Silk fibroin, pleated-sheet structure

Silk is produced from the spun threads from silkworms (the larvae of the moth Bombyx mori and related species). The main protein in silk, fibroin, consists of antiparallel pleated sheet structures arranged one on top of the other in numerous layers (1). Since the amino acid side chains in pleated sheets point either straight up or straight down (see p. 68), only compact side chains fit between the layers. In fact, more than 80% of fibroin consists of glycine, alanine, and serine, the three amino acids with the shortest side chains. A typical repetitive amino acid sequence is (Gly-Ala-Gly-Ala-Gly-Ser). The individual pleated sheet layers in fibroin are found to lie alternately 0.35 nm and 0.57 nm apart. In the first case, only glycine residues (R = H) are opposed to one another. The slightly greater distance of 0.57 nm results from repulsion forces between the side chains of alanine and serine residues (2). 

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. 

Figure 36.3. Pleated sheet structure (hefa arrangement) proposed by Pauling for silk fibroin. Chains contracted to make room for small side chains. Adjacent chains head in opposite directions hydrogen bonding between adjacent chains.

The /7-pleated-sheet structure present In silk fibroin. 

The -pleated-sheet structure present in silk fibroin. 

The principal component of wool is the protein keratin, which is a classic example of a-helical structure. The principal component of silk is the protein fibroin, which is a classic example of p-pleated sheet structure. The statement is somewhat of an oversimplification, but it is fundamentally valid. 

It should be pointed out that in both silk fibroins the OOZ reflections are quite diffuse, whereas the kOO and kOl reflections are relatively sharp. This is readily understandable in terms of a pleated-sheet structure the strong hydrogen bonding within the sheets would be expected to lead to well-defined repeat distances and, hence, sharp reflections in the direction of the a axis, whereas any disorder in the packing of adjacent sheets would result in diffuse reflections in the direction of the c axis. 

In contrast to the a-helical structure of the a-K. discussed above, the -K. have -pleated sheet structure. The most prominent representative of this class is silk fibroin (iff, 365,000, 2 subunits). Here the chains run antiparallel rather than parallel, and form a zig-zag structure. The formation of hydrogen bonds between the -CH(=0) and -NH- groups of neighboring chains stabilizes the pleated sheet structure. Together with weak hydrophobic interactions, the hydrogen bonds link pairs of polypeptides into a three-dimensional protein complex. These are additionally stabilized, in silk, by a water-soluble protein, sericin. The resultant fiber is very resistant and flexible, but only slightly elastic. The amino acid sequence which repeats over long stretches of the chain is, for silk fibroin, (Gly-Ser-Gly-Ala-Gly-Ala-) . 

Collagen (see) has a specialized structure containing interchain, hydrogen-bonded, left-handed helices. Otherwise, all known P. helices are right-handed, but the possibility remains that left-handed helices might be found in P. which have not yet been analysed by X-ray diffraction, e.g. membrane P. In the pleated sheet structure, the polypeptide chain is more or less stretched out, and neighboring lengths of chain can be parallel or antiparallel (with respect to their N- and C-termini), e.g. the pleated sheet structure of silk fibroin (Fig. 8) consists of antiparallel polypeptide chains. 

Figure 10 Pleated sheet structure (P-arrangement) of silk fibroin.

Another popular crystalline structure for protein is the structure formed by extended protein molecules. A pleated sheet structure is formed by hydrogen bonds between adjacent extended molecules aligned in parallel. Piezoelectricily in B-form protein was first observed by Fukada in 1956 for silk fibroin [3]. Since there is no intrinsic pt -zation in the B structure, pyroelectricity and ten piezoelectricity are not observed. However, the piezoelectricity due to shear is observed. If the shear is applied such as to cause slip between oriented molecules, polarizatioo is induced in the direction perpendicular to the plane of the shear. Shear piezoelectricity is observed for most natural biopolymers, not only keratin but abo trumy other proteins. 

The pleated sheet structure applies to proteins of the silk fibroin-/3-keratin group. For silk fibroin itself it may be assumed that the structure has been proved correct by Pauling s calculations (antiparallel chains). A pleated sheet structure with parallel chains is proposed for stretched hair (/3-keratin). Investigations on some other natural representatives have not yet been concluded. Possibly, amorphous or differently arranged sections occur in combination with pleated-sheet areas. 

Figure 25-13 Hydrogen-bonded structure of silk fibroin. Notice that the peptides run in different directions in alternate chains. This structure is called an antiparallel /3-pleated sheet.

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. 

Fibroin, the fibrous protein found in silk, has a secondary structure called a beta- (/8-) pleated sheet, in which a polypeptide chain doubles back on itself after a hairpin bend. The two sections of the chain on either side of the bend line up in a parallel arrangement held together by hydrogen bonds (Figure 24.8). Although not as common as the a-helix, small pleated-sheet regions are often found in proteins. 

Silk is made of fibroin, a fibrous protein with a pleated-sheet secondary structure. 

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). 

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