Polypeptides silk fibroin

Figure 14.9 Spicier fibers are composite materials formed by large silk fibroin polypeptide chains with repetitive sequences that form p sheets. Some regions of the chains participate in forming 100-nm crystals, while other regions are part of a less-ordered mesh-work in which the crystals are embedded. The diagram shows a model of the current concepts of how these fibers are built up, which probably will be modified and extended as new knowledge is gained. (Adapted from F. Vollrath, Sci. Am. p. 54-58, March 1992 and A.H. Simmons, Science 271 84-87, 1996. Photograph courtesy of Science Photo Library.)...

The beta arrangement, or pleated sheet conformation (see Figure 14.2), is predominant when small pendant groups are present in the chain, as in silk fibroin. The silk fibroins, which are spun by various species of silkworms, are monofilament polypeptides with extensive secondary interchain bonding. The crystalline portion of the fibroin is a polymer of a hexapeptide. The... 

In Nautilus, the CP has a ratio of GLY ALA SER = 3 2 1. This ratio is identical to that described for the -configuration of silk fibroin, where serine supplants alanine in the layer structure in concentrations up to 15 mole %229. In silk, polypeptide chains are arranged in a zig-zag configuration and so-called pleated sheets are generated (Fig. 26). 

The (3-strand sequences are stretched out conformations of these polypeptide sections and are typically stabilized by inter-strand hydrogen bonds between keto (C = 0) oxygens and peptide bond NHs, the strands being arrayed in an antiparallel fashion. This type of secondary structure is favoured by amino acid residues with small R groups (such as Gly, Ala and Ser) that minimize steric overlap between chains. Thus a well-known protein having this type of secondary structure is silk fibroin that has a high proportion of repeated sequences involving Gly, Ala and Ser and an extensive antiparallel (3-pleated sheet structure. The macroscopic properties of silk fibroin (flexibility but lack of stretchability) reflect this type of secondary structure at the molecular level. 

The fibers of silk that are spun by silkworms and spiders consist mainly of the protein called silk fibroin. Different species of silkworms and spiders produce different kinds of silk fibroin, which differ somewhat from one another in their molecular architecture. All of the kinds of silk fibroin contain long zigzag polypeptide chains lying parallel to one another, as shown in the adjacent drawing. The chains extend in the "direction of the fiber or thread (vertically in the drawing). 

The -structure has the amino acids in an extended confirmation with a distance between adjacent residues of 0.35 nm (in the a-helix, the distance along the axis is 0.15 nm). The structure is stabilized by intermolecular hydrogen bonds between the -NH and -CO groups of adjacent polypeptide chains. The -structure can occur between separate peptide chains (e.g., silk fibroin) or be-... 

Some natural fibrous proteins and sequential model polypeptides have been used as follows (1) Tussah Antheraea perni silk fibroin [a-helix form], (2) Bombyx mori silk fibroin [silk I and II forms], (3) poly(L-alanyL-glycine) [Ala-Gly]i2 [silk I and II], (4) collagen fibril [triple-strand helix], and (5) poly(L-prolyl-L-alanyL-glycine) [Pro-Ala-Gly [triple-strand helix]. 

CRAMPS NMR study of a silk fibroin model polypeptide... 

Now it is clear that the H chemical shift reflects the conformation of model polypeptide [Ala-Gly] 12 and natural silk fibroins such as Tussah Antheraea pernyi and Bombyx mori silk fibroins. It is confirmed that the well-defined [Ala-Gly] 12 is a suitable model for the structural study of natural silk fibroins (silk I and silk II forms) using high-resolution solid-state NMR. As a result, the H peak assignment of the silk fibroins on the basis of the conformation-dependent H chemical shifts of model polypeptides can be determined utilizing H CRAMPS NMR and H- C 2D HETCOR NMR, as described in this section. The chemical shift results of model polypeptides [Ala-Gly] 12 synthesized by Shoji et al. play an important role in determining new structures for silk I and silk II forms, as very recently proposed by Lazo and Downing. ... 

It would seem reasonable to expect that the displacements of the chemical shifts could be used as an intrinsic probe of local environment of a given amino acid residue, if the transferability of the conformation-dependent shifts of polypeptides to more complicated protein systems is guaranteed. Fibrous proteins such as silk fibroin, collagen and collagen-like polypeptides can serve as ideal systems to justify this view, because several crystalline polymorphs are available depending on a variety of physical treatments and the spectral pattern is very simple as compared with those of globular proteins because of the limited numbers of amino acid residues involved. 

Some fibrous proteins are composed of p-pleated sheets. For example, the silkworm produces silk fibroin, a protein whose structure is an antiparallel p-pleated sheet (Figure 19.9). The polypeptide chains of a p-pleated sheet are almost completely extended, and silk does not stretch easily. Glycine accounts for nearly half of the amino acids of silk fibroin. Alanine and serine account for most of the others. The methyl groups of alanines and the hydroxymethyl groups of serines lie on opposite sides of the sheet. Thus the stacked sheets nestle comfortably, like sheets of corrugated cardboard, because the R groups are small enough to allow the stacked-sheet superstructure. 

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