Even if you don’t follow materials research closely you may have come across the amazing properties of spider silk. The stuff is stronger than steel, yet more elastic than most artificial fibres, despite being made of proteins only. It owes its remarkable strength to hydrogen bonds and its microstructure of amorphous and crystalline domains.
But the really amazing thing about spider silk is its assembly. The precursor protein solution is stored as a stable liquid within the spider’s body, but when it exits through the spinning gland it immediately precipitates into a solid protein thread. This shows not only what amazing tricks are possible with those versatile biomolecules, but also that we have only just begun to unravel their secrets. There have been, however, two recent publications in Nature going a long way to explaining the controlled transition from solution to silk. The one I found most inspiring deals with the molecular details of a pH switch at the N-terminus of silk proteins.
One amazing thing is that the basic principle is actually very simple: Proteins are precipitated from their highly saturated solution by changes of pH and salt concentration. On the other hand the details how those switches work and how they enable self-organisation into the complete silk thread are fascinating. It is known that the conditions in the silk gland point change significantly and contribute a range of chemical and mechanical stimuli to the development of the silk thread.
Silk proteins consist of three distinct areas: A region of repetitive elements in the middle that actually form the main structural component of the silk, and two evolutionally conserved regions that cap the protein at the N- and the C-terminus. Knight et al. studied several recombinant proteins assembled from various combinations of the three domains. Since recombinant proteins made up of only repetitive regions spontaneously coagulate into filaments under almost all circumstances, they focused on the capping regions, especially the five-helix structure that caps the protein at the N-terminus.
This structure alone, it turns out, forms a stable and highly soluble dimer as soon as the pH rises significantly above 7. At pH 6.3, however, the Proteins readily assemble into supramolecular fibres. The results indicate, according to the scientists, that assembly of silk fires requires complementary surfaces, which only exists within a narrow range of conditions. The search for the exact structure that is responsible for this behavior turned up two amino acids (Asp40 and Glu84) within the protein sequence which are conserved in all spider silk proteins.
Interestingly their pka values don’t really match the pH at which silk formation is initiated in vivo, while another amino acid (His6) is a good match pka-wise but is neither conserved nor involved in binding. Apparently the pka values of the Asp and Glu residues in question are shifted by some kind of collective action with other acidic amino acids.
As regular readers may have noticed I am quite a fan of advanced materials of all sorts. That usually means unusual alloys, rare metal oxides or oddly functionalized polymers. But spider silk teaches us that the most amazing and versatile materials only require C, H, O, N and S in a few well-known and inoffensive configurations. We are only starting to understand what proteins can do.
Askarieh, G., Hedhammar, M., Nordling, K., Saenz, A., Casals, C., Rising, A., Johansson, J., & Knight, S. (2010). Self-assembly of spider silk proteins is controlled by a pH-sensitive relay Nature, 465 (7295), 236-238 DOI: 10.1038/nature08962