Zusammenfassung
Thermotoga maritima XynA is an extremely thermostable modular enzyme with five domains (A1-A2-B-C1-C2), Its catalytic domain (-B-) is flanked by duplicated non-catalytic domains, The C-terminal repeated domains represent cellulose-binding domains (CBDs), Xylanase domains related to the N-terminal domains of XynA (A1-A2) are called thermostabilizing domains because their deletion normally leads to ...
Zusammenfassung
Thermotoga maritima XynA is an extremely thermostable modular enzyme with five domains (A1-A2-B-C1-C2), Its catalytic domain (-B-) is flanked by duplicated non-catalytic domains, The C-terminal repeated domains represent cellulose-binding domains (CBDs), Xylanase domains related to the N-terminal domains of XynA (A1-A2) are called thermostabilizing domains because their deletion normally leads to increased thermosensitivity of the enzymes. It was found that a glutathione-S-transferase (GST) hybrid protein (GST-A1A2) containing both A-domains of XynA can interact with various soluble xylan preparations and with mixed-linkage beta-1,3/beta-1,4-glucans, GST-A1A2 showed no affinity for insoluble microcrystalline cellulose, whereas, vice versa, GST-CP, which contains the C-terminal CBD of XynA, did not interact with soluble xylan, Another hybrid protein, GST-AS, displayed the same binding properties as GST-A1A2, indicating that A2 alone can also promote xylan binding. The dissociation constants for the binding of xylose, xylobiose, xylotriose, xylotetraose and xylopentaose by GST-AP, as determined at 20 degrees C by fluorescence quench experiments, were 8.1 x 10(-3) M, 2.3 x 10(-4) M, 2.3 x 10(-5) M, 2.5 x 10(-6) M and 1.1 x 10(-6) M respectively. The A-domains of XynA, which are designated as xylan binding domains (XBD), are, from the structural as well as the functional point of view, prototypes of a novel class of binding domains. More than 50 related protein segments with hitherto unknown function were detected in about 30 other multidomain beta-glycanases, among them putative plant (Arabidopsis thaliana) xylanases, It is argued that polysaccharide binding and not thermostabilization is the main function of A-like domains.
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