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Contributed by Nancy Pierce on 06/08/2012
Andy Smith. Keynote speaker at the 1st International Conference on Biological and Biomimetic Adhesives in Lisbon Portugal (May 9-11, 20112).
His talk was titled "Multiple metal-based cross-links: protein oxidation and metal coordination in a biological glue".
Metal ions provide a powerful mechanism for cross-linking adhesives and other biomaterials, especially in aqueous environments. Metal ions can cross-link polymers directly through coordination, and some metals can catalyze redox reactions that drive the formation of other cross-links. It is likely that many biomaterials depend on multiple metal-based cross-links, involving variations of direct and oxidative mechanisms. The glue of gastropod mollusks demonstrates this complexity well. The terrestrial slug Arion subfuscus utilizes both types of interactions to strengthen their defensive glue. This glue contains substantial amounts of calcium, zinc, iron and copper. In addition, there are metal-binding proteins that are unique to the glue that have gel-stiffening activity. These proteins bind to both iron and zinc, and likely other metals. The function of these proteins, and the integrity of the glue overall, depends on metals. One mechanism that appears to play a central role is metal-catalyzed oxidation. Several prominent proteins in the glue are heavily oxidized, and experimental work has provided evidence that the resulting carbonyl groups link with primary amines to form imine bonds. Specific disruption of these bonds decreases glue stiffness significantly. These findings are noteworthy because common amino acids such as lysine can be readily oxidized by metals. While it has been known that oxidation of the rare amino acid 3,4-dihydroxyphenylalanine plays a role in other biomaterials, these results suggest an even broader role for protein oxidation. In addition to oxidative cross-links, metals directly cross-link slug glue, likely through coordinate covalent bonds. Evidence suggests that calcium directly cross-links the gel through interactions with sulfate on polysaccharides. Surprisingly, zinc does not strengthen the gel, though it is present in large quantities in slug glue associated with key proteins, and it is a common cross-linker in other biomaterials. Thus, it may play a different role. Overall, slug glue demonstrates interesting variations on the metal-dependent mechanisms that have been described thus far. Slug glue is also interesting because it is a dilute gel. It typically contains 97% water, and appears to be a modification of the normal lubricating slime. The ability to convert a dilute, lubricating gel into a strong glue demonstrates the power of metal-based cross-links.