![]() ![]() Amino acid abbreviations are A, alanine G, glycine P, proline Q, glutamine and S, serine.Īll spider silks begin as liquid protein solutions, termed dopes. Secondary structures are indicated to the right and include various β-sheets that stack to form crystals, 3 1 helices that can link molecules together (GGX) and elastic nanosprings that bond intra-molecularly (GPGX). The relative abundance of each type of functional motif (black = common and white = absent) are indicated for each of the five fibrous silks spun by orb spiders (MaSp, major ampullate MiSp, minor ampullate Flag, flagelliform TuSp, tubuliform and AcSp, aciniform). ![]() (C) At least five different functional motifs are indentified in various spider silks. Each repetitive module is composed of short runs of amino acids, termed functional motifs, which are often predicted to form specific secondary structures in the silk fibre. (B) Most spider silk fibroins have a modular structure consisting of ∼ 12–20 modular units or ‘ensemble repeats’ whose amino acid sequences are largely similar to one another. The now solid fibre passes through a muscled valve that can further control molecular alignment until the fibre passes out of the spigot. The micellar configuration of the fibroins is squeezed as they pass through the narrowing of the funnel and into the duct where counter-current exchange allows for ion uptake and water resorption. The fibroins are stored as micelles in a concentrated liquid solution in the lumen. The tail contains secretory cells that excrete silk fibroins into the gland. (A) Major ampullate glands in orb spiders are composed of at least six functional elements. Structure of spider silk glands and silk proteins. In contrast, the ensemble repeats of different types of silks are incredibly divergent, to the degree that they cannot be easily homologized ( Gatesy et al., 2001).įig. The amino acid sequences of these terminal regions also provide the primary data to describe the evolutionary origin of silks ( Garb et al., 2010). The high degree of conservation in the amino acid sequences of the termini among different types of silk, some of which diverged hundreds of millions of years ago, argues for conserved function of the termini in the production of different silks ( Rising et al., 2006 Sponner et al., 2004). Ten to 100 of these repetitive modules are linked together, forming ∼ 90% of the total protein, and are flanked on either end by n (amino) and c (carboxyl) terminal runs of 100–200 amino acids ( Ayoub et al., 2007 Fig. Amino acid motifs are combined into larger repetitive units, sometimes called ensemble repeats or repetitive modules, that range from < 50 to over 200 amino acids in length ( Ayoub et al., 2007). These motifs are short sequences of amino acids hypothesized to form the specific secondary structures that ultimately determine the overall shapes of individual proteins, and therefore how silk proteins interact to form whole fibres ( Guerette et al., 1996 Hayashi et al., 1999 Fig. Most spider silk proteins consist largely of internal regions of highly repetitive amino acid ‘motifs’. For example, most studies on silk blending with other synthetic polymers and biomacromolecules have been omitted from this chapter. We have also focused mainly on some of the more compelling biomaterial-related studies and applications with silk proteins, necessitating the omission of many studies and many areas of potential interest in the biomaterials field. While we have emphasized more routine processing approaches, such as methanol and temperature treatments to crystallize silks, many other physical methods, such as electrical/dielectric field and electromagnetic fields, can induce beta-sheet crystallization in silk proteins, leading to new processing for silk-based biomaterials. Further, the myriad of processing tools to control the structural state of silk proteins provide direct control over the mechanical properties and degradation lifetime of silk-based biomaterials. The versatility of silk proteins in terms of processing, including aqueous solutions, the biocompatibility, controllable in vivo biodegradation rate, along with the remarkably robust mechanical properties, prompt interest in the biomaterials generated from silk proteins. ![]() By processing into a diverse set of morphologies, such as films, nanofibers, microspheres, nanoparticles, hydrogels, or different micro-/nanopatterned devices, the potential of silk-based biomaterials has expanded for potential biomedical applications. Silk proteins have been exploited recently in a wide range of biomaterials. Kaplan, in Comprehensive Biomaterials, 2011 2.212.9 Conclusions ![]()
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