Silks, /3-keratin structure

For many years has been assumed that in silk fibroin, stretched hair and muscle, and other proteins with the jS-keratin structure the polypeptide chains are extended to nearly their maximum length, about 3.6 A per residue, and during the last decade it has been assumed also that the chains form lateral hydrogen bonds with adjacent chains, which have the opposite orientation. A hydrogen-bonded layer of this sort is represented diagram-matically in figure l. ... 

The apparent identity distance in the fiber X-ray diagrams of silk is somewhat smaller than corresponds to a completely extended polypeptide chain. We accordingly concluded that the polypeptide chains have a zigzag conformation in silk and the beta-keratin structure. We reported in detail three proposed sheet structures. The first one, which we... 

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. 

Fibrous proteins are long-chain polymers that are used as structural materials. Most contain specific repetitive amino acid sequences and fall into one of three groups coiled-coil a helices as in keratin and myosin triple helices as in collagen and p sheets as in silk and amyloid fibrils. 

Some protein structures limit the kinds of amino acids that can occur in the J3 sheet. When two or more /3 sheets are layered close together within a protein, the R groups of the amino acid residues on the touching surfaces must be relatively small. J3-Keratins such as silk fibroin and the fibroin of spider webs have a very high content of Gly and Ala residues, the two amino acids with the smallest R groups. Indeed, in silk fibroin Gly and Ala alternate over large parts of the sequence. 

Compare structural features and properties of the following proteins silk fibroin, a-keratin, collagen, and bovine serum albumin. 

In many cases there are important interactions between protein molecules that may lead to highly organized structures such as the pleated sheet of silk fibroin (Figure 25-13) or the coiling of a helices, as found in a-keratins, the fibrous proteins of hair, horn, and muscles (Figure 25-17). This sort of organization of protein molecules is called quaternary structure and is an important feature of many proteins that associate into dimers, tetramers, and so on. The tetrameric structure of hemoglobin is an important example. 

Coils such as those found in alpha-keratin are not the only structural motifs present in fibrous proteins. Silk, for example, is largely composed of fibrous proteins whose structures resemble interleaved sheets, see also Quaternary Structure Secondary Structure Tertiary Structure. 

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. 

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. 

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... 

Proteins can be divided into two main types, based on their overall shape. Fibrous proteins, as their name implies, have fiberlike structures and are used for structure or support. They are found, for example, in collagen (skin, tendon cartilage, fish scales), elastin (connective tissue), and keratins (silk, feathers, horn, and hair). They are tough and insoluble macromolecules, often having several a-helical chains wound together into ropelike bundles. 

Fiber diffraction is a technique used to determine the structures of molecules that are oriented to form fibers by virtue of a parallel assembly of molecules. Certain materials, such as cellulose, keratin and fibroin (the silk protein) occur naturally in this form. Some polymers can be drawn out to form fibers in which the same type of orientation occurs. This was the technique used by Rosalind Franklin to study DNA. More recently. 

---