Silk-like polypeptide

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. 

D-and L-Poly(lysine) both occur in a helical conformation in aqueous solution above pH 10. A 1 1 mixture of both helices in water precipitates in the form of regular, silk-like P-sheets (Fig. 9.5.3). This is a polypeptide example of the chiral bilayer effect. It demonstrates the importance of chiral uniformity not only for noncovalent, soft-membrane systems, but also for covalent polyamides. Helical fibers in aqueous gels do not survive the addition of enantiomeric fibers with the single exception of DNA (see Fig. 8.6.1). The precipitation of enantiomeric peptides containing more than one kind of amino acid has, however, not been reported so far. 

Explain the following three observations, (a) Silk, like most polypeptides with sheet structures, is water insoluble, (b) Globular proteins such as myoglobin generally dissolve readily in water, (c) Disruption of the tertiary structure of a globular protein (dena-turation) leads to precipitation from aqueous solution. 

The /S-pleated structure is markedly different from the a helix in that it is like a sheet rather than a rod. The polypeptide chain is almost fully extended, and each chain forms many intermolecular hydrogen bonds with adjacent chains. Figure 25.13 shows the two different types of /S-pleated structures, called parallel and antiparallel. Silk molecules possess the /S structure. Because its polypeptide chains are already in extended form, silk lacks elasticity and extensibility, but it is quite strong due to the many intermolecular hydrogen bonds. 

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. 

Secondary structures can take two forms an a-helix is a spiral shape, whereas the pleated sheet looks like a piece of paper folded many times. The parallel polypeptide chains cross-linked by hydrogen bonds form an extremely tough structure. Silk is an example (Hale et al., 1995). 

Not all proteins, however, form helical structures. If the substituent groups on the amino acids are small, as found in silk fibroin, then the polypeptide chains can line up side by side and form sheet-like arrangements. The chains tend to contract to acconunodate hydrogen bonding and form pleated sheets. This is called a -arrangement. Such an arrangement can be parallel and antiparallel. The identity period of the parallel one is 6.5 A and that of the antiparallel, 7.0 A. 

The two pleated sheets provide satisfactory structures for proteins and for polypeptides composed exclusively of L amino-acid residues (or d amino-acid residues). We think that it is likely that silk fibroin, for which the... 

The question whether silk fibroin filaments are resorbable or permanent is open to interpretation. Having a polypeptide chemical structure, silk fibroin, like any other protein, is susceptible to proteolytic degradation, and will become weaker and eventually over a period of 2 years will be totally resorbed in vzvo. However, given the definition for an absorbable suture in the United States Pharmacopeia as a material that loses most of its tensile strength within 60 days post-implantation silk can therefore be classified as a permanent biomaterial. 

Natural polymers include RNA and DNA that are so important in genes and life processes. In fact, messenger RNA is what makes possible proteins, peptides, and enzymes. Enzymes do the chemistiy inside living organisms and peptides make up some of the most interesting stmctural components of skin, hair, and even the horns of rhinos. Other natural polymers include polysaccharides (sugar polymers) and polypeptides like silk, keratin, and hair. Natural mbber is a natural polymer, made from just carbon and hydrogen. 

A second form of keratin, known as fi-keratin, is produced by stretching the a-keratin in hair to about twice its original length. The X-ray diffraction pattern of -keratin is similar to that of silk fibroin (page 56) and its structure, like that of fibroin, is based on the /5-pleated sheet. There is a difference, however, in that polypeptide chains are in a parallel (Figure 5.2a) and not an antiparallel arrangement. 

All of us are held together by fibrous proteins. A quarter of our body protein is collagen, possibly the most abundant protein on earth. There are actually at least twenty collagens, all very similar in structure, but differently distributed in the animal body. Like silk, collagen is rich in glycine, which occupies every third position in the sequence, and the other abundant amino acid is proline. This amino acid is unique in that its side chain makes a loop, attached at its other end to the nitrogen atom of the polypeptide backbone thus ... 

Chimeric (fusion) proteins that incorporate the R5 peptide have been synthesized to control and precipitate silica nanoparticles. Po Foo and coworkers have utilized a two-component chimeric protein consisting of the R5 polypeptide (from C. fusiformis) and the self-assembling domain based on the consensus repeat in the major ampullate spidroin protein 1 (MaSpl) of Nephila clavipes spider dragline silk [64]. MaSpl forms highly stable P-sheet secondary stmctures that can be spun into intricate fibers which, when fused with the sihca-templating R5-peptide, allow for the formation of film-like and fibrous silica structures (Figure 1.18). 

In a P-pleated sheet, hydrogen bonds are formed between —NH and—CO groups in different polypeptide chains or different areas of the same polypeptide chain. Figure 28.9 shows the P-pleated sheet in the structural protein, silk. A fairly flat sheet-like structure is formed. 

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