Harry Brumer

The chemist′s guide to the galactosyl unit: A chemo-enzymatic process is developed for the multivalent functionalization of cellulose surfaces via regioselective oxidation of heteropolysaccharides with galactose 6-oxidase. Reductive amination, surface sorption, and click chemistry enable the assembly of (bio)chemically active cellulose surfaces for applications ranging from functional biocomposites to in vitro diagnostics.

The desire for improved methods of biomass conversion into fuels and feedstocks has re-awakened interest in the enzymology of plant cell wall degradation. The complex polysaccharide xyloglucan is abundant in plant matter, where it may account for up to 20% of the total primary cell wall carbohydrates. Despite this, few studies have focused on xyloglucan saccharification, which requires a consortium of enzymes including endo-xyloglucanases, α-xylosidases, β-galactosidases and α-L-fucosidases, among others. In the present paper, we show the characterization of Xyl31A, a key α-xylosidase in xyloglucan utilization by the model Gram-negative soil saprophyte Cellvibrio japonicus. CjXyl31A exhibits high regiospecificity for the hydrolysis of XGOs (xylogluco-oligosaccharides), with a particular preference for longer substrates. Crystallographic structures of both the apo enzyme and the trapped covalent 5-fluoro-β-xylosyl-enzyme intermediate, together with docking studies with the XXXG heptasaccharide, revealed, for the first time in GH31 (glycoside hydrolase family 31), the importance of a PA14 domain insert in the recognition of longer oligosaccharides by extension of the active-site pocket. The observation that CjXyl31A was localized to the outer membrane provided support for a biological model of xyloglucan utilization by C. japonicus, in which XGOs generated by the action of a secreted endo-xyloglucanase are ultimately degraded in close proximity to the cell surface. Moreover, the present study diversifies the toolbox of glycosidases for the specific modification and saccharification of cell wall polymers for biotechnological applications.

Background: The microbes Escherichia coli and Pichia pastoris are convenient prokaryotic and eukaryotic hosts, respectively, for the recombinant production of proteins at laboratory scales. A comparative study was performed to evaluate a range of constructs and process parameters for the heterologous intra- and extracellular expression of genes encoding the industrially relevant enzyme galactose 6-oxidase (EC 1.1.3.9) from the fungus Fusarium graminearum. In particular, the wild-type galox gene from F. graminearum, an optimized variant for E. coli and a codon-optimized gene for P. pastoris were expressed without the native pro-sequence, but with a His-tag either at the N- or the C-terminus of the enzyme. Results: The intracellular expression of a codon-optimized gene with an N-terminal His₁₀-tag in E. coli, using the pET16b⁺ vector and BL21DE3 cells, resulted in a volumetric productivity of 180 U·L^-1·h^-1. The intracellular expression of the wild-type gene from F. graminearum, using the pPIC3.5 vector and the P. pastoris strain GS115, was poor, resulting in a volumetric productivity of 120 U·L^-1·h^-1. Furthermore, this system did not tolerate an N-terminal His₁₀-tag, thus rendering isolation of the enzyme from the complicated mixture difficult. The highest volumetric productivity (610 U·L^-1·h^-1) was achieved when the wild-type gene from F. graminearum was expressed extracellularly in the P. pastoris strain SMD1168H using the pPICZα-system. A C-terminal His₆-tag did not significantly affect the production of the enzyme, thus enabling simple purification by immobilized metal ion affinity chromatography. Notably, codon-optimisation of the galox gene for expression in P. pastoris did not result in a higher product yield (g protein·L^-1 culture). Effective activation of the enzyme to generate the active-site radical copper complex could be equally well achieved by addition of CuSO₄ directly in the culture medium or post-harvest. Conclusions: The results indicate that intracellular production in E. coli and extracellular production in P. pastoris comprise a complementary pair of systems for the production of GalOx. The prokaryotic host is favored for high-throughput screening, for example in the development of improved enzymes, while the yeast system is ideal for production scale-up for enzyme applications.

Background: Molecular evolution of carbohydrate binding modules (CBM) is a new approach for the generation of glycan-specific molecular probes. To date, the possibility of performing affinity maturation on CBM has not been investigated. In this study we show that binding characteristics such as affinity can be improved for CBM generated from the CBM4-2 scaffold by using random mutagenesis in combination with phage display technology. Results: Two modified proteins with greatly improved affinity for xyloglucan, a key polysaccharide abundant in the plant kingdom crucial for providing plant support, were generated. Both improved modules differ from other existing xyloglucan probes by binding to galactose-decorated subunits of xyloglucan. The usefulness of the evolved binders was verified by staining of plant sections, where they performed better than the xyloglucan-binding module from which they had been derived. They discriminated non-fucosylated from fucosylated xyloglucan as shown by their ability to stain only the endosperm, rich in non-fucosylated xyloglucan, but not the integument rich in fucosylated xyloglucan, on tamarind seed sections. Conclusion: We conclude that affinity maturation of CBM selected from molecular libraries based on the CBM4-2 scaffold is possible and has the potential to generate new analytical tools for detection of plant carbohydrates.

There currently exists a diverse array of molecular probes for the in situ localization of polysaccharides, nucleic acids, and proteins in plant cells, including reporter enzyme strategies (e.g. protein-glucuronidase fusions). In contrast, however, there is a paucity of methods for the direct analysis of endogenous glycoside hydrolases and transglycosidases responsible for cell wall remodeling. To exemplify the potential of fluorogenic resorufin glycosides to address this issue, a resorufin β-glycoside of a xylogluco-oligosaccharide (XXXG-β-Res) was synthesized as a specific substrate for in planta analysis of XEH activity. The resorufin aglycone is particularly distinguished for high sensitivity in muro assays due to a low pK_a (5.8) and large extinction coefficient (ϵ 62,000 m⁻¹cm⁻¹), long-wavelength fluorescence (excitation 571 nm/emission 585 nm), and high quantum yield (0.74) of the corresponding anion. In vitro analyses demonstrated that XXXG-β-Res is hydrolyzed by the archetypal plant XEH, nasturtium (Tropaeolum majus) NXG1, with classical Michaelis-Menten substrate saturation kinetics and a linear dependence on both enzyme concentration and incubation time. Further, XEH activity could be visualized in real time by observing the localized increase in fluorescence in germinating nasturtium seeds and Arabidopsis (Arabidopsis thaliana) inflorescent stems by confocal microscopy. Importantly, this new in situ XEH assay provides an essential complement to the in situ xyloglucan endotransglycosylase assay, thus allowing delineation of the disparate activities encoded by xyloglucanendotransglycosylase/hydrolase genes directly in plant tissues. The observation that XXXG-β-Res is also hydrolyzed by diverse microbial XEHs indicates that this substrate, and resorufin glycosides in general, may find broad applicability for the analysis of wall restructuring by polysaccharide hydrolases during morphogenesis and plant-microbe interactions.

Glycosynthases, hydrolytically inactive mutant glycosidases that catalyze glycosylation reactions using glycosyl fluoride donor substrates, are emerging as useful tools for the synthesis of large, complex polysaccharides [Faijes, M.; Planas, A. Carbohydr. Res. 2007, 342, 1581-1594]. Guided by wild-type xyloglucanase activity, we have produced and characterized new glycosynthases for the synthesis of xyloglucan oligo- and polysaccharides, based on family GH7, GH12, and GH16 scaffolds. The Humicola insolens GH7 glycosynthase, HiCe17B E197S, is capable of synthesizing nongalactosylated, XXXG-based homoxyloglucan up to M(w) 60000 [G = Glc beta(1 -> 4); X = Xyl alpha(1 -> 6)Glc beta(1 -> 4); L = Gal beta(1 -> 2)Xyl alpha(1 -> 6)Glc beta(1 -> 4)], which is among the largest products so far obtained with glycosynthase technology. Novel glycosynthases based on the GH16 xyloglucan hydrolase from Tropaeolum majus (nasturtium), TmNXG1, are capable of synthesizing XLLG-based xyloglucan oligosaccharides at rates feasible for preparative synthesis, thus providing an essential expansion of product range. Finally, a new glycosynthase based on the recently characterized GH12 xyloglucanase from Bacillus licheniformis, BlXG12 E155A, can perform the condensation of xyloglucosyl fluorides, albeit at poor rates. Altogether, the high catalytic efficiency demonstrated by HiCe17B E197S and the extended product range provided by TmNXG1 E94A are key achievements toward a robust and versatile method for the preparative synthesis of homogeneous xyloglucans with regular substitution patterns not available in nature. Such compounds enable in vitro experimental studies regarding the role of particular structural elements for xyloglucan properties and its interaction with cellulose.

Reorganization and degradation of the wall crosslinking and seed storage polysaccharide xyloglucan by glycoside hydrolase family 16 (GH16) endo‐transglycosylases and hydrolases are crucial to the growth of the majority of land plants, affecting processes as diverse as germination, morphogenesis, and fruit ripening. A high‐resolution, three‐dimensional structure of a nasturtium (Tropaeolum majus) endo‐xyloglucanase loop mutant, TmNXG1‐ΔYNIIG, with an oligosaccharide product bound in the negative active‐site subsites, has been solved by X‐ray crystallography. Comparison of this novel complex to that of the strict xyloglucan endo‐transglycosylase PttXET16‐34 from hybrid aspen (Populus tremula x tremuloides), previously solved with a xylogluco‐oligosaccharide bound in the positive subsites, highlighted key protein structures that affect the disparate catalytic activities displayed by these closely related enzymes. Combination of these “partial” active‐site complexes through molecular dynamics simulations in water allowed modeling of wild‐type TmNXG1, TmNXG1‐ΔYNIIG, and wild‐type PttXET16‐34 in complex with a xyloglucan octadecasaccharide spanning the entire catalytic cleft. A comprehensive analysis of these full‐length complexes underscored the importance of various loops lining the active site. Subtle differences leading to a tighter hydrogen bonding pattern on the negative (glycosyl donor) binding subsites, together with loop flexibility on the positive (glycosyl acceptor) binding subsites appear to favor hydrolysis over transglycosylation in GH16 xyloglucan‐active enzymes. Proteins 2009.