The molecular packing in β-chitin unit cells was experimentally determined by a combination of unidirectional degradation by Bacillus circulans chitinase A1 and microdiffraction electron crystallography using highly crystalline β-chitin microfibrils from the protective tubes secreted by Lamellibrachia satsuma. The mode of chain packing was found to be identical with that of the previously published crystal model for β-chitin, despite a controversial definition of the unit cell parameters. Here, a 'parallel-down' packing was determined, where the reducing ends of chains point in an opposite direction to the crystallographic c-axis. Microdiffraction analyses of nascent β-chitin microfibrils generated from diatom Thalassiosira sp. showed that the c-axis of the crystal was directed toward the diatoms, and therefore the reducing end of a growing chain pointed away from the locus of biosynthesis. This mechanism agreed well with what we found recently in the cellulose biosynthesis system, and provides strong evidence that the polymerization by the processive glycosyl transferase takes place at the non-reducing end of the growing polysaccharide chains.
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