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Novel Xylose Dehydrogenase in the Halophilic Archaeon Haloarcula marismortui. Ulrike Johnsen, 2004.During growth of the halophilic archaeon Haloarcula marismortui on D-xylose, a specific D-xylose dehydrogenase was induced.The enzyme was purified to homogeneity . It constitutes a homotetramerof about 175 kDa and catalyzed the oxidation of xylose withboth NADP+ and NAD+ as cosubstrates with 10-fold higher affinityfor NADP+ . In addition to D-xylose, D-ribose was oxidized atsimilar kinetic constants, whereas D-glucose was used with about70-fold lower catalytic efficiency [kcat/Km] . With the N-terminalamino acid sequence of the subunit, an open reading frame [ORF]—codingfor a 39.9-kDA protein—was identified in the partiallysequenced genome of H . marismortui . The function of the ORFas the gene designated xdh and coding for xylose dehydrogenasewas proven by its functional overexpression in Escherichia coli.The recombinant enzyme was reactivated from inclusion bodiesfollowing solubilization in urea and refolding in the presenceof salts, reduced and oxidized glutathione, and substrates.Xylose dehydrogenase showed the highest sequence similarityto glucose-fructose oxidoreductase from Zymomonas mobilis andother putative bacterial and archaeal oxidoreductases . Activitiesof xylose isomerase and xylulose kinase, the initial reactionsof xylose catabolism of most bacteria, could not be detectedin xylose-grown cells of H . marismortui, and the genes thatencode them, xylA and xylB, were not found in the genome ofH . marismortui . Thus, we propose that this first characterizedarchaeal xylose dehydrogenase catalyzes the initial step inxylose degradation by H . marismortui.
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