Fibroin strands

Other fibrous proteins contain extensive regions of pleated sheets. The fibers spun by a silkworm, for example, are made almost entirely of fibroin, a protein composed primarily of just three amino acids glycine (45%), alanine (30%), and serine (12%). Each chain of fibroin contains extensive regions where a sequence of six amino acids occurs repeatedly . .. -Gly-Ser-Gly-Ala-Gly-Ala-. .. Notice that every other amino acid is glycine, which is the smallest amino acid. This alternating arrangement is an important feature in the packing of the strands that make up the pleated sheet. 

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 silk fiber protein is synthesized by the silk gland cells and stored in the lumen of silk gland. Subsequently it is converted into silk fibers. Each strand of silk fiber is a double stmeture with 2 fibroin filaments covered by sericin (Figure 18). 

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]. 

Right-handed P-rolls are formed by stacked cods of two or more P-strands built of short repeats. The silkworm fibroin consists of antiparallel P-sheets with 50 repeated - (GA)2GSGAAG-(SGAG)gY-sequences. The P-sheets are stacked on top of each other with Gly tending in contact with each other alternating with contacting Ala and Ser to give close-packed sheets. 

All silks are extrusion spun into a protective cocoon before the larva undergoes metamorphosis. This cocoon consists of a continuous double strand of fibroin protein, conglutinated by the sericin protein, which acts as a binder to glue the threads into the cocoon shell. 

Fig. 2 Supramolecular natural polymeric hydrogels discussed in this chapter, (a) Chemical structure of the most-repeated sequence in collagen, forming the a-chain that folds in a three-stranded superhelix [135]. These superhelices bundle to fram the collagen fiber, (b) Representative chemical structure of fibroin and the antiparallel p-sheet formation connected by hydrophilic linkers, (c) Chemical structure of alginic acid, cross-linked by calcium ions (highlighted), (d) Left Top view of two a-helixes of keratin forming a coiled coll by hydrophobic interactions. Right Overview of subsequent formation of the fibril. The left part is adapted from [57] with permission of The Royal Society of Chemistry...

From WAXS and SAED data of both ProNectin F lyophilized powder and sprayed fibrils, the current model indicates that ProNectin F crystallizes into a chain folded pleated sheet of beta strands (Anderson et a/. 1994). The strands are oriented antiparallel. The beta strands are not fully extended, but have a more compressed crankshaft conformation. This conformation agrees with the predicted conformation of unoriented silk fibroin protein, the Silk I structure (Lotz and Keith 1971). The crystal dimension in the c direction (along the peptide backbone) is consistent with a theoretical length of 11.6 nm for nine SEP blocks (54 amino acids) in this conformation. This predicts that the width of the ProNectin F tile is controlled at least in part by the number of amino acids in the silklike block domains. 

The P-pleated sheet is an important secondary structure in proteins that are rich in amino acids with small side chains such as H (glycine), CHj (alanine), and CH2OH (serine). The model in Figure 25.13 is a portion of the calculated structure for a sheet composed of antiparallel strands containing only glycine and alanine in alternating order (Gly-Ala-Gly-Ala-, etc.). It was designed to resemble fibroin, the major protein of silk. 

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