MS-6-P-2374 Folding Structures of Polypeptide Chains in Silk

Fibrous structures represent a basic construction principle in nature and are of interest in research aimed at the development of advanced materials for biomimetic and other applications. The amino acid sequences and the environment determine the ability of a peptide chain to fold in a particular manner and assemble into a complex protein. This talk presents an overview of the structural organization in fibers and films of natural and regenerated silk as revealed by 200-400 kV electron microscopy and diffraction under cryo-protection.
The high mechanical stability of silk is provided by stiff nano-sized crystallites acting as reinforcing elements within an oriented fiber network [1], helical chain conformations, e.g., 31 helices, are responsible for the elastic and contractional behavior. The β-structure of spidroin from spiders is formed by peptide chains with dominating alanine groups in a pleated-sheet arrangement. Silkworm fibroin differs in its contributing amino acids through the replacement of parts of alanine repeats by glycine resulting in additional small and poor crystals and an increased number of random molecules. The silk-I phase and metastable hydrogen-bonded linker regions control the amazing reversible contraction behavior. Increasing the degree of silk-II β-crystallinity induces dramatic changes which cause irreversible supercontraction due to lamellar overgrowth of pre-existing β-crystals.
Silk from ants, wasps, bees and hornets exemplify a different fiber type that is dominated by α-helices in a coiled coil superstructure (Fig. 1) [2]. Characteristic diffraction features are the sharp reflection arc on the meridian, belonging to the 5.1 Å spacing of one turn of the α-helix, and the broad equatorial maximum related to a spacing of ~9 Å (Fig. 2a). The pitch of the supercoil of 172 Å gathered from the off-equatorial diffraction intensity distribution as well as from the orders of meridional scattering fits well into the range predicted for four-stranded coiled coils. Only a minor component forms pleated β-sheet structures (Fig. 2b).
[1] Schaper, A.K., Yoshioka, T., Kawahara, Y.: Fascinating silk - electrospinning, contraction and diffraction experiments. Imaging & Microscopy 14, 25-27 (2012).
[2] Kameda, T., Nemoto, T., Ogawa, T., Tosaka, M., Kurata, H., Schaper, A.K.: Evidence of α-helical coiled coils and β-sheets in hornet silk. J. Struct. Biol. 185, 303-308 (2014).