Crystallographic fragment screening-based study of a novel FAD-dependent oxidoreductase from Chaetomium thermophilum

The FAD-dependent oxidoreductase from Chaetomium thermophilum (CtFDO) is a novel thermostable glycoprotein from the glucose–methanol–choline (GMC) oxidoreductase superfamily. However, CtFDO shows no activity toward the typical substrates of the family and high-throughput screening with around 1000 compounds did not yield any strongly reacting substrate. Therefore, protein crystallography, including crystallographic fragment screening, with 42 fragments and 37 other compounds was used to describe the ligand-binding sites of CtFDO and to characterize the nature of its substrate. The structure of CtFDO reveals an unusually wide-open solvent-accessible active-site pocket with a unique His–Ser amino-acid pair putatively involved in enzyme catalysis. A series of six crystal structures of CtFDO complexes revealed five different subsites for the binding of aryl moieties inside the active-site pocket and conformational flexibility of the interacting amino acids when adapting to a particular ligand. The protein is capable of binding complex polyaromatic substrates of molecular weight greater than 500 Da.

Mechanisms of Manganese(II) Oxidation by Filamentous Ascomycete Fungi Vary With Species and Time as a Function of Secretome Composition

Manganese (Mn) oxides are among the strongest oxidants and sorbents in the environment, and Mn(II) oxidation to Mn(III/IV) (hydr)oxides includes both abiotic and microbially-mediated processes. While white-rot Basidiomycete fungi oxidize Mn(II) using laccases and manganese peroxidases in association with lignocellulose degradation, the mechanisms by which filamentous Ascomycete fungi oxidize Mn(II) and a physiological role for Mn(II) oxidation in these organisms remain poorly understood. Here we use a combination of chemical and in-gel assays and bulk mass spectrometry to demonstrate secretome-based Mn(II) oxidation in three phylogenetically diverse Ascomycetes that is mechanistically distinct from hyphal-associated Mn(II) oxidation on solid substrates. We show that Mn(II) oxidative capacity of these fungi is dictated by species-specific secreted enzymes and varies with secretome age, and we reveal the presence of both Cu-based and FAD-based Mn(II) oxidation mechanisms in all 3 species, demonstrating mechanistic redundancy. Specifically, we identify candidate Mn(II)-oxidizing enzymes as tyrosinase and glyoxal oxidase in Stagonospora sp. SRC1lsM3a, bilirubin oxidase in Stagonospora sp. and Paraconiothyrium sporulosum AP3s5-JAC2a, and GMC oxidoreductase in all 3 species, including Pyrenochaeta sp. DS3sAY3a. The diversity of the candidate Mn(II)-oxidizing enzymes identified in this study suggests that the ability of fungal secretomes to oxidize Mn(II) may be more widespread than previously thought.

Targeting Penicillium expansum GMC Oxidoreductase with High Affinity Small Molecules for Reducing Patulin Production

Flavine adenine dinucleotide (FAD) dependent glucose methanol choline oxidoreductase (GMC oxidoreductase) is the terminal key enzyme of the patulin biosynthetic pathway. GMC oxidoreductase catalyzes the oxidative ring closure of (E)-ascladiol to patulin. Currently, no protein involved in the patulin biosynthesis in Penicillium expansum has been experimentally characterized or solved by X-ray diffraction. Consequently, nothing is known about P. expansum GMC oxidoreductase substrate-binding site and mode of action. In the present investigation, a 3D comparative model for P. expansum GMC oxidoreductase has been described. Furthermore, a multistep computational approach was used to identify P. expansum GMC oxidoreductase residues involved in the FAD binding and in substrate recognition. Notably, the obtained 3D comparative model of P. expansum GMC oxidoreductase was used for performing a virtual screening of a chemical/drug library, which allowed to predict new GMC oxidoreductase high affinity ligands to be tested in in vitro/in vivo assays. In vitro assays performed in presence of 6-hydroxycoumarin and meticrane, among the highly affinity predicted binders, confirmed a dose-dependent inhibition (17–81%) of patulin production by 6-hydroxycoumarin (10 µM–1 mM concentration range), whereas the approved drug meticrane inhibited patulin production by 43% already at 10 µM. Furthermore, 6-hydroxycoumarin and meticrane caused a 60 and 41% reduction of patulin production, respectively, in vivo on apples at 100 µg/wound.

High-resolution study of fungal enzymes

Current processes for lignocellulose deconstruction are unspecific and produce some constituents in poor quality. Specific biocatalysts could achieve optimal segregation together with minimal damage to cellulose and lignin and provide high-quality feedstocks for industry. Naturally occurring fungal oxidoreductases perform this task, but their characterisation - and hence their optimisation for industrial application - is difficult because of the experimental challenges. The mission of OXIDISE to develop appropriate methods to characterise lignocellulose degrading oxidoreductases, i.e. elucidate their conversions rates and to resolve their distribution and interaction in vicinity of their polymeric substrates. High-resolution techniques will be adapted to specifically detect fungal oxidoreductases like lytic polysaccharide monooxygenase, cellobiose dehydrogenase, laccase, lignin peroxidase, or members of the GMC oxidoreductase superfamily. These enzymes are all involved in the oxidative attack of recalcitrant biopolymers and are present in over 90% of fungal genomes. To overcome problems of current assaying techniques such as their low spatial and temporal resolution, OXIDISE will develop and apply techniques based on microelectrodes, scanning electron microscopy, surface plasmon resonance and fluorescence microscopy thereby pursuing three objectives: 1) study the interaction of all major oxidoreductases secreted by fungi in regard to electron transfer, regeneration of redox species and substrate cascading; 2) resolve the distribution of secreted oxidoreductases on cellulosic and lignocellulosic substrates at high resolution; 3) transfer the developed techniques to natural lignocellulose samples with growing fungal hyphae and study the secreted oxidoreductase activities. OXIDISE strives to establish new techniques to elucidate the kinetics and interactions of oxidoreductases - a long neglected enzyme class for lignocellulose depolymerisation.

Independently recruited oxidases from the glucose-methanol-choline oxidoreductase family enabled chemical defences in leaf beetle larvae (subtribe Chrysomelina) to evolve

Larvae of the leaf beetle subtribe Chrysomelina sensu stricto repel their enemies by displaying glandular secretions that contain defensive compounds. These repellents can be produced either de novo (iridoids) or by using plant-derived precursors (e.g. salicylaldehyde). The autonomous production of iridoids, as in Phaedon cochleariae , is the ancestral chrysomeline chemical defence and predates the evolution of salicylaldehyde-based defence. Both biosynthesis strategies include an oxidative step of an alcohol intermediate. In salicylaldehyde-producing species, this step is catalysed by salicyl alcohol oxidases (SAOs) of the glucose-methanol-choline (GMC) oxidoreductase superfamily, but the enzyme oxidizing the iridoid precursor is unknown. Here, we show by in vitro as well as in vivo experiments that P. cochleariae also uses an oxidase from the GMC superfamily for defensive purposes. However, our phylogenetic analysis of chrysomeline GMC oxidoreductases revealed that the oxidase of the iridoid pathway originated from a GMC clade different from that of the SAOs. Thus, the evolution of a host-independent chemical defence followed by a shift to a host-dependent chemical defence in chrysomeline beetles coincided with the utilization of genes from different GMC subfamilies. These findings illustrate the importance of the GMC multi-gene family for adaptive processes in plant–insect interactions.

Crystal structure of pyridoxine 4-oxidase from Mesorhizobium loti

Mesorhizobium loti MAFF303099, a nitrogen-fixing symbiotic bacterium, harbors degradation pathway I for pyridoxine (PN); a free form of vitamin B6. Pyridoxine 4-oxidase (PNOX), a monomeric glucose-methanol-choline (GMC) oxidoreductase family enzyme, is the first enzyme in the pathway. It catalyzes FAD-dependent oxidation of pyridoxine (PN) into pyridoxal. PNOX with a C-terminal His6 tag was overexpressed in E.coli JM109 cells and purified with a Ni-NTA agarose column and a QA52 column. The tertiary structures of PNOX and a complex of PNOX with pyridoxamine (PM), which is a substrate analog, were determined at 2.2 Å and at 2.1 Å resolutions, respectively. The overall structure consisted of FAD-binding and substrate-binding domains. The FAD interacts with the PNOX protein through a network of hydrogen bonds, which are mainly found in the ribose and pyrophosphate moieties of the FAD molecule. The surface structure of PNOX molecule showed that it had an opening socket for access of substrates. The opening was followed by a tunnel that was linked to the active site cavity. In the active site, His460, His462, and Pro504 were located on the re-face of the isoalloxazine ring of FAD. PM binds to the active site through several hydrogen bonds. The side chains of His462 and His460 are located at 2.7 and 3.1 Å from the N4′ atom of PM. The activities of H460A and H462A mutant PNOXs were very low, and H460A/H462A double mutant PNOX exhibited no activity. His462 may act as a general base for abstraction of a proton from the 4′-hydroxyl of PN. His460 may play a role in the binding and positioning of PN. The C4′ atom in PM is located at 3.2 Å, and the hydride ion from the C4′ atom may be transferred to the N5 atom of the isoalloxazine ring. The comparison of active site residues in GMC oxidoreductase family shows that Pro504 in PNOX corresponds to Asn or His of the conserved His-Asn or His-His pair in other GMC oxidoreductases.