Structural dynamics and catalytic properties of a multimodular xanthanase

The precise catalytic strategies used for the breakdown of the complex bacterial polysaccharide xanthan, an increasingly frequent component of processed human foodstuffs, have remained a mystery. Here, we present characterization of an endo-xanthanase from Paenibacillus nanensis. We show that it is a CAZy family 9 glycoside hydrolase (GH9) responsible for the hydrolysis of the xanthan backbone capable of generating tetrameric xanthan oligosaccharides from polysaccharide lyase family 8 (PL8) xanthan lyase-treated xanthan. Three-dimensional structure determination reveals a complex multimodular enzyme in which a catalytic (α/α) 6 barrel is flanked by an N-terminal "immunoglobulin-like" (Ig-like) domain (frequently found in GH9 enzymes) and by four additional C-terminal all β-sheet domains that have very few homologues in sequence databases and at least one of which functions as a new xanthan-binding domain, now termed CBM84. The solution-phase conformation and dynamics of the enzyme in the native calcium-bound state and in the absence of calcium were probed experimentally by hydrogen/deuterium exchange mass spectrometry. Measured conformational dynamics were used to guide the protein engineering of enzyme variants with increased stability in the absence of calcium; a property of interest for the potential use of the enzyme in cleaning detergents. The ability of hydrogen/deuterium exchange mass spectrometry to pinpoint dynamic regions of a protein under stress (e.g., removal of calcium ions) makes this technology a strong tool for improving protein catalyst properties by informed engineering

Tetraploids do not form cushions:association of ploidy level, growth form and ecology in the High Arctic Saxifraga oppositifolia L. s. lat. (Saxifragaceae) in Svalbard

<p>Saxifraga oppositifolia L. is a common circumpolar plant species that displays considerable morphological and genetic variation throughout its range. It is mainly diploid, but tetraploids are reported from several regions. The growth form varies from prostate to cushion-shaped, and the plant thrives in wet snow beds as well as on dry ridges. This variation has triggered the curiosity of many researchers, but as yet, no one has explained the observed morphological variation using ecological and/or genetic factors. However, the ploidy level has rarely been taken into account. This is the first study that demonstrates a significant correlation between ploidy level, ecology and growth form in S. oppositifolia. We successfully analysed 193 individuals of S. oppositifolia from 15 locations in Svalbard to investigate possible relationships among growth forms (prostrate, intermediate and cushion), ecological factors (vegetation and soil characteristics) and ploidy level. Results from flow cytometry reported 106 diploids, eight triploids and 79 tetraploids. Tetraploids almost exclusively showed prostrate growth, while the diploids displayed all three growth forms, evidence that growth form is at least partly genetically determined. Our analyses of environmental and vegetation data in relation to ploidy level indicated overlapping niches, but the tetraploids showed a narrower niche, and one shifted towards more benign habitats characterized by higher pH, higher soil temperatures and higher cover of vascular plants. The latter may suggest that tetraploids are slightly better competitors, but less hardy. Thus, autopolyploidy in S. oppositifolia has expanded the ecological amplitude of this species complex.</p>