Keratin helix structure

Pauling and Corey proposed the a-helix structure for a-keratins. 

In alpha-keratin, shown in Figure 5, the entire length of the protein has an a-helix structure. However, other proteins will have only sections that are a-helixes. Different sections of the same protein may have a pleated sheet secondary structure. These different sections of a protein can fold in different directions. These factors, combined with the inter-molecular forces acting between side chains give each protein a distinct three-dimensional shape. This shape is the tertiary structure of the protein. 

The peptide arrangement in protein fibre has been investigated since the first half of the twentieth century. Astbury used X-rays to demonstrate the nature of a crystalline phase in hair. The X-ray dilfraction pattern of animal hairs shows a meridian reflection at 0.51 mn and an equatorial reflection at 0.98 nm. Interpreting these results Pauling et al. proposed the a-helix structure to give account of the secondary structure of the keratin fibre, shown in Figure 9.6.5. 

Fibrous proteins, being insoluble in water often have a structural or protective function. The most familiar fibrous proteins are the keratins and collagen. a-Keratin (Figure 25.18) is based on the a-helix secondary structure and is the protein structural component of hair, wool, nails, claws, quills, horns, and the outer layer of skin. P-Keratin is based on the P-sheet secondary structure and occurs in silk as fibroin. L-Cysteine is especially abundant in keratins, where it can account for more than 20% of the amino acids present. Collagen occurs mainly in connective tissue (cartilage and tendons) and has a triple helix structure. 

The secondary structures can also unite the supersecondary structures or secondary structure aggregates. Two a helices are enmeshed with each other via the side chains in what are called double a helices they wind about each other with a period of about 18 nm. Such double a helices are found, for example, with a-keratin, myosin, paramyosin, and tropomyosin. The pleated-sheet-helix structure is another supersecondary structure it consists of three pleated-sheet strands and two a helices. Such supersecondary structures occur with phosphorylases, phosphoglycerate-kinases, and some dehydrogenases. 

Proteins with helix structures (a structures) can be drawn or stretched to about double their length and possess unusually high elasticity. Wool keratin, myosin (a muscle protein), fibrinogen, and collagen are in this group. 

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. 

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. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Alpha farnesene, structure of, 207 Alpha helix (protein), 1038 Alpha-keratin, molecular model of, 1039... 

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). 

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