Production of Bioactive Deapiosylated Platycosides from Glycosylated Platycosides in Balloon Flower Root Using the Crude Enzyme from the Food-Available Fungus Rhizopus oryzae

Extract from balloon flower root (Platycodi radix) containing platycosides as saponins is a beneficial food additive and is used for their savory taste and the alleviation of respiratory diseases. Deglycosylated platycosides show greater pharmacological effects than glycosylated platycosides. However, there are no reports on the conversion of glycosylated platycosides into deapiosylated platycosides. In this study, we showed that the crude enzyme from Rhizopus oryzae, a generally recognized as safe (GRAS) fungus isolated from meju (fermented soybean brick), completely converted glycosylated platycosides in Platycodi radix extract into deapiosylated platycosides: deapiosylated platycodin D (deapi-PD), deapiosylated platycodin A (deapi-PA), deapiosylated polygalacin D (deapi-PGD), and deapiosylated platyconic acid A (deapi-PCA). Among these, deapi-PA and deapi-PCA were first identified using liquid chromatography/mass spectrometry. The anti-inflammatory and antioxidant effects of deapiosylated platycosides were greater than those of the precursor glycosylated platycosides. These deapiosylated platycosides could improve the properties of functional food additives.

Recent advances in high-throughput mass spectrometry that accelerates enzyme engineering for biofuel research

Enzymes play indispensable roles in producing biofuels, a sustainable and renewable source of transportation fuels. Lacking rational design rules, the development of industrially relevant enzyme catalysts relies heavily on high-throughput screening. However, few universal methods exist to rapidly characterize large-scale enzyme libraries. Therefore, assay development is necessary on an ad hoc basis to link enzyme properties to spectrophotometric signals and often requires the use of surrogate, optically active substrates. On the other hand, mass spectrometry (MS) performs label-free enzyme assays that utilize native substrates and is therefore generally applicable. But the analytical speed of MS is considered rate limiting, mainly due to the use of time-consuming chromatographic separation in traditional MS analysis. Thanks to new instrumentation and sample preparation methods, direct analyte introduction into a mass spectrometer without a prior chromatographic step can be achieved by laser, microfluidics, and acoustics, so that each sample can be analyzed within seconds. Here we review recent advances in MS platforms that improve the throughput of enzyme library screening and discuss how these advances can potentially facilitate biofuel research by providing high sensitivity, selectivity and quantitation that are difficult to obtain using traditional assays. We also highlight the limitations of current MS assays in studying biofuel-related enzymes and propose possible solutions.

A unique xylose reductase from Thermomyces lanuginosus: Effect of lignocellulosic substrates and inhibitors and applicability in lignocellulosic bioconversion

In this study, the xylose reductase gene (XRTL) from Thermomyces lanuginosus SSBP was expressed in Pichia pastoris GS115 and Saccharomyces cerevisiae Y294. The purified 39.2 kDa monomeric enzyme was optimally active at pH 6.5 and 50 °C and showed activity over a wide range of temperatures (30-70 °C) and pH (4.0-9.0), with a half-life of 1386 min at 50 °C. The enzyme preferred NADPH as cofactor and showed broad substrate specificity. The enzyme was inhibited by Cu²⁺, Fe²⁺ and Zn²⁺, while ferulic acid was found to be the most potent lignocellulosic inhibitor. Recombinant S. cerevisiae with the XRTL gene showed 34% higher xylitol production than the control strain. XRTL can therefore be used in a cell-free xylitol production process or as part of a pathway for utilization of xylose from lignocellulosic waste.

Crystal structure of yeast xylose reductase in complex with a novel NADP-DTT adduct provides insights into substrate recognition and catalysis

Aldose reductases (ARs) belonging to the aldo-keto reductase (AKR) superfamily catalyze the conversion of carbonyl substrates into their respective alcohols. Here we report the crystal structures of the yeast Debaryomyces nepalensis xylose reductase (DnXR, AKR2B10) in the apo form and as a ternary complex with a novel NADP-DTT adduct. Xylose reductase, a key enzyme in the conversion of xylose to xylitol, has several industrial applications. The enzyme displayed the highest catalytic efficiency for l-threose (138 ± 7 mm^-1 ·s^-1 ) followed by d-erythrose (30 ± 3 mm^-1 ·s^-1 ). The crystal structure of the complex reveals a covalent linkage between the C4N atom of the nicotinamide ring of the cosubstrate and the S1 sulfur atom of DTT and provides the first structural evidence for a protein mediated NADP-low-molecular-mass thiol adduct. We hypothesize that the formation of the adduct is facilitated by an in-crystallo Michael addition of the DTT thiolate to the specific conformation of bound NADPH in the active site of DnXR. The interactions between DTT, a four-carbon sugar alcohol analog, and the enzyme are representative of a near-cognate product ternary complex and provide significant insights into the structural basis of aldose binding and specificity and the catalytic mechanism of ARs. DATABASE: Structural data are available in the PDB under the accession numbers 5ZCI and 5ZCM.