cellodextrin and laminaran

Members of the periplasmic binding protein (PBP) superfamily utilize a highly conserved inter-domain ligand binding site that adapts to specifically bind a chemically diverse range of ligands. This paradigm of PBP ligand binding specificity was recently altered when the structure of the Thermotoga maritima cellobiose-binding protein (tmCBP) was solved. The tmCBP binding site is bipartite, comprising a canonical solvent-excluded region (subsite one), adjacent to a solvent-filled cavity (subsite two) where specific and semi-specific ligand recognition occur, respectively.. A molecular level understanding of binding pocket adaptation mechanisms that simultaneously allow both ligand specificity at subsite one and promiscuity at subsite two has potentially important implications in ligand binding and drug design studies. We sought to investigate the determinants of ligand binding selectivity in tmCBP through biophysical characterization of tmCBP in the presence of varying β-glucan oligosaccharides. Crystal structures show that whilst the amino acids that comprise both the tmCBP subsite one and subsite two binding sites remain fixed in conformation regardless of which ligands are present, the rich hydrogen bonding potential of water molecules may facilitate the ordering and the plasticity of this unique PBP binding site.. The identification of the roles these water molecules play in ligand recognition suggests potential mechanisms that can be utilized to adapt a single ligand binding site to recognize multiple distinct ligands.

Topics: Amino Acid Sequence; Bacterial Proteins; beta-Glucans; Binding Sites; Carrier Proteins; Cellulose; Circular Dichroism; Crystallography, X-Ray; Dextrins; Glucans; Hydrogen Bonding; Lectins; Ligands; Models, Molecular; Polysaccharides; Protein Conformation; Protein Denaturation; Protein Stability; Protein Structure, Secondary; Protein Structure, Tertiary; Substrate Specificity; Thermotoga maritima

1. Cell-free extracts from Epidinium ecaudatum (Crawley) hydrolysed the three hemicellulose fractions of pasture plants, but at different rates. 2. All of the constituent monosaccharides are released from the hemicellulose fractions, galactose and uronic acids being liberated at much slower rates than pentoses. 3. An arabinofuranosidase, which removes arabinose from highly branched arabinoxylan before the xylan chain can be hydrolysed, was isolated free from other pentosanases. 4. A xylanase hydrolysing xylan (by random cleavage) and xylodextrins of degree of polymerization (D.P.) > 3 to xylotriose and xylobiose was isolated free from other pentosanases. 5. A separate xylodextrinase hydrolysing (by random cleavage) xylodextrins of D.P. > 2 to xylobiose and xylose was also obtained; this enzyme did not hydrolyse xylan or xylobiose and the original extracts themselves possessed very weak xylobiase activity. 6. The epidinial extracts hydrolysed laminaribiose, laminarin, lichenin and cellodextrins of D.P. < 7 rapidly, cellobiose and gentiobiose slowly but cellulose not at all. 7. Polysaccharide glucose associated with plant linear B hemicellulose was liberated with cellobiose and possibly laminaribiose as intermediates. 8. The cellodextrinase hydrolysed cellopentaose initially to cellobiose plus cellotriose and is a distinctly different enzyme from the xylanase and xylodextrinase. 9. Extracts from Entodinium species and Eremoplastron bovis also hydrolysed all three types of plant hemicellose.

Topics: Animals; Carbohydrate Metabolism; Cellulose; Chemistry Techniques, Analytical; Chromatography; Ciliophora; Dextrins; Dialysis; Galactose; Glucans; Glycoside Hydrolases; Hydrolysis; Oligosaccharides; Plants; Polymers; Polysaccharides; Renal Dialysis; Research; Rumen; Xylans