title: The secondary substrate binding site of the Pseudoalteromonas haloplanktis GH8 xylanase is relevant for activity on insoluble but not soluble substrates.
abstract: Previously, it has been demonstrated that the glycoside hydrolase family 8 xylanase from the psychrophylic bacterium Pseudoalteromonas haloplanktis (XPH) can bind substrate non-catalytically on the surface of its catalytic module. In the present study, the functional relevance of this secondary binding site (SBS) for the enzyme is investigated by site-directed mutagenesis and evaluation of activity and binding properties of mutant variants on a range of structurally different homoxylan and heteroxylan substrates. The SBS had an impact on the activity on insoluble substrates, whereas the activity on soluble substrates remained unaffected. Unexpectedly, the activity on a soluble oligomeric substrate was also affected for some mutants and results on a chromophoric polymeric model substrate were in contrast with the trends observed on the corresponding natural substrate. All in all, results show that the impact of the SBS on the activity of XPH is in part analogous to the functioning of some carbohydrate-binding modules in modular enzymes.
authors.affiliation: Laboratory of Food Chemistry and Biochemistry & Leuven Food Science and Nutrition Research Centre (LFoRCe), Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, box 2463, B-3001, Leuven, Belgium.
authors.forename: Sven
authors.initials: S
authors.lastname: Cuyvers
authors.forename: Emmie
authors.initials: E
authors.lastname: Dornez
authors.forename: Jan A
authors.initials: JA
authors.lastname: Delcour
authors.forename: Christophe M
authors.initials: CM
authors.lastname: Courtin
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D031161:Pseudoalteromonas / Q000201:enzymology* / Q000235:genetics; D013379:Substrate Specificity; D014995:Xylosidases / Q000235:genetics / Q000378:metabolism*
chemical_list: D050505:Mutant Proteins; D014995:Xylosidases