samples with 5% sodium sulfite.Proteins may also contribute to the formation of ordered regions in polymers/protein blends. DSC analysis of crystal formation in soy protein/ poly(butylene succinate) (PBS) blends showed soy protein both induced and accelerated PBS crystallization. ... 

Keratins - ot-Keratins are the major proteins of hair and fingernails and a compose a major fraction of animal skin, oi-keratins are classified in the broad group of intermediate filament proteins, which play important structural roles in nuclei, cytoplasm, and cell surfaces. Their secondary structure is composed predominantly of -helices. Figure 6.11 shows the coiled-coil structure of the ot-keratin in hair. The chemical composition of the cysteine residues in ot-keratin affects its macromolecular structure and function. For example, hair has relatively few cysteine cross-links, whereas fingernails have many such cross-links, / -keratins, on the other hand, contain much more pleated sheet secondary strucure than ot-keratins and are found in feathers and scales. 

Alpha keratin is found in sheep wool. The springy nature of wool is based on its composition of alpha helices that are coiled around and cross-linked to each other through cystine residues. Chemical reduction of the cystine in keratin to form cysteines breaks the cross-links. Subsequent oxidation of the cysteines allows new cross-links to form. This simple chemical reaction sequence is used in beauty shops and home permanent products to restructure the curl of human hair—the reducing agent accounts for the characteristic odor of these products. Beta keratin is found in bird feathers and human fingernails. The more brittle, flat structure of these body parts is determined by beta keratin being composed of beta sheets almost exclusively. 

In humans, perhaps the best-known example of an aggregated P-sheet containing fibrous protein is P-keratin, an aggregated P-sheet-containing protein. P-Keratin is the major constituent of human nail, bird feather calamus and rachis, and reptile scales and claws. This differs from amyloids by having a P-structure in which the peptide chains are parallel, rather than perpendicular, to the direction of the fibril axis. 

The X-ray diffraction photographs of feather rachis keratin, which have previously been interpreted as involving P-keratin pleated sheets and a-keratin helixes in molecular distribution, may show a superimposed P-keratin pattern and a-keratin pattern. If this is correct, the a-keratin is different in nature from hair and horn a-keratin, probably consisting of AB cables with ropes (three a-helixes coiled about one another) in the interstices. This structure accounts in a striking way for the characteristic features of the X-ray photographs. 

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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