Structural basis for substrate binding and catalytic mechanism of the key enzyme VioD in the violacein synthesis pathway.

Violacein is a pigment synthesized by gram-negative bacteria with various biological activities such as antimicrobial, antiviral, and anticancer activities. VioD is a key oxygenase converting protodeoxyviolaceinic acid to protoviolaceinic acid in violacein biosynthesis. To elucidate the catalytic mechanism of VioD, here, we resolved two crystal structures of VioD, a binary complex structure containing VioD and a FAD and a ternary complex structure composed of VioD, a FAD and a 2-ethyl-1-hexanol (EHN). Structural analysis revealed a deep funnel like binding pocket with wide entrance, this pocket is positively charged. The EHN is located at the deep bottom of the binding pocket near isoalloxazine ring. Further docking simulation help us to propose the mechanism of the hydroxylation of the substrate catalyzed by VioD. Bioinformatic analysis suggested and emphasized the importance of the conserved residues involved in substrate binding. Our results provide a structural basis for the catalytic mechanism of VioD.

Biochemical and structural investigation of sulfoacetaldehyde reductase from Klebsiella oxytoca.

Sulfoacetaldehyde reductase (IsfD) is a member of the short-chain dehydrogenase/reductase (SDR) family, involved in nitrogen assimilation from aminoethylsulfonate (taurine) in certain environmental and human commensal bacteria. IsfD catalyzes the reversible NADPH-dependent reduction of sulfoacetaldehyde, which is generated by transamination of taurine, forming hydroxyethylsulfonate (isethionate) as a waste product. In the present study, the crystal structure of Klebsiella oxytoca IsfD in a ternary complex with NADPH and isethionate was solved at 2.8 Å, revealing residues important for substrate binding. IsfD forms a homotetramer in both crystal and solution states, with the C-terminal tail of each subunit interacting with the C-terminal tail of the diagonally opposite subunit, forming an antiparallel ß sheet that constitutes part of the substrate-binding site. The sulfonate group of isethionate is stabilized by a hydrogen bond network formed by the residues Y148, R195, Q244 and a water molecule. In addition, F249 from the diagonal subunit restrains the conformation of Y148 to further stabilize the orientation of the sulfonate group. Mutation of any of these four residues into alanine resulted in a complete loss of catalytic activity for isethionate oxidation. Biochemical investigations of the substrate scope of IsfD, and bioinformatics analysis of IsfD homologs, suggest that IsfD is related to the promiscuous 3-hydroxyacid dehydrogenases with diverse metabolic functions.

Boosting the Efficiency of Perovskite/Organic Tandem Solar Cells via Enhanced Near-Infrared Absorption and Minimized Energy Losses.

The compatibility of perovskite and organic photovoltaic materials in solution processing provides a significant advantage in the fabrication of high-efficiency perovskite/organic tandem solar cells. However, additional recombination losses can occur during exciton dissociation in organic materials, leading to energy losses in the near-infrared region of tandem devices. Consequently, a ternary organic rear subcell is designed containing two narrow-bandgap non-fullerene acceptors to enhance the absorption of near-infrared light. Simultaneously, a unique diffusion-controlled growth technique is adopted to optimize the morphology of the ternary active layer, thereby improving exciton dissociation efficiency. This innovation not only broadens the absorption range of near-infrared light but also facilitates the generation and effective dissociation of excitons. Owing to these technological improvements, the power conversion efficiency (PCE) of organic solar cells increased to 19.2%. Furthermore, a wide-bandgap perovskite front subcell is integrated with a narrow-bandgap organic rear subcell to develop a perovskite/organic tandem solar cell. Owing to the reduction in near-infrared energy loss, the PCE of this tandem device significantly improved, reaching 24.5%.

Bioinformatics-Structural Approach to the Search for New D-Amino Acid Oxidases.

D-amino acid oxidase (DAAO, EC 1.2.1.2) plays an important role in the functioning of prokaryotes as well as of lower (yeast and fungi) and higher eukaryotes (mammals). DAAO genes have not yet been found in archaean genomes. D-amino acid oxidase is increasingly used in various fields, which requires the development of new variants of the enzyme with specific properties. However, even within one related group (bacteria, yeasts and fungi, mammals), DAAOs show very low homology between amino acid sequences. In particular, this fact is clearly observed in the case of DAAO from bacteria. The high variability in the primary structures of DAAO severely limits the search for new enzymes in known genomes. As a result, many (if not most) DAAO genes remain either unannotated or incorrectly annotated. We propose an approach that uses bioinformatic methods in combination with general 3D structure and active center structure analysis to confirm that the gene found encodes D-amino acid oxidase and to predict the possible type of its substrate specificity. Using a homology search, we obtained a set of candidate sequences, modelled the tertiary structure of the selected enzymes, and compared them with experimental and model structures of known DAAOs. The effectiveness of the proposed approach for discrimination of DAAOs and glycine oxidases is shown. Using this approach, new DAAO genes were found in the genomes of six strains of extremophilic bacteria, and for the first time in the world, one gene was identified in the genome of halophilic archaea. Preliminary experiments confirmed the predicted specificity of DAAO from Natronosporangium hydrolyticum ACPA39 with D-Leu and D-Phe.

Interface Tuning between Two Connecting Bulk Heterojunctions in Small Molecule Bilayer Ternary Solar Cells.

Bilayer ternary solar cells are a kind of novel organic photovoltaic device with a triple-component active layer but are different from the ternary bulk heterojunction (BHJ) blend. Two binary BHJs with a common acceptor (or donor) are deposited sequentially in this kind of device. Here, we study the fabrication and optimization of bilayer ternary solar cells using metal phthalocyanine donors and fullerene acceptor. The device power conversion efficiency (PCE) shows a significant dependence on the interface between the two binary BHJs. The interface formed by stacking two BHJs directly demonstrates severe restrictions on the device efficiency. We find that the photovoltaic performance of bilayer ternary cells can be improved by inserting a C60 molecular monolayer between the two binary BHJs. The effect of the C60 interfacial layer on charge transport is analyzed based on their transport characteristics under negative bias. A relationship between the C60 interfacial layer and recombination under illumination is discussed. This work reveals a particular influence due to the interface facing three materials in organic solar cells.

Morphology Control Enables Efficient Ternary Organic Solar Cells.

Ternary organic solar cells are promising alternatives to the binary counterpart due to their potential in achieving high performance. Although a growing number of ternary organic solar cells are recently reported, less effort is devoted to morphology control. Here, ternary organic solar cells are fabricated using a wide-bandgap polymer PBT1-C as the donor, a crystalline fused-ring electron acceptor ITIC-2Cl, and an amorphous fullerene derivative indene-C60 bisadduct (ICBA) as the acceptor. It is found that ICBA can disturb π-π interactions of the crystalline ITIC-2Cl molecules in ternary blends and then help to form more uniform morphology. As a result, incorporation of 20% ICBA in the PBT1-C:ITIC-2Cl blend enables efficient charge dissociation, negligible bimolecular recombination, and balanced charge carrier mobilities. An impressive power conversion efficiency (PCE) of 13.4%, with a high fill factor (FF) of 76.8%, is eventually achieved, which represents one of the highest PCEs reported so far for organic solar cells. The results manifest that the adoption of amorphous fullerene acceptor is an effective approach to optimizing the ternary blend morphology and thereby increases the solar cell performance.

A new tetragonal structure type for Li2B2C.

The ternary dilithium diboron carbide, Li2B2C (tetragonal, space group P-4m2, tP10), crystallizes as a new structure type and consists of structural fragments which are typical for structures of elemental lithium and boron or binary borocarbide B13C2. The symmetries of the occupied sites are .m. and 2mm. for the B and C atoms, and -4m2 and 2mm. for the Li atoms. The coordination polyhedra around the Li atoms are cuboctahedra and 15-vertex distorted pseudo-Frank-Kasper polyhedra. The environment of the B atom is a ten-vertex polyhedron. The nearest neighbours of the C atom are two B atoms, and this group is surrounded by a deformed cuboctahedron with one centred lateral facet. Electronic structure calculations using the TB-LMTO-ASA method reveal strong B...C and B...B interactions.