Ngày tải lên :
11/08/2014, 15:20
... 2.Glucanases
β-1,3-glucan,orlaminarin,isapolymerofd-glucoseinaβ-1,3configuration,arranged
ashelicalcoils.Fungalcellwallscontainmorethan60%laminarin.Whereaschitinis
arrangedinregularlyorderedlayers,laminarinfibrilsarearrangedinanamorphicmanner.
Therearechemicalbondsbetweenthelaminarinandchitin,andtogethertheyforma
complexnetofglucanandGlcNAcoligomers(103).Laminarinishydrolyzedmainlybyβ-
1,3-glucanases,alsoknownaslaminarinases.Theseenzymes,describedinfungi,bacteria,
actinomycetes,algae,mollusks,andhigherplants,arefurtherclassifiedasexo-andendo-
β-glucanases.Exo-β-1,3-glucanases(β-1,3-glucanglucanohydrolase,[EC3.2.1.58])hy-
drolyzelaminarinbysequentiallycleavingglucoseresiduesfromthenonreducingendsof
polymersoroligomers.Consequently,thesolehydrolysisproductsareglucosemonomers.
Endo-β-1,3-glucanases(β-1,3-glucanglucanohydrolase[EC3.2.1.6orEC3.2.1.39])
cleaveβ-1,3linkagesatrandomsitesalongthepolysaccharidechain,releasingsmaller
oligosaccharides.Bothenzymetypesarenecessaryforthefulldigestionoflaminarin
(104).Theseenzymeshaveseveralfunctionsinfungiincludingnutritioninsaprotropism,
mobilizationofβ-glucansunderconditionsofC-andenergy-sourceexhaustion,anda
physiologicalroleinmorphogeneticprocessesduringfungaldevelopmentanddifferentia-
tion(105).
Glucanaseshavebeensuggestedasanothergroupofkeyenzymesinvolvedinthe
mycoparasitismofGliocladiumandTrichodermaspp.againstfungalplantpathogens(Ta-
ble1).Thesubstrateoftheseenzymes,β-1,3-glucan,isoneofthemajorcomponentsof
fungal ... Spodoptera
exigua. These and similar results obtained by other authors (225) suggested the usefulness
of introducing chitinase-encoding genes into B. thuringiensis to increase the toxicity of
the bacterium ... in two other strains of E. agglomerans tested in this work (157). The
chitinolytic activity was induced when the strains were grown in the presence of colloidal
chitin as the sole C source; the...