Comparative Physicochemical and Biochemical Characterization of Small-Molecule Glucosides

Glycosylation of small molecules can significantly improve their physicochemical and biological properties. Only recently, decisive improvements in the biotechnological production of small-molecule glucosides (SMGs) have resulted in a large number of these compounds now being commercially available. In this study, we have analyzed a number of physical, chemical, and biological parameters of 31 SMGs, including solubility, stability, melting and pyrolysis points, partition coefficient log P, minimum inhibitory concentration against Escherichia coli (MIC), and enzymatic degradability. The properties such as water solubility, pH stability, and MICs of the glycosides were strongly dependent on the structures of the respective aglycones, which is why the SMG clustered according to their aglycones in most cases. Phenolic and furanone glucosides were readily hydrolyzed by saliva and skin microflora, whereas monoterpenol glycosides were poorer substrates for the enzymes involved. The results of this comparative analysis of SMGs provide valuable information for elucidating the biological functions of SMGs and the future technological applications of these useful natural products.

Enzymatic Synthesis of Modified Alternaria Mycotoxins Using a Whole-Cell Biotransformation System

Reference standards for Alternaria mycotoxins are rarely available, especially the modified mycotoxins alternariol-3-glucoside (AOH-3-G), alternariol-9-glucoside (AOH-9-G), and alternariol monomethylether-3-glucoside (AME-3-G). To obtain these three glucosides as analytical standards for method development and method validation, alternariol and alternariol monomethylether were enzymatically glycosylated in a whole-cell biotransformation system using a glycosyltransferase from strawberry (Fragaria x ananassa), namely UGT71A44, expressed in Escherichia coli (E. coli). The formed glucosides were isolated, purified, and structurally characterized. The exact amount of the isolated compounds was determined using high-performance liquid chromatography with UV-detection (HPLC-UV) and quantitative nuclear resonance spectroscopy (qNMR). This method has proved to be highly effective with biotransformation rates of 58% for AOH-3-G, 5% for AOH-9-G, and 24% for AME-3-G.

Dirigent Protein Mode of Action Revealed by the Crystal Structure of AtDIR6

Dirigent proteins impart stereoselectivity to phenoxy radical coupling reactions in plants and, thus, play an essential role in the biosynthesis of biologically active natural products. This includes the regioselective and enantioselective coupling and subsequent cyclization of two coniferyl alcohol radicals to pinoresinol as the committed step of lignan biosynthesis. The reaction is controlled by dirigent proteins, which, depending on the species and protein, direct the reaction to either (+)- or (-)-pinoresinol. We present the crystal structure of the (-)-pinoresinol forming DIRIGENT PROTEIN6 (AtDIR6) from Arabidopsis (Arabidopsis thaliana) with data to 1.4 Å resolution. The structure shows AtDIR6 as an eight-stranded antiparallel β-barrel that forms a trimer with spatially well-separated cavities for substrate binding. The binding cavities are two lobed, exhibiting two opposing pockets, each lined with a set of hydrophilic and potentially catalytic residues, including essential aspartic acids. These residues are conserved between (+) and (-)-pinoresinol-forming DIRs and required for activity. The structure supports a model in which two substrate radicals bind to each of the DIR monomers. With the aromatic rings fixed in the two pockets, the propionyl side chains face each other for radical-radical coupling, and stereoselectivity is determined by the exact positioning of the side chains. Extensive mutational analysis supports a previously unrecognized function for DIRs in catalyzing the cyclization of the bis-quinone methide reaction intermediate to yield (+)- or (-)-pinoresinol.

Expression in Pichia pastoris and characterization of two novel dirigent proteins for atropselective formation of gossypol

We established an efficient fed-batch fermentation process for two novel dirigent proteins from cotton plants, GbDIR2 from Gossypium barbadense and GhDIR3 from G. hirsutum, using the engineered Pichia pastoris GlycoSwitch® SuperMan₅ strain to prevent hyperglycosylation. The two (His)₆-tagged proteins were purified by metal-chelate affinity chromatography and obtained in quantities of 12 and 15 mg L^-1 of culture volume, respectively. Glycosylation sites were identified for the native and for the enzymatically deglycosylated proteins by mass spectrometry, confirming five to six of the seven predicted glycosylation sites in the NxS/T sequence context. The predominant glycan structure was Man₅GlcNAc₂ with, however, a significant contribution of Man_4-10GlcNAc₂. Both dirigent proteins (DIRs) mediated the formation of (+)-gossypol by atropselective coupling of hemigossypol radicals. Similar to previously characterized DIRs, GbDIR2 and GhDIR3 lacked oxidizing activity and depended on an oxidizing system (laccase/O₂) for the generation of substrate radicals. In contrast to DIRs involved in the biosynthesis of lignans, glycosylation was not essential for function. Quantitative enzymatic deglycosylation yielded active GbDIR2 and GhDIR3 in excellent purity. The described fermentation process in combination with enzymatic deglycosylation will pave the way for mechanistic and structural studies and, eventually, the application of cotton DIRs in a biomimetic approach towards atropselective biaryl synthesis.

Dirigent Proteins from Cotton (Gossypium sp.) for the Atropselective Synthesis of Gossypol

Gossypol is a defense compound in cotton plants for protection against pests and pathogens. Gossypol biosynthesis involves the oxidative coupling of hemigossypol and results in two atropisomers owing to hindered rotation around the central binaphthyl bond. (+)-Gossypol predominates in vivo, thus suggesting stereochemically controlled biosynthesis. The aim was to identify the factors mediating (+)-gossypol formation in cotton and to investigate their potential for asymmetric biaryl synthesis. A dirigent protein from Gossypium hirsutum (GhDIR4) was found to confer atropselectivity to the coupling of hemigossypol in presence of laccase and O2 as an oxidizing agent. (+)-Gossypol was obtained in greater than 80% enantiomeric excess compared to racemic gossypol in the absence of GhDIR4. The identification of GhDIR4 highlights a broader role for DIRs in plant secondary metabolism and may eventually lead to the development of DIRs as tools for the synthesis of axially chiral binaphthyls.