Crystallization and preliminary X-ray diffraction analysis of Lin1840, a putative β-glucosidase from Listeria innocua

Synthesis of three deoxy-sophorose derivatives for evaluating the requirement of hydroxy groups at position 3 and/or 3' of sophorose by 1,2-β-oligoglucan phosphorylases

Sophorose (Sop₂) is known as a powerful inducer of cellulases in Trichoderma reesei, and in recent years 1,2-β-D-oligoglucan phosphorylase (SOGP) has been found to use Sop₂ in synthetic reactions. From the structure of the complex of SOGP with Sop₂, it was predicted that both the 3-hydroxy group at the reducing end glucose moiety of Sop₂ and the 3'-hydroxy group at the non-reducing end glucose moiety of Sop₂ were important for substrate recognition. In this study, three kinds of 3- and/or 3'-deoxy-Sop₂ derivatives were synthesized to evaluate this mechanism. The deoxygenation of the 3-hydroxy group of D-glucopyranose derivative was performed by radical reduction using a toluoyl group as a leaving group. The utilization of a toluoyl group that plays two roles (a leaving group for the deoxygenation and a protecting group for a hydroxy group) resulted in efficient syntheses of the three target compounds. The NMR spectra of the two final compounds (3-deoxy- and 3,3'-dideoxy-Sop₂) suggested that the glucose moiety of the reducing end of Sop₂ can easily take on a furanose structure (five-membered ring structure) by deoxygenation of the 3-hydroxy group of Sop₂. In addition, the ratio of the five- and six-membered ring structures changed depending on the temperature. The SOGPs exhibited remarkably lower specific activity for 3'-deoxy- and 3,3'-dideoxy-Sop₂, indicating that the 3'-hydroxy group of Sop₂ is important for substrate recognition by SOGPs.

Functional and Structural Analysis of a β-Glucosidase Involved in β-1,2-Glucan Metabolism in Listeria innocua

Despite the presence of β-1,2-glucan in nature, few β-1,2-glucan degrading enzymes have been reported to date. Recently, the Lin1839 protein from Listeria innocua was identified as a 1,2-β-oligoglucan phosphorylase. Since the adjacent lin1840 gene in the gene cluster encodes a putative glycoside hydrolase family 3 β-glucosidase, we hypothesized that Lin1840 is also involved in β-1,2-glucan dissimilation. Here we report the functional and structural analysis of Lin1840. A recombinant Lin1840 protein (Lin1840r) showed the highest hydrolytic activity toward sophorose (Glc-β-1,2-Glc) among β-1,2-glucooligosaccharides, suggesting that Lin1840 is a β-glucosidase involved in sophorose degradation. The enzyme also rapidly hydrolyzed laminaribiose (β-1,3), but not cellobiose (β-1,4) or gentiobiose (β-1,6) among β-linked gluco-disaccharides. We determined the crystal structures of Lin1840r in complexes with sophorose and laminaribiose as productive binding forms. In these structures, Arg572 forms many hydrogen bonds with sophorose and laminaribiose at subsite +1, which seems to be a key factor for substrate selectivity. The opposite side of subsite +1 from Arg572 is connected to a large empty space appearing to be subsite +2 for the binding of sophorotriose (Glc-β-1,2-Glc-β-1,2-Glc) in spite of the higher K_m value for sophorotriose than that for sophorose. The conformations of sophorose and laminaribiose are almost the same on the Arg572 side but differ on the subsite +2 side that provides no interaction with a substrate. Therefore, Lin1840r is unable to distinguish between sophorose and laminaribiose as substrates. These results provide the first mechanistic insights into β-1,2-glucooligosaccharide recognition by β-glucosidase.