Aurone synthase is a catechol oxidase with hydroxylase activity and provides insights into the mechanism of plant polyphenol oxidases

Identification of two key genes involved in flavonoid catabolism and their different roles in apple resistance to biotic stresses

Biosynthesis of flavonoid aglycones and glycosides is well established. However, key genes involved in their catabolism are poorly understood, even though the products of hydrolysis and oxidation play important roles in plant resistance to biotic stress. Here, we report on catabolism of dihydrochalcones (DHCs), the most abundant flavonoids in domesticated apple and wild Malus. Two key genes, BGLU13.1 and PPO05, were identified by activity-directed protein purification. BGLU13.1-A hydrolyzed phlorizin, (the most abundant DHC in domesticated apple) to produce phloretin which was then oxidized by PPO05. The process differed in some wild Malus, where trilobatin (a positional isomer of phlorizin) was mainly oxidized by PPO05. The effects of DHC catabolism on apple resistance to biotic stresses was investigated using transgenic plants. Either directly or indirectly, phlorizin hydrolysis affected resistance to the phytophagous pest two-spotted spider mite, while oxidation of trilobatin was involved in resistance to the biotrophic fungus Podosphaera leucotricha. DHC catabolism did not affect apple resistance to necrotrophic pathogens Valsa mali and Erwinia amylovara. These results suggest that different DHC catabolism pathways play different roles in apple resistance to biotic stresses. The role of DHC catabolism on apple resistance appeared closely related to the mode of invasion/damage used by pathogen/pest.

A Promising Drug Candidate as Potent Therapeutic Approach for Neuroinflammation and Its In Silico Justification of Chalcone Congeners: a Comprehensive Review

Multiple genetic, environmental, and immunological variables cause neuropsychiatric disorders (NPDs). The induced inflammatory immune response is also connected to the severity and treatment outcomes of various NPDs. These reactions also significantly impact numerous brain functions such as GABAergic signaling and neurotransmitter synthesis through inflammatory cytokines and chemokines. Chalcones (1,3-diaryl-2-propen-1-ones) and their heterocyclic counterparts are flavonoids with various biological characteristics including anti-inflammatory activity. Several pure chalcones have been clinically authorized or studied in humans. Chalcones are favored for their diagnostic and therapeutic efficacy in neuroinflammation due to their tiny molecular size, easy manufacturing, and flexibility for changes to adjust lipophilicity ideal for BBB penetrability. These compounds reached an acceptable plasma concentration and were well-tolerated in clinical testing. As a result, they are attracting increasing attention from scientists. However, chalcones' therapeutic potential remains largely untapped. This paper is aimed at highlighting the causes of neuroinflammation, more potent chalcone congeners, their mechanisms of action, and relevant structure-activity relationships.

An intramolecular macrocyclase in plant ribosomal peptide biosynthesis

The biosynthetic dogma of ribosomally synthesized and posttranslationally modified peptides (RiPP) involves enzymatic intermolecular modification of core peptide motifs in precursor peptides. The plant-specific BURP-domain protein family, named after their four founding members, includes autocatalytic peptide cyclases involved in the biosynthesis of side-chain-macrocyclic plant RiPPs. Here we show that AhyBURP, a representative of the founding Unknown Seed Protein-type BURP-domain subfamily, catalyzes intramolecular macrocyclizations of its core peptide during the sequential biosynthesis of monocyclic lyciumin I via glycine-tryptophan crosslinking and bicyclic legumenin via glutamine-tyrosine crosslinking. X-ray crystallography of AhyBURP reveals the BURP-domain fold with two type II copper centers derived from a conserved stapled-disulfide and His motif. We show the macrocyclization of lyciumin-C(sp3)-N-bond formation followed by legumenin-C(sp3)-O-bond formation requires dioxygen and radical involvement based on enzyme assays in anoxic conditions and isotopic labeling. Our study expands enzymatic intramolecular modifications beyond catalytic moiety and chromophore biogenesis to RiPP biosynthesis.

Exploring the Evolvability of Plant Specialized Metabolism: Uniqueness Out Of Uniformity and Uniqueness Behind Uniformity

The huge structural diversity exhibited by plant specialized metabolites has primarily been considered to result from the catalytic specificity of their biosynthetic enzymes. Accordingly, enzyme gene multiplication and functional differentiation through spontaneous mutations have been established as the molecular mechanisms that drive metabolic evolution. Nevertheless, how plants have assembled and maintained such metabolic enzyme genes and the typical clusters that are observed in plant genomes, as well as why identical specialized metabolites often exist in phylogenetically remote lineages, is currently only poorly explained by a concept known as convergent evolution. Here, we compile recent knowledge on the co-presence of metabolic modules that are common in the plant kingdom but have evolved under specific historical and contextual constraints defined by the physicochemical properties of each plant specialized metabolite and the genetic presets of the biosynthetic genes. Furthermore, we discuss a common manner to generate uncommon metabolites (uniqueness out of uniformity) and an uncommon manner to generate common metabolites (uniqueness behind uniformity). This review describes the emerging aspects of the evolvability of plant specialized metabolism that underlie the vast structural diversity of plant specialized metabolites in nature.

Selective sensing of catechol based on a fluorescent nanozyme with catechol oxidase activity

Nanozymes, an unusual category of nanomaterials possessing enzymatic properties, and have generated considerable interest regarding their application feasibilities on several important fronts. In the present work, an innovative sensing device for catechol was established ground on a fluorescent nanozyme (Cu-BDC-NH2) that exhibited catechol oxidase activity. The fluorescent nanozyme combines both functions of catechol recognition and response signal output, and can realize the sensing of catechol without the addition of other chromogenic agents. In the existence of Cu-BDC-NH2, catechol can be oxidized efficiently to produce quinones or polymers with strong electron absorption capacity, which immediately results in efficient fluorescence quenching of Cu-BDC-NH2. However, other common phenolic compounds, such as phenol, the other two diphenols (hydroquinone and resorcinol), phloroglucinol, and chlorophenol, do not result in efficient fluorescence quenching of Cu-BDC-NH2. The method shows a nice linear relationship between catechol concentration prep the fluorescence intensity of Cu-BDC-NH2 in the scope of 0-10 μM, with a detection limit of 0.997 μM. The detection of catechol in actual water samples has also achieved the satisfactory consequences, which provides a new strategy for the convenient and selective detection of catechol.

Current Advances of Atomically Dispersed Metal-Centered Nanozymes for Tumor Diagnosis and Therapy

Nanozymes, which combine enzyme-like catalytic activity and the biological properties of nanomaterials, have been widely used in biomedical fields. Single-atom nanozymes (SANs) with atomically dispersed metal centers exhibit excellent biological catalytic activity due to the maximization of atomic utilization efficiency, unique metal coordination structures, and metal-support interaction, and their structure-activity relationship can also be clearly investigated. Therefore, they have become an emerging alternative to natural enzymes. This review summarizes the examples of nanocatalytic therapy based on SANs in tumor diagnosis and treatment in recent years, providing an overview of material classification, activity modulation, and therapeutic means. Next, we will delve into the therapeutic mechanism of SNAs in the tumor microenvironment and the advantages of synergistic multiple therapeutic modalities (e.g., chemodynamic therapy, sonodynamic therapy, photothermal therapy, chemotherapy, photodynamic therapy, sonothermal therapy, and gas therapy). Finally, this review proposes the main challenges and prospects for the future development of SANs in cancer diagnosis and therapy.

Unprecedented Benzoquinone Motifs Reveal Post-Oligomerizational Modification of Proanthocyanidins

Proanthocyanidins (PACs) are complex flavan-3-ol polymers with stunning chemical complexity due to oxygenation patterns, oxidative phenolic ring linkages, and intricate stereochemistry of their heterocycles and inter-flavan linkages. Being promising candidates for dental restorative biomaterials, trace analysis of dentin bioactive cinnamon PACs now yielded novel trimeric (1 and 2) and tetrameric (3) PACs with unprecedented o- and p-benzoquinone motifs (benzoquinonoid PACs). Challenges in structural characterization, especially their absolute configuration, prompted the development of a new synthetic-analytical approach involving comprehensive spectroscopy, including NMR with quantum mechanics-driven 1H iterative functionalized spin analysis (HifSA) plus experimental and computational electronic circular dichroism (ECD). Vital stereochemical information was garnered from synthesizing 4-(2,5-benzoquinone)flavan-3-ols and a truncated analogue of trimer 2 as ECD models. Discovery of the first natural benzoquinonoid PACs provides new evidence to the experimentally elusive PAC biosynthesis as their formation requires two oxidative post-oligomerizational modifications (POMs) that are distinct and occur downstream from both quinone-methide-driven oligomerization and A-type linkage formation. While Nature is known to achieve structural diversity of many major compound classes by POMs, this is the first indication of PACs also following this common theme.

The Crystal Structure of Tyrosinase from Verrucomicrobium spinosum Reveals It to Be an Atypical Bacterial Tyrosinase

Tyrosinases belong to the type-III copper enzyme family, which is involved in melanin production in a wide range of organisms. Despite similar overall characteristics and functions, their structures, activities, substrate specificities and regulation vary. The tyrosinase from the bacterium Verrucomicrobium spinosum (vsTyr) is produced as a pre-pro-enzyme in which a C-terminal extension serves as an inactivation domain. It does not require a caddie protein for copper ion incorporation, which makes it similar to eukaryotic tyrosinases. To gain an understanding of the catalytic machinery and regulation of vsTyr activity, we determined the structure of the catalytically active "core domain" of vsTyr by X-ray crystallography. The analysis showed that vsTyr is an atypical bacterial tyrosinase not only because it is independent of a caddie protein but also because it shows the highest structural (and sequence) similarity to plant-derived members of the type-III copper enzyme family and is more closely related to fungal tyrosinases regarding active site features. By modelling the structure of the pre-pro-enzyme using AlphaFold, we observed that Phe453, located in the C-terminal extension, is appropriately positioned to function as a "gatekeeper" residue. Our findings raise questions concerning the evolutionary origin of vsTyr.

Genome-Wide Analysis of the Polyphenol Oxidase Gene Family in Olea europaea Provides Insights into the Mechanism of Enzymatic Browning in Olive Fruit

Browning of olive (Olea europaea L.) fruit reduces the sensory and nutritional qualities of olive oil, thereby increasing production costs. Polyphenol oxidases (PPOs) are the key enzymes that catalyze phenolic substance oxidation and mediate enzymatic browning in olive fruit, but the exact regulatory mechanism remains unclear. The main challenge is the lack of comprehensive information on OePPOs at the genome-wide level. In this study, 18 OePPO genes were identified. Subsequently, we performed a bioinformatic analysis on them. We also analyzed the expression patterns and determined the relationship among browning degree, PPO activity, and expression of OePPOs in the fruits of three olive varieties. Based on our analysis, we identified the four most conserved motifs. OePPOs were classified into two groups, with OePPOs from Group 1 showing only diphenolase activity and OePPOs from Group 2 exhibiting both mono-/diphenolase activities. Seven pairs of gene duplication events were identified, and purifying selection was found to have played a critical role in the evolution of the OePPO gene family. A positive correlation was observed between the browning degree of olive fruit and PPO activity across different olive varieties. Moreover, two important genes were found: OePPO-5 the main effector gene responsible for fruit browning, and OePPO-8, a key gene associated with specialized metabolite synthesis in the olive fruit. In short, our discoveries provide a basis for additional functional studies on OePPO genes and can help elucidate the mechanism of enzymatic browning in olive fruit in the future.

An update of aurones: food resource, health benefit, biosynthesis and application

Aurones are a subclass of active flavonoids characterized with a scaffold of 2-benzylidene-3(2H)-benzofuranone. This type of chemicals are widely distributed in fruit, vegetable and flower, and contribute to human health. In this review, we summarize the natural aurones isolated from dietary plants. Their positive effects on immunomodulation, antioxidation, cancer prevention as well as maintaining the health status of cardiovascular, nervous system and liver organs are highlighted. The biosynthesis strategies of plant-derived aurones are elaborated to provide solutions for their limited natural abundance. The potential application of natural aurones in food coloration are also discussed. This paper combines the up-to-date information and gives a full image of dietary aurones.

Phenoloxidases: catechol oxidase - the temporary employer and laccase - the rising star of vascular plants

Phenolics are vital for the adaptation of plants to terrestrial habitats and for species diversity. Phenoloxidases (catechol oxidases, COs, and laccases, LACs) are responsible for the oxidation and polymerization of phenolics. However, their origin, evolution, and differential roles during plant development and land colonization are unclear. We performed the phylogeny, domain, amino acids, compositional biases, and intron analyses to clarify the origin and evolution of COs and LACs, and analysed the structure, selective pressure, and chloroplast targeting to understand the species-dependent distribution of COs. We found that Streptophyta COs were not homologous to the Chlorophyta tyrosinases (TYRs), and might have been acquired by horizontal gene transfer from bacteria. COs expanded in bryophytes. Structural-functionality and selective pressure were partially responsible for the species-dependent retention of COs in embryophytes. LACs emerged in Zygnemaphyceae, having evolved from ascorbate oxidases (AAOs), and prevailed in the vascular plants and strongly expanded in seed plants. COs and LACs coevolved with the phenolic metabolism pathway genes. These results suggested that TYRs and AAOs were the first-stage phenoloxidases in Chlorophyta. COs might be the second key for the early land colonization. LACs were the third one (dominating in the vascular plants) and might be advantageous for diversified phenol substrates and the erect growth of plants. This work provided new insights into how phenoloxidases evolved and were devoted to plant evolution.

A Polyphenol Oxidase Catalyzes Aurone Synthesis in Marchantia polymorpha

Aurones constitute one of the major classes of flavonoids, with a characteristic furanone structure that acts as the C-ring of flavonoids. Members of various enzyme families are involved in aurone biosynthesis in different higher plants, suggesting that during evolution plants acquired the ability to biosynthesize aurones independently and convergently. Bryophytes also produce aurones, but the biosynthetic pathways and enzymes involved have not been determined. The present study describes the identification and characterization of a polyphenol oxidase (PPO) that acts as an aureusidin synthase (MpAS1) in the model liverwort, Marchantia polymorpha. Crude enzyme assays using an M. polymorpha line overexpressing MpMYB14 with high accumulation of aureusidin showed that aureusidin was biosynthesized from naringenin chalcone and converted to riccionidin A. This activity was inhibited by N-phenylthiourea, an inhibitor specific to enzymes of the PPO family. Of the six PPOs highly induced in the line overexpressing MpMyb14, one, MpAS1, was found to biosynthesize aureusidin from naringenin chalcone when expressed in Saccharomyces cerevisiae. MpAS1 also recognized eriodictyol chalcone, isoliquiritigenin and butein, showing the highest activity for eriodictyol chalcone. Members of the PPO family in M. polymorpha evolved independently from PPOs in higher plants, indicating that aureusidin synthases evolved in parallel in land plants.

Polyphenol oxidases regulate pollen development through modulating flavonoids homeostasis in tobacco

Pollen development is critical in plant reproduction. Polyphenol oxidases (PPOs) genes encode defense-related enzymes, but the role of PPOs in pollen development remains largely unexplored. Here, we characterized NtPPO genes, and then investigated their function in pollen via creating NtPPO9/10 double knockout mutant (cas-1), overexpression 35S::NtPPO10 (cosp) line and RNAi lines against all NtPPOs in Nicotiana tabacum. NtPPOs were abundantly expressed in the anther and pollen (especially NtPPO9/10). The pollen germination, polarity ratio and fruit weights were significantly reduced in the NtPPO-RNAi and cosp lines, while they were normal in cas-1 likely due to compensation by other NtPPO isoforms. Comparisons of metabolites and transcripts between the pollen of WT and NtPPO-RNAi, or cosp showed that decreased enzymatic activity of NtPPOs led to hyper-accumulation of flavonoids. This accumulation might reduce the content of ROS. Ca²⁺ and actin levels also decreased in pollen of the transgenic lines.Thus, the NtPPOs regulate pollen germination through the flavonoid homeostasis and ROS signal pathway. This finding provides novel insights into the native physiological functions of PPOs in pollen during reproduction.

A comprehensive review of polyphenol oxidase in tea (Camellia sinensis): Physiological characteristics, oxidation manufacturing, and biosynthesis of functional constituents

Polyphenol oxidase (PPO) is a metalloenzyme with a type III copper core that is abundant in nature. As one of the most essential enzymes in the tea plant (Camellia sinensis), the further regulation of PPO is critical for enhancing defensive responses, cultivating high-quality germplasm resources of tea plants, and producing tea products that are both functional and sensory qualities. Due to their physiological and pharmacological values, the constituents from the oxidative polymerization of PPO in tea manufacturing may serve as functional foods to prevent and treat chronic non-communicable diseases. However, current knowledge of the utilization of PPO in the tea industry is only available from scattered sources, and a more comprehensive study is required to reveal the relationship between PPO and tea obviously. A more comprehensive review of the role of PPO in tea was reported for the first time, as its classification, catalytic mechanism, and utilization in modulating tea flavors, compositions, and nutrition, along with the relationships between PPO-mediated enzymatic reactions and the formation of functional constituents in tea, and the techniques for the modification and application of PPO based on modern enzymology and synthetic biology are summarized and suggested in this article.

In Silico Characterization of the Physicochemical and Biological Properties of the Pink (Pleurotus djamor var. salmoneostramineus) Oyster Mushroom Chromoprotein

Cap color is an important commercial trait for oyster mushrooms. Various pigment constituents determine a diverse color. However, the pigments of oyster mushrooms are still ambiguous. The pink oyster mushroom (Pleurotus salmoneostramineus or Pleurotus djamor) chromoprotein is one of the few proteins belonging to this fungus that has a record of its sequence of amino acid residues. However, even though there are studies about this chromoprotein isolation, purification, and crystallization, the current information focused on its 3-dimensional model and the cofactor and prosthetic group (3H-indol-3-one) binding sites is unreliable and fragmented. Therefore, in this study, using free online servers such as Prot pi, GalaxyWEB, MIB, and CB-Dock2, a structural analysis and the prediction of its physicochemical and biological properties were conducted, to understand the possible function of this chromoprotein. The obtained results showed that this molecule is a protein with a molecular weight of 23 712.5 Da, an isoelectric point of 7.505, with oligomerization capacity in a dimer and glycation in the Ser6 residue. In addition, the participation of the residues Leu5, Leu8, Lys211, Ala214, and Gln215 in the binding of the prosthetic group to the protein was highlighted; as well as Ser6 and Pro7 are important residues for the interaction of the Mg²⁺ ion and eumelanin. Likewise, morphological changes based on different culture conditions (light/dark) showed that this protein is constitutive expressed and independent of blue light. The findings in this study demonstrate that pink chromoprotein is a melanosomal protein, and it possibly has a critical role in melanogenesis and the melanin polymerization. However, more experimental studies are needed to predict a possible mechanism of action and type of enzymatic activity.

Purification and comparison of soluble and membrane-bound polyphenol oxidase from potato (Solanum tuberosum) tubers

Enzymatic browning greatly affects the quality of potato products. Polyphenol oxidase (PPO) is the enzyme mainly responsible for potato enzymatic browning. PPO has soluble polyphenol oxidase (sPPO) and membrane-bound polyphenol oxidase (mPPO) forms. In this study, the properties of sPPO and mPPO were investigated in potato tubers. The molecular weight of potato sPPO and mPPO were estimated to be 69 kDa in the form of homodimers in vivo. The mass spectrometry results showed that the purified sPPO and mPPO protein in potato tubers was mainly tr|M1BMR6 (Uniprot). The optimum pH for sPPO and mPPO was 6.5, and the optimum temperatures were 20 and 30 °C, respectively. The Michaelis constant (K_m) and maximum unit enzyme activity (V_max) of sPPO were 6.08 mM and 2161 U/S when catechol was used as the substrate, whereas those of mPPO were 2.95 mM and 2129.53 U/S, respectively. The mPPO had stronger affinity to the substrate catechol than sPPO, whereas pyrogallic acid was stronger affinity for sPPO. Ascorbic acid and sodium sulfite were inhibitors of sPPO and mPPO, respectively. After understanding the different binding states of polyphenol oxidase, different inhibitors and treatment methods can be used to treat the enzyme according to different enzymatic properties, so as to achieve a greater degree of Browning control. These results will provide a theoretical basis for regulating PPO activity to reduce enzymatic browning during potato processing.

Cell death regulation: A new way for natural products to treat osteoporosis

Osteoporosis is a common metabolic bone disease that results from the imbalance of homeostasis within the bone. Intra-bone homeostasis is dependent on a precise dynamic balance between bone resorption by osteoclasts and bone formation by mesenchymal lineage osteoblasts, which comprises a series of complex and highly standardized steps. Programmed cell death (PCD) (e.g., apoptosis, autophagy, ferroptosis, pyroptosis, and necroptosis) is a cell death process that involves a cascade of gene expression events with tight structures. These events play a certain role in regulating bone metabolism by determining the fate of bone cells. Moreover, existing research has suggested that natural products derived from a wide variety of dietary components and medicinal plants modulate the PCDs based on different mechanisms, which show great potential for the prevention and treatment of osteoporosis, thus revealing the emergence of more acceptable complementary and alternative drugs with lower costs, fewer side effects and more long-term application. Accordingly, this review summarizes the common types of PCDs in the field of osteoporosis. Moreover, from the perspective of targeting PCDs, this review also discussed the roles of currently reported natural products in the treatment of osteoporosis and the involved mechanisms. Based on this, this review provides more insights into new molecular mechanisms of osteoporosis and provides a reference for developing more natural anti-osteoporosis drugs in the future.

Biochemical characterization of Dimocarpus longan polyphenol oxidase provides insights into its catalytic efficiency

The "dragon-eye" fruits produced by the tropical longan tree are rich in nutrients and antioxidants. They suffer from post-harvest enzymatic browning, a process for which mainly the polyphenol oxidase (PPO) family of enzymes is responsible. In this study, two cDNAs encoding the PPO have been cloned from leaves of Dimocarpus longan (Dl), heterologously expressed in Escherichia coli and purified by affinity chromatography. The prepro-DlPPO1 contains two signal peptides at its N-terminal end that facilitate transportation of the protein into the chloroplast stroma and to the thylakoid lumen. Removal of the two signal peptides from prepro-DlPPO1 yields pro-DlPPO1. The prepro-DlPPO1 exhibited higher thermal tolerance than pro-DlPPO1 (unfolding at 65 °C vs. 40 °C), suggesting that the signal peptide may stabilize the fold of DlPPO1. DlPPO1 can be classified as a tyrosinase because it accepts both monophenolic and diphenolic substrates. The pro-DlPPO1 exhibited the highest specificity towards the natural diphenol (-)-epicatechin (k_cat/K_M of 800 ± 120 s^-1 mM^-1), which is higher than for 4-methylcatechol (590 ± 99 s^-1 mM^-1), pyrogallol (70 ± 9.7 s^-1 mM^-1) and caffeic acid (4.3 ± 0.72 s^-1 mM^-1). The kinetic efficiencies of prepro-DlPPO1 are 23, 36, 1.7 and 4.7-fold lower, respectively, than those observed with pro-DlPPO1 for the four aforementioned diphenolic substrates. Additionally, docking studies showed that (-)-epicatechin has a lower binding energy than any other investigated substrate. Both kinetic and in-silico studies strongly suggest that (-)-epicatechin is a good substrate of DlPPO1 and ascertain the affinity of PPOs towards specific flavonoid compounds.

The structure characteristics, biosynthesis and health benefits of naturally occurring rare flavonoids

Rare flavonoids, a special subclass of naturally occurring flavonoids with diverse structures including pterocarpans, aurones, neoflavonoids, homoisoflavones, diphenylpropanes, rotenoids and 2-phenylethyl-chromones. They are mainly found in legumes with numerous health benefits. Rare flavonoids are regarded as minor flavonoids due to their very limited abundance in nature. This review gives an overview of the natural occurrences of rare flavonoids from previous literatures. Recent findings on the biosynthesis of rare flavonoids have been updated by describing their structural characteristics and classifications. Recent findings on the health benefits of rare flavonoids have also been compiled and discussed. Natural rare flavonoids with various characteristics from different subclasses from plant-based food sources are stated. They show a wide range of health benefits, including antibacterial, anticancer, anti-osteoporosis and antiviral activities. Studies reviewed suggest that rare flavonoids possessing different skeletons demonstrate different characteristic bioactivities by discussing their mechanism of actions and structure-activity relationships. Besides, recent advances on the biosynthesis of rare flavonoids, such as pterocarpans, rotenoids and aurones are well-known, while the biosynthesis of other subclasses remain unknown. The perspectives and further applications of rare flavonoids using metabolic engineering strategies also be expected.

A novel aldo-keto reductase gene is involved in 6'-deoxychalcone biosynthesis in dahlia (Dahlia variabilis)

A novel gene belonging to the aldo-keto reductase 13 family is involved in isoliquiritigenin biosynthesis in dahlia. The yellow pigments of dahlia flowers are derived from 6'-deoxychalcones, which are synthesized via a two-step process, involving the conversion of 3-malonyl-CoA and 4-coumaloyl-CoA into isoliquiritigenin in the first step, and the subsequent generation of butein from isoliquiritigenin. The first step reaction is catalyzed by chalcone synthase (CHS) and aldo-keto reductase (AKR). AKR has been implicated in the isoflavone biosynthesis in legumes, however, isolation of butein biosynthesis related AKR members are yet to be reported. A comparative RNA-seq analysis between two dahlia cultivars, 'Shukuhai' and its butein-deficient lateral mutant 'Rinka', was used in this study to identify a novel AKR gene involved in 6'-deoxychalcone biosynthesis. DvAKR1 encoded a AKR 13 sub-family protein with significant differential expression levels, and was phylogenetically distinct from the chalcone reductases, which belongs to the AKR 4A sub-family in legumes. DNA sequence variation and expression profiles of DvAKR1 gene were correlated with 6'-deoxychalcone accumulation in the tested dahlia cultivars. A single over-expression analysis of DvAKR1 was not sufficient to initiate the accumulation of isoliquiritigenin in tobacco, in contrast, its co-overexpression with a chalcone 4'-O-glucosyltransferase (Am4'CGT) from Antirrhinum majus and a MYB transcription factor, CaMYBA from Capsicum annuum successfully induced isoliquiritigenin accumulation. In addition, DvAKR1 homologous gene expression was detected in Coreopsideae species accumulating 6'-deoxychalcone, but not in Asteraceae species lacking 6'-deoxychalcone production. These results not only demonstrate the involvement of DvAKR1 in the biosynthesis of 6'-deoxychalcone in dahlia, but also show that 6'-deoxychalcone occurrence in Coreopsideae species developed evolutionarily independent from legume species.

The (Bio)chemical Base of Flower Colour in Bidens ferulifolia

Bidens ferulifolia is a yellow flowering plant, originating from Mexico, which is increasingly popular as an ornamental plant. In the past few years, new colour combinations ranging from pure yellow over yellow-red, white-red, pure white and purple have emerged on the market. We analysed 16 Bidens ferulifolia genotypes to provide insight into the (bio)chemical base underlying the colour formation, which involves flavonoids, anthochlors and carotenoids. In all but purple and white genotypes, anthochlors were the prevalent pigments, primarily derivatives of okanin, a 6′-deoxychalcone carrying an unusual 2′3′4′-hydroxylation pattern in ring A. The presence of a cytochrome-P450-dependent monooxygenase introducing the additional hydroxyl group in position 3′ of both isoliquiritigenin and butein was demonstrated for the first time. All genotypes accumulate considerable amounts of the flavone luteolin. Red and purple genotypes additionally accumulate cyanidin-type anthocyanins. Acyanic genotypes lack flavanone 3-hydroxylase and/or dihydroflavonol 4-reductase activity, which creates a bottleneck in the anthocyanin pathway. The carotenoid spectrum was analysed in two Bidens genotypes and showed strong variation between the two cultivars. In comparison to anthochlors, carotenoids were present in much lower concentrations. Carotenoid monoesters, as well as diesters, were determined for the first time in B. ferulifolia flower extracts.

Biochemistry and regulation of aurone biosynthesis

Aurones are a group of flavonoids that confer a bright yellow coloration to certain ornamental flowers and are a promising structural target for the development of new therapeutic drugs. Since the first identification of the snapdragon aurone synthase as a polyphenol oxidase (PPO) in 2000, several important advances in the biochemistry and regulation of aurone biosynthesis have been achieved. For example, several other aurone synthases have been identified in distantly related plants, which not only include PPOs but also peroxidases. Elucidation of the subcellular localization of aurone biosynthesis in snapdragon led to the establishment of a method to genetically engineer novel yellow flowers. The crystal structure of an aurone-producing PPO was clarified and provided important insights into the structure-function relationship of aurone-producing PPOs. A locus (SULFUREA) that negatively regulates aurone biosynthesis in snapdragon was identified, illustrating the evolution of flower color pattern through selection on regulatory small RNAs.

Transcriptomic and proteomic data provide new insights into cold-treated potato tubers with T- and D-type cytoplasm

Tuber-omics in potato with the T- and D-types of cytoplasm showed different sets of differentially expressed genes and proteins in response to cold storage. For the first time, we report differences in gene and protein expression in potato (Solanum tuberosum L.) tubers possessing the T- or D-type cytoplasm. Two F1 diploid reciprocal populations, referred to as T and D, were used. The pooling strategy was applied for detection of differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) in tubers consisting of extreme chip colour after cold storage. RNA and protein bulks were constructed from contrasting phenotypes. We recognized 48 and 15 DEGs for the T and D progenies, respectively. DEPs were identified in the amyloplast and mitochondrial fractions. In the T-type cytoplasm, only 2 amyloplast-associated and 5 mitochondria-associated DEPs were detected. Of 37 mitochondria-associated DEPs in the D-type cytoplasm, there were 36 downregulated DEPs in the dark chip colour bulks. These findings suggest that T- and D-type of cytoplasm might influence sugar accumulation in cold-stored potato tubers in different ways. We showed that the mt/nucDNA ratio was higher in D-possessing tubers after cold storage than in T progeny. For the D-type cytoplasm, the pt/nucDNA ratio was higher for tubers characterized by dark chip colour than for those with light chip colour. Our findings suggest that T- and D-type cytoplasm might influence sugar accumulation in cold-stored potato tubers in different ways.

Plant Copper Metalloenzymes As Prospects for New Metabolism Involving Aromatic Compounds

Copper is an important transition metal cofactor in plant metabolism, which enables diverse biocatalysis in aerobic environments. Multiple classes of plant metalloenzymes evolved and underwent genetic expansions during the evolution of terrestrial plants and, to date, several representatives of these copper enzyme classes have characterized mechanisms. In this review, we give an updated overview of chemistry, structure, mechanism, function and phylogenetic distribution of plant copper metalloenzymes with an emphasis on biosynthesis of aromatic compounds such as phenylpropanoids (lignin, lignan, flavonoids) and cyclic peptides with macrocyclizations via aromatic amino acids. We also review a recent addition to plant copper enzymology in a copper-dependent peptide cyclase called the BURP domain. Given growing plant genetic resources, a large pool of copper biocatalysts remains to be characterized from plants as plant genomes contain on average more than 70 copper enzyme genes. A major challenge in characterization of copper biocatalysts from plant genomes is the identification of endogenous substrates and catalyzed reactions. We highlight some recent and future trends in filling these knowledge gaps in plant metabolism and the potential for genomic discovery of copper-based enzymology from plants.

Speciation of Transition-Metal-Substituted Keggin-Type Silicotungstates Affected by the Co-crystallization Conditions with Proteinase K

We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts Na₁₄[(A-α-SiW₁₀O₃₇)₂{Co₄(OH)₂(H₂O)₂}]·37H₂O (Na{SiW₁₀Co₂}₂) and Na₁₄[(A-α-SiW₁₀O₃₇)₂{Ni₄(OH)₂(H₂O)₂}]·77.5H₂O (Na{SiW₁₀Ni₂}₂), which were prepared by applying a new synthesis protocol and characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Proteinase K was applied as a model protein and the polyoxotungstate (POT)–protein interactions of Na{SiW₁₀Co₂}₂ and Na{SiW₁₀Ni₂}₂ were studied side by side with the literature-known K₅Na₃[A-α-SiW₉O₃₄(OH)₃{Co₄(OAc)₃}]·28.5H₂O ({SiW₉Co₄}) featuring the same number of transition metals. Testing the solution behavior of applied POTs under the crystallization conditions (sodium acetate buffer, pH 5.5) by time-dependent UV/vis spectroscopy and electrospray ionization mass spectrometry speciation studies revealed an initial dissociation of the sandwich POTs to the disubstituted Keggin anions H_xNa_5–x[SiW₁₀Co₂O₃₈]^3– and H_xNa_5–x[SiW₁₀Ni₂O₃₈]^3– ({SiW₁₀M₂}, M = Co^II and Ni^II) followed by partial rearrangement to the monosubstituted compounds (α-{SiW₁₁Co} and α-{SiW₁₁Ni}) after 1 week of aging. The protein crystal structure analysis revealed monosubstituted α-Keggin POTs in two conserved binding positions for all three investigated compounds, with one of these positions featuring a covalent attachment of the POT anion to an aspartate carboxylate. Despite the presence of both mono- and disubstituted anions in a crystallization mixture, proteinase K selectively binds to monosubstituted anions because of their preferred charge density for POT–protein interaction.

We report on the development of a new synthesis protocol to prepare the Na salts of the tetrasubstituted sandwich-type Keggin derivatives Na₁₄[(A-α-SiW₁₀O₃₈)₂{Co₄(OH)₂(H₂O)₂}]·37H₂O (Na{SiW₁₀Co₂}₂) and Na₁₄[(A-α-SiW₁₀O₃₈)₂{Ni₄(OH)₂(H₂O)₂}]·77.5H₂O (Na{SiW₁₀Ni₂}₂). Following a thorough characterization of the polyoxotungstate (POT) dimers involving single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis in the solid state and UV/vis spectroscopy and electrospray ionization mass spectrometry in solution, the water-soluble compounds (>5 mM) were applied as additives for the crystallization of proteinase K along with the tetrasubstituted monomeric Keggin-type analogue K₅Na₃[A-α-SiW₉O₃₄(OH)₃{Co₄(OAc)₃}]·28.5H₂O ({SiW₉Co₄}). Crystallographic studies on the obtained protein crystals revealed monosubstituted Keggin derivatives in all three cases bound to conserved sites of the protein, which highlights a selectivity of proteinase K toward monosubstituted Keggin POTs within a narrow range of surface charge density.

Major Flower Pigments Originate Different Colour Signals to Pollinators

Flower colour is mainly due to the presence and type of pigments. Pollinator preferences impose selection on flower colour that ultimately acts on flower pigments. Knowing how pollinators perceive flowers with different pigments becomes crucial for a comprehensive understanding of plant-pollinator communication and flower colour evolution. Based on colour space models, we studied whether main groups of pollinators, specifically hymenopterans, dipterans, lepidopterans and birds, differentially perceive flower colours generated by major pigment groups. We obtain reflectance data and conspicuousness to pollinators of flowers containing one of the pigment groups more frequent in flowers: chlorophylls, carotenoids and flavonoids. Flavonoids were subsequently classified in UV-absorbing flavonoids, aurones-chalcones and the anthocyanins cyanidin, pelargonidin, delphinidin, and malvidin derivatives. We found that flower colour loci of chlorophylls, carotenoids, UV-absorbing flavonoids, aurones-chalcones, and anthocyanins occupied different regions of the colour space models of these pollinators. The four groups of anthocyanins produced a unique cluster of colour loci. Interestingly, differences in colour conspicuousness among the pigment groups were almost similar in the bee, fly, butterfly, and bird visual space models. Aurones-chalcones showed the highest chromatic contrast values, carotenoids displayed intermediate values, and chlorophylls, UV-absorbing flavonoids and anthocyanins presented the lowest values. In the visual model of bees, flowers with UV-absorbing flavonoids (i.e., white flowers) generated the highest achromatic contrasts. Ours findings suggest that in spite of the almost omnipresence of floral anthocyanins in angiosperms, carotenoids and aurones-chalcones generates higher colour conspicuousness for main functional groups of pollinators.

Phenoloxidases in Plants-How Structural Diversity Enables Functional Specificity

The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.

Toward more specific inhibitor for Solanum tuberosum polyphenol oxidase through a structural insight into its activities and inhibition

To prevent enzymatic browning, applying a polyphenol oxidase (PPO) inhibitor is more desirable, especially when the freshness of the product matters. Most of the inhibition studies were done on mushroom tyrosinase (MT) while the literature indicates that MT and PPO of Solanum tuberosum (PPO_sol ) respond differently to the same modulator despite their similar active sites. This research was conducted to deepen our knowledge about PPO_sol and introduce a more specific inhibitor for this enzyme to be used in controlling the enzymatic browning of potatoes. A modified procedure was developed for PPO_sol purification. The enzyme was subjected to some essential physicochemical and kinetics studies. In parallel to the comparable physicochemical properties, homology modeling revealed high structural similarity between Solanum lycopersicum PPO (PPO_sly ) and PPO_sol except for their active site pockets. Accordingly, PPO_sol showed 5.1- and 34-fold higher affinity toward chlorogenic acid compared with two PPO_sly isozymes. Alike PPO_sly , PPO_sol showed monophenolase activity but it was inactive toward L-tyrosine and p-coumaric acid. Based on structural criteria, phthalic acid, cinnamic acid, ferulic acid, and vanillin were selected and thoroughly examined for inhibition of the catecholase activity of PPO_sol . Although all these substances inhibited PPO_sol in mixed-inhibition mode, the results were strongly in favor of vanillin with IC₅₀ < 1.37 mM and K_i < 1.2 mM. PRACTICAL APPLICATIONS: There are subtle structural differences in the active site pockets of polyphenol oxidase (PPOs) of various fruits, vegetables, and crops. Consequently, to introduce an efficient inhibitor for hindering enzymatic browning of crop products, it is essential to have detailed knowledge about the structure and activity of its PPO as the main player of this undesirable phenomenon. Results of this study not only shed light on the physicochemical properties of PPO_sol but can also be used in making various formulations for safe controlling enzymatic browning of potatoes, especially fresh-cut and minimally processed products, and similar crops products during postharvest and the processes of products preparations.

The basicity of an active-site water molecule discriminates between tyrosinase and catechol oxidase activity

Tyrosinase (Ty) and catechol oxidase (CO) are members of type-3 copper enzymes. While Ty catalyzes both phenolase and catecholase reactions, CO catalyzes only the latter reaction. In the present study, Ty was found to catalyze the catecholase reaction, but hardly the phenolase reaction in the presence of the metallochaperon called "caddie protein (Cad)". The ability of the substrates to dissociate the motif shielding the active-site pocket seems to contribute critically to the substrate specificity of Ty. In addition, a mutation at the N191 residue, which forms a hydrogen bond with a water molecule near the active center, decreased the inherent ratio of phenolase versus catecholase activity. Unlike the wild-type complex, reaction intermediates were not observed when the catalytic reaction toward the Y98 residue of Cad was progressed in the crystalline state. The increased basicity of the water molecule may be necessary to inhibit the proton transfer from the conjugate acid to a hydroxide ion bridging the two copper ions. The deprotonation of the substrate hydroxyl by the bridging hydroxide seems to be significant for the efficient catalytic cycle of the phenolase reaction.

Considerations Regarding Activity Determinants of Fungal Polyphenol Oxidases Based on Mutational and Structural Studies

Polyphenol oxidases (PPOs) are an industrially relevant family of enzymes, being involved in the postharvest browning of fruits and vegetables, as well as in human melanogenesis. Their involvement lies in their ability to oxidize phenolic or polyphenolic compounds, which subsequently form pigments. The PPO family includes tyrosinases and catechol oxidases, which, in spite of their high structural similarity, exhibit different catalytic activities. Long-standing research efforts have not yet managed to decipher the structural determinants responsible for this differentiation, as every new theory is disproved by a more recent study. In the present work, we combined biochemical along with structural data in order to better understand the function of a previously characterized PPO from Thermothelomyces thermophila (TtPPO). The crystal structure of a TtPPO variant, determined at 1.55 Å resolution, represents the second known structure of an ascomycete PPO. Kinetic data for structure-guided mutants prove the implication of "gate" residue L306, residue HB1+1 (G292), and HB2+1 (Y296) in TtPPO function against various substrates. Our findings demonstrate the role of L306 in the accommodation of bulky substrates and show that residue HB1+1 is unlikely to determine monophenolase activity, as was suggested from previous studies.IMPORTANCE PPOs are enzymes of biotechnological interest. They have been extensively studied both biochemically and structurally, with a special focus on the plant-derived counterparts. Even so, explicit description of the molecular determinants of their substrate specificity is still pending. For ascomycete PPOs, only one crystal structure has been determined so far, thus limiting our knowledge on this tree branch of the family. In the present study, we report the second crystal structure of an ascomycete PPO. Combined with site-directed mutagenesis and biochemical studies, we depict the amino acids in the vicinity of the active site that affect enzyme activity and perform a detailed analysis on a variety of substrates. Our findings improve current understanding of structure-function relations of microbial PPOs, which is a prerequisite for the engineering of biocatalysts of desired properties.

Production of a novel slow-release coal fly ash microbial fertilizer for restoration of mine vegetation

Driven by a need for economic development, a large number of mines have been exploited, resulting in the destruction of large areas of vegetation and a significant deterioration in local ecological environment. In order to restore vegetation of mines in a timely manner, a new type of organic fertilizer needs to be developed. However, until now, there has been a lack of organic fertilizer with slow-release suitable for mine virescence. As the largest amount of solid waste in coal-fired power plants, coal fly ash presents a promising basis as a bioresource for developing this type of organic fertilizer. In our study, for the first time, fly ash was demonstrated to be an effective carrier matrix via hydrothermal-alkali treatment sintering process for solving the problem of low efficiency of fly ash adsorption for microorganisms via sintering process. Then, a novel slow-release microbial fertilizer which can adsorb a variety of microorganisms was produced using ethyl cellulose as a solvent adhesive. Finally, the pot experiment showed that the soil fertility of abandoned mines can be improved after applying the fly ash microbial fertilizer, and demonstrated the regreening effects with Pseudodrynaria coronans and Buxus microphylla. Our study provides a green engineering approach to recycle fly ash for regreening mines, as well as a new development direction for high-value green recyclable pathway of fly ash.

Chalcone and its analogs: Therapeutic and diagnostic applications in Alzheimer's disease

Chalcone [(E)-1,3-diphenyl-2-propene-1-one], a small molecule with α, β unsaturated carbonyl group is a precursor or component of many natural flavonoids and isoflavonoids. It is one of the privileged structures in medicinal chemistry. It possesses a wide range of biological activities encouraging many medicinal chemists to study this scaffold for its usefulness to oncology, infectious diseases, virology and neurodegenerative diseases including Alzheimer's disease (AD). Small molecular size, convenient and cost-effective synthesis, and flexibility for modifications to modulate lipophilicity suitable for blood brain barrier (BBB) permeability make chalcones a preferred candidate for their therapeutic and diagnostic potential in AD. This review summarizes and highlights the importance of chalcone and its analogs as single target small therapeutic agents, multi-target directed ligands (MTDLs) as well as molecular imaging agents for AD. The information summarized here will guide many medicinal chemist and researchers involved in drug discovery to consider chalcone as a potential scaffold for the development of anti-AD agents including theranostics.

Dahlia variabilis cultivar 'Seattle' as a model plant for anthochlor biosynthesis

We investigated the bi-colored dahlia cultivar 'Seattle', which exhibits bright yellow petals with white tips, for its potential use as a model system for studies of the anthochlor biosynthesis. The yellow base contained high amounts of the 6'-deoxychalcones and the structurally related 4-deoxyaurones, as well as flavones. In contrast, only traces of anthochlors and flavones were detected in the white tips. No anthocyanins, flavonols, flavanones or dihydroflavonols were found in the petals. Gene expression studies indicated that the absence of anthocyanins in the petals is caused by a lack of flavanone 3-hydroxylase (FHT) expression, which is accompanied by a lack of expression of the bHLH transcription factor IVS. Expression of other genes involved in anthocyanidin biosynthesis such as dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS) was not affected. The yellow and white petal parts showed significant differences in the expression of chalcone synthase 2 (CHS2), which is sufficient to explain the absence of yellow pigments in the white tips. Transcriptomes of both petal parts were de novo assembled and three candidate genes for chalcone reductase (CHR) were identified. None of them showed a significantly higher expression in the yellow base compared to the white tips. In summary, it was shown that the bicolouration is most likely caused by a bottleneck in chalcone formation in the white tip. The relative prevalence of flavones compared to the anthochlors in the white tips could be an indication for the presence of a so far unknown differentially expressed CHR.

Similar but Still Different: Which Amino Acid Residues Are Responsible for Varying Activities in Type-III Copper Enzymes?

Type-III copper enzymes like polyphenol oxidases (PPOs) are ubiquitous among organisms and play a significant role in the formation of pigments. PPOs comprise different enzyme groups, including tyrosinases (TYRs) and catechol oxidases (COs). TYRs catalyze the o-hydroxylation of monophenols and the oxidation of o-diphenols to the corresponding o-quinones (EC 1.14.18.1). In contrast, COs only catalyze the oxidation of o-diphenols to the corresponding o-quinones (EC 1.10.3.1). To date (August 2020), 102 PDB entries encompassing 18 different proteins from 16 organisms and several mutants have been reported, identifying key residues for tyrosinase activity. The structural similarity between TYRs and COs, especially within and around the active center, complicates the elucidation of their modes of action on a structural basis. However, mutagenesis studies illuminate residues that influence the two activities and show that crystallography on its own cannot elucidate the enzymatic activity mode. Several amino acid residues around the dicopper active center have been proposed to play an essential role in the two different activities. Herein, we critically review the role of all residues identified so far that putatively affect the two activities of PPOs.

Identification of Amino Acid Residues Responsible for C-H Activation in Type-III Copper Enzymes by Generating Tyrosinase Activity in a Catechol Oxidase

Tyrosinases (TYRs) catalyze the hydroxylation of phenols and the oxidation of the resulting o-diphenols to o-quinones, while catechol oxidases (COs) exhibit only the latter activity. Aurone synthase (AUS) is not able to react with classical tyrosinase substrates, such as tyramine and l-tyrosine, while it can hydroxylate its natural substrate isoliquiritigenin. The structural difference of TYRs, COs, and AUS at the heart of their divergent catalytic activities is still a puzzle. Therefore, a library of 39 mutants of AUS from Coreopsis grandiflora (CgAUS) was generated and the activity studies showed that the reactivity of the three conserved histidines (HisA2 , HisB1 , and HisB2 ) is tuned by their adjacent residues (HisB1 +1, HisB2 +1, and waterkeeper residue) either to react as stronger bases or / and to stabilize a position permissive for substrate proton shuffling. This provides the understanding for C-H activation based on the type-III copper center to be used in future biotechnological processes.

Neurotrophic, anti-neuroinflammatory, and redox balance mechanisms of chalcones

The passage of time that evoke aging; the tilted redox balance that contribute oxidative entropy; the polarization of microglia cells that produce inflammatory phenotype; all represent the intricacies of CNS-dependent disease progression. Neurological diseases that result from CNS injury raise social concerns and the available therapeutic strategies are frustrated by low efficacy, high toxicity, and multiple side effects. However, emergent studies have shown the neuroprotective role of natural compounds - including chalcones - with high efficacy in the protection of CNS structures. These compounds reportedly demonstrate neurotrophic mechanism through the upregulation of neurotrophic factors, anti-apoptotic Bcl-2, and downregulation of Bax protein; anti-neuroinflammatory mechanism via the inhibition of neuroinflammatory pathways, attenuated secretion of pro-inflammatory cytokines, prevention of blood brain barrier (BBB) disruption, and protection against nerve senescence; antioxidant mechanism through the upregulation of Nrf2 activities, inhibition of Keap1, synthesis of antioxidant enzymes, and maintenance of high antioxidant/oxidant ratio. All these mechanisms represent chalcones' neuroprotective mechanisms. In this review, we highlight different pathways involved in CNS-related diseases and elucidate various mechanisms by which chalcones can perturb these shunts as a potential therapeutic modality.

Monophenolase and catecholase activity of Aspergillus oryzae catechol oxidase: insights from hybrid QM/MM calculations

Catechol oxidase from Aspergillus oryzae (AoCO4) can not only catalyze oxidation of o-diphenols to o-quinones, but can also catalyze monooxygenation of small phenolics. To gain insight into the catecholase and monophenolase activities of AoCO4, the reaction mechanism of catechol oxidation was investigated by means of hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. The oxy-form of AoCO4 was found to be a μ-η2:η2 side-on peroxo dicopper(ii) complex, which can undergo a proton coupled electron transfer from the substrate rather than a proton transfer from the nearby Ser302 residue to generate a hydroperoxide. The μ-1,1-OOH Cu2(i,ii) complex is thermodynamically more stable than the μ-η1:η2 hydroperoxide. Moreover, the cleavage of the O-O bond in the μ-1,1-OOH Cu2(i,ii) intermediate has a much lower barrier than that in the μ-η1:η2 hydroperoxide species. In both cases, the O-O bond cleavage is the rate-limiting step, generating the reactive (μ-O˙)(μ-OH) dicopper(ii) complex. In addition, our results demonstrated that the oxidation of catechol to quinone is much more preferred than the hydroxylation reaction. These findings may provide useful information for understanding the reactivity of the Cu2O2 active site of coupled binuclear copper enzymes.

Polyphenol oxidases exhibit promiscuous proteolytic activity

Tyrosinases catalyse both the cresolase and catecholase reactions for the formation of reactive compounds which are very important for industrial applications. In this study, we describe a proteolytic activity of tyrosinases. Two different tyrosinases originating from mushroom and apple are able to cleave the carboxylesterase EstA. The cleavage reaction correlates with the integrity of the active site of tyrosinase and is independent of other possible influencing factors, which could be present in the reaction. Therefore, the cleavage of EstA represents a novel functionality of tyrosinases. EstA was previously reported to degrade synthetic polyesters, albeit slowly. However, the EstA truncated by tyrosinase shows higher degradation activity on the non-biodegradable polyester polyethylene terephthalate (PET), which is a well-established environmental threat.

Characterization of membrane-bound polyphenol oxidase from Granny Smith apple (Malus × domestica Borkh.)

Membrane-bound polyphenol oxidase (mPPO) from the Granny Smith apple was purified and characterized. The enzyme was purified by a factor of 20.53 with a recovery of 1.8%. The molecular weight of purified mPPO was determined to be 65 kDa by electrophoresis and nano-electrospray ionization mass spectrometry. mPPO exhibited its highest activity at a temperature of 35 °C and a pH of 7.0 and can be regarded as a diphenol oxidase. A low concentration of SDS (≤0.5 mM) enhanced the enzymatic activity, whereas mPPO was activated at high concentration EDTA (≥2 mM). The thermal transition temperature of mPPO was 76.98 °C. The circular dichroism spectrum showed that mPPO contains high α-helix content, the fluorescence spectroscopy indicated that the tryptophan residues of mPPO are partially buried. The particle size of mPPO was 5-10 nm with a complete structure. The structural characterization of mPPO provided better insights into the regions responsible for its activity.

Inhibitory Effect and Mechanism of Action of Quercetin and Quercetin Diels-Alder anti-Dimer on Erastin-Induced Ferroptosis in Bone Marrow-Derived Mesenchymal Stem Cells

In this study, the anti-ferroptosis effects of catecholic flavonol quercetin and its metabolite quercetin Diels-Alder anti-dimer (QDAD) were studied using an erastin-treated bone marrow-derived mesenchymal stem cell (bmMSCs) model. Quercetin exhibited higher anti-ferroptosis levels than QDAD, as indicated by 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (C11-BODIPY), 2',7'-dichlorodihydrofluoroscein diacetate (H2DCFDA), lactate dehydrogenase (LDH) release, cell counting kit-8 (CCK-8), and flow cytometric assays. To understand the possible pathways involved, the reaction product of quercetin with the 1,1-diphenyl-2-picrylhydrazyl radical (DPPH^●) was measured using ultra-performance liquid-chromatography coupled with electrospray-ionization quadrupole time-of-flight tandem mass spectrometry (UHPLC-ESI-Q-TOF-MS). Quercetin was found to produce the same clusters of molecular ion peaks and fragments as standard QDAD. Furthermore, the antioxidant effects of quercetin and QDAD were compared by determining their 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide radical-scavenging, Cu²⁺-reducing, Fe³⁺-reducing, lipid peroxidation-scavenging, and DPPH^●-scavenging activities. Quercetin consistently showed lower IC₅₀ values than QDAD. These findings indicate that quercetin and QDAD can protect bmMSCs from erastin-induced ferroptosis, possibly through the antioxidant pathway. The antioxidant pathway can convert quercetin into QDAD-an inferior ferroptosis-inhibitor and antioxidant. The weakening has highlighted a rule for predicting the relative anti-ferroptosis and antioxidant effects of catecholic flavonols and their Diels-Alder dimer metabolites.

Purification of Polyphenol Oxidase from Potato and Investigation of the Inhibitory Effects of Phenolic Acids on Enzyme Activity

Background:

Polyphenol Oxidase (PPO) belongs to the oxidoreductase enzyme family.

Methods:

Here, PPO was purified from potato using Sepharose 4B-L-tyrosine-p-aminobenzoic acid affinity chromatography. It determined the interactions between some phenolic acids and the enzyme.

Results:

The enzyme was obtained with a specific activity of 15333.33 EU/mg protein and 7.87- fold purification. It was found that phenolic acids exhibited inhibitory properties for PPO. The IC50 values of the phenolic acids were found in the range of 0.36-2.12 mM, and their Ki values were found in the range of 0.28± 0.07-1.72±0.32 mM. It was determined that all studied compounds displayed a competitive inhibition effect. Among these compounds, 3-hydroxybenzoic acid was found to be the most effective PPO inhibitor (Ki: 0.28±0.07 mM).

Conclusion:

Investigating the inhibition kinetics of the enzyme will simplify the testing of PPO inhibitor candidates.

The Evolution of Flavonoid Biosynthesis: A Bryophyte Perspective

The flavonoid pathway is one of the best characterized specialized metabolite pathways of plants. In angiosperms, the flavonoids have varied roles in assisting with tolerance to abiotic stress and are also key for signaling to pollinators and seed dispersal agents. The pathway is thought to be specific to land plants and to have arisen during the period of land colonization around 550-470 million years ago. In this review we consider current knowledge of the flavonoid pathway in the bryophytes, consisting of the liverworts, hornworts, and mosses. The pathway is less characterized for bryophytes than angiosperms, and the first genetic and molecular studies on bryophytes are finding both commonalities and significant differences in flavonoid biosynthesis and pathway regulation between angiosperms and bryophytes. This includes biosynthetic pathway branches specific to each plant group and the apparent complete absence of flavonoids from the hornworts.

Conversion of walnut tyrosinase into a catechol oxidase by site directed mutagenesis

Polyphenol oxidases (PPOs) comprise tyrosinases (TYRs) and catechol oxidases (COs), which catalyse the initial reactions in the biosynthesis of melanin. TYRs hydroxylate monophenolic (monophenolase activity) and oxidize diphenolic (diphenolase activity) substrates, whereas COs react only with diphenols. In order to elucidate the biochemical basis for the different reactions in PPOs, cDNA from walnut leaves was synthesized, the target gene encoding the latent walnut tyrosinase (jrPPO1) was cloned, and the enzyme was heterologously expressed in Escherichia coli. Mutations targeting the two activity controller residues (Asn240 and Leu244) as well as the gatekeeper residue (Phe260) were designed to impair monophenolase activity of jrPPO1. For the first time, monophenolase activity of jrPPO1 towards L-tyrosine was blocked in two double mutants (Asn240Lys/Leu244Arg and Asn240Thr/Leu244Arg) while its diphenolase activity was partially preserved, thereby converting jrPPO1 into a CO. Kinetic data show that recombinant jrPPO1 resembles the natural enzyme, and spectrophotometric investigations proved that the copper content remains unaffected by the mutations. The results presented herein provide experimental evidence that a precisely tuned interplay between the amino acids located around the active center controls the substrate specificity and therewith the mono- versus diphenolase activity in the type-III copper enzyme jrPPO1.

Laccase Properties, Physiological Functions, and Evolution

Discovered in 1883, laccase is one of the first enzymes ever described. Now, after almost 140 years of research, it seems that this copper-containing protein with a number of unique catalytic properties is widely distributed across all kingdoms of life. Laccase belongs to the superfamily of multicopper oxidases (MCOs)—a group of enzymes comprising many proteins with different substrate specificities and diverse biological functions. The presence of cupredoxin-like domains allows all MCOs to reduce oxygen to water without producing harmful byproducts. This review describes structural characteristics and plausible evolution of laccase in different taxonomic groups. The remarkable catalytic abilities and broad substrate specificity of laccases are described in relation to other copper-containing MCOs. Through an exhaustive analysis of laccase roles in different taxa, we find that this enzyme evolved to serve an important, common, and protective function in living systems.