Cloning of a novel L-amino acid oxidase from Trichoderma harzianum ETS 323 and bioactivity analysis of overexpressed L-amino acid oxidase

Ancestral l-amino acid oxidase: From substrate scope exploration to phenylalanine ammonia-lyase assay

In this study we assessed the applicability of the recently reported ancestral l-amino acid oxidase (AncLAAO), for the development of an enzyme-coupled phenylalanine ammonia-lyase (PAL) activity assay. Firstly, the expression and isolation of the AncLAAO-N1 was optimized, followed by activity tests of the obtained octameric N-terminal His-tagged enzyme towards various phenylalanine analogues to assess the compatibility of its substrate scope with that of the well-characterized PALs. AncLAAO-N1 showed high catalytic efficiency towards phenylalanines mono-, di-, or multiple-substituted in the meta- or para-positions, with ortho- substituted substrates being poorly transformed, these results highlighting the significant overlap between its substrate scope and those of PALs. After successful set-up of the AncLAAO-PAL coupled solid phase assay, in a 'proof of concept' approach we demonstrated its applicability for the high-throughput activity screens of PAL-libraries, by screening the saturation mutagenesis-derived I460NNK variant library of PAL from Petroselinum crispum, using p-MeO-phenylalanine as model substrate. Notably, the hits revealed by the coupled assay comprised all the active PAL variants: I460V, I460T, I460S, I460L, previously identified from the tested PAL-library by other assays. Our results validate the applicability of AncLAAO for coupled enzyme systems with phenylalanine ammonia-lyases, including cell-based assays suitable for the high-throughput screening of directed evolution-derived PAL-libraries.

Trichoderma spp. Genes Involved in the Biocontrol Activity Against Rhizoctonia solani

Rhizoctonia solani is a pathogen that causes considerable harm to plants worldwide. In the absence of hosts, R. solani survives in the soil by forming sclerotia, and management methods, such as cultivar breeding, crop rotations, and fungicide sprays, are insufficient and/or inefficient in controlling R. solani. One of the most challenging problems facing agriculture in the twenty-first century besides with the impact of global warming. Environmentally friendly techniques of crop production and improved agricultural practices are essential for long-term food security. Trichoderma spp. could serve as an excellent example of a model fungus to enhance crop productivity in a sustainable way. Among biocontrol mechanisms, mycoparasitism, competition, and antibiosis are the fundamental mechanisms by which Trichoderma spp. defend against R. solani, thereby preventing or obstructing its proliferation. Additionally, Trichoderma spp. induce a mixed induced systemic resistance (ISR) or systemic acquired resistance (SAR) in plants against R. solani, known as Trichoderma-ISR. Stimulation of every biocontrol mechanism involves Trichoderma spp. genes responsible for encoding secondary metabolites, siderophores, signaling molecules, enzymes for cell wall degradation, and plant growth regulators. Rhizoctonia solani biological control through genes of Trichoderma spp. is summarized in this paper. It also gives information on the Trichoderma-ISR in plants against R. solani. Nonetheless, fast-paced current research on Trichoderma spp. is required to properly utilize their true potential against diseases caused by R. solani.

Isolation of Polysaccharides from Trichoderma harzianum with Antioxidant, Anticancer, and Enzyme Inhibition Properties

In this work, a total of six polysaccharides were isolated from culture filtrate (EPS1, EPS2) and mycelia (IPS1-IPS4) of Trichoderma harzianum. The HPLC analysis results showed that EPS1, EPS2, IPS1, and IPS2 were composed of mannose, ribose, glucose, galactose, and arabinose. The FT-IR, 1H, and 13C NMR chemical shifts confirmed that the signals in EPS1 mainly consist of (1→4)-linked α-d-glucopyranose. EPS1 and IPS1 showed a smooth and clean surface, while EPS2, IPS2, and IPS3 exhibited a microporous structure. Among polysaccharides, EPS1 displayed higher ABTS+ (47.09 ± 2.25% and DPPH (26.44 ± 0.12%) scavenging activities, as well as higher α-amylase (69.30 ± 1.28%) and α-glucosidase (68.22 ± 0.64%) inhibition activity than the other polysaccharides. EPS1 exhibited high cytotoxicity to MDA-MB293 cells, with an IC50 of 0.437 mg/mL, and this was also confirmed by cell staining and FACS assays. These results report the physicochemical and bioactive properties of polysaccharides from T. harzianum.

Analysis of N-glycosylation in fungal l-amino acid oxidases expressed in the methylotrophic yeast Pichia pastoris

l-amino acid oxidases (LAAOs) catalyze the oxidative deamination of l-amino acids to corresponding α-keto acids. Here, we describe the heterologous expression of four fungal LAAOs in Pichia pastoris. cgLAAO1 from Colletotrichum gloeosporioides and ncLAAO1 from Neurospora crassa were able to convert substrates not recognized by recombinant 9His-hcLAAO4 from the fungus Hebeloma cylindrosporum described earlier thereby broadening the substrate spectrum for potential applications. 9His-frLAAO1 from Fibroporia radiculosa and 9His-laLAAO2 from Laccaria amethystine were obtained only in low amounts. All four enzymes were N-glycosylated. We generated mutants of 9His-hcLAAO4 lacking N-glycosylation sites to further understand the effects of N-glycosylation. All four predicted N-glycosylation sites were glycosylated in 9His-hcLAAO4 expressed in P. pastoris. Enzymatic activity was similar for fully glycosylated 9His-hcLAAO4 and variants without one or all N-glycosylation sites after acid activation of all samples. However, activity without acid treatment was low in a variant without N-glycans. This was caused by the absence of a hypermannosylated N-glycan on asparagine residue N54. The lack of one or all of the other N-glycans was without effect. Our results demonstrate that adoption of a more active conformation requires a specific N-glycosylation during biosynthesis.

Expression of L-amino acid oxidase of Trichoderma harzianum in tobacco confers resistance to Sclerotinia sclerotiorum and Botrytis cinerea

L-amino acid oxidase (ThLAAO) secreted by Trichoderma harzianum ETS323 is a flavoenzyme with antimicrobial characteristics. In this study, we transformed the ThLAAO gene into tobacco to elucidate whether ThLAAO can activate defense mechanisms and confer resistance against phytopathogens. Transgenic tobacco overexpressing ThLAAO showed enhanced resistance against Sclerotinia sclerotiorum and Botrytis cinerea and activated the expression of defense-related genes and the genes involved in salicylic acid, jasmonic acid, and ethylene biosynthesis accompanied by substantial accumulation of H2O2 in chloroplasts, cytosol around chloroplasts, and cell membranes of transgenic tobacco. Scavenge of H2O2 with ascorbic acid abolished disease resistance against B. cinerea infection and decreased the expression of defense-related genes. ThLAAO-FITC application on tobacco protoplast or overexpression of ThLAAO-GFP in tobacco revealed the localization of ThLAAO in chloroplasts. Chlorophyll a/b binding protein (CAB) was isolated through ThLAAO-ConA affinity chromatography. The pull down assay results confirmed ThLAAO-CAB binding. Application of ThLAAO-Cy5.5 on cabbage roots promptly translocated to the leaves. Treatment of ThLAAO on cabbage roots induces systemic resistance against B. cinerea. Overall, these results demonstrate that ThLAAO may target chloroplast and activate defense mechanisms via H2O2 signaling to confer resistance against S. sclerotiorum and B. cinerea.

Expression, characterization, and site-specific covalent immobilization of an L-amino acid oxidase from the fungus Hebeloma cylindrosporum

L-Amino acid oxidases (LAAOs) are flavoproteins, which use oxygen to deaminate L-amino acids and produce the corresponding α-keto acids, ammonia, and hydrogen peroxide. Here we describe the heterologous expression of LAAO4 from the fungus Hebeloma cylindrosporum without signal sequence as fusion protein with a 6His tag in Escherichia coli and its purification. 6His-hcLAAO4 could be activated by exposure to acidic pH, the detergent sodium dodecyl sulfate, or freezing. The enzyme converted 14 proteinogenic L-amino acids with L-glutamine, L-leucine, L-methionine, L-phenylalanine, L-tyrosine, and L-lysine being the best substrates. Methyl esters of these L-amino acids were also accepted. Even ethyl esters were converted but with lower activity. K_m values were below 1 mM and v_max values between 19 and 39 U mg^-1 for the best substrates with the acid-activated enzyme. The information for an N-terminal aldehyde tag was added to the coding sequence. Co-expressed formylglycine-generating enzyme was used to convert a cysteine residue in the aldehyde tag to a C^α-formylglycine residue. The aldehyde tag did not change the properties of the enzyme. Purified Ald-6His-hcLAAO4 was covalently bound to a hexylamine resin via the C^α-formylglycine residue. The immobilized enzyme could be reused repeatedly to generate phenylpyruvate from L-phenylalanine with a total turnover number of 17,600 and was stable for over 40 days at 25 °C.

Recombinant expression and characterization of a L-amino acid oxidase from the fungus Rhizoctonia solani

L-Amino acid oxidases (L-AAOs) catalyze the oxidative deamination of L-amino acids to the corresponding α-keto acids, ammonia, and hydrogen peroxide. L-AAOs are homodimeric enzymes with FAD as a non-covalently bound cofactor. They are of potential interest for biotechnological applications. However, heterologous expression has not succeeded in producing large quantities of active recombinant L-AAOs with a broad substrate spectrum so far. Here, we report the heterologous expression of an active L-AAO from the fungus Rhizoctonia solani in Escherichia coli as a fusion protein with maltose-binding protein (MBP) as a solubility tag. After purification, it was possible to remove the MBP-tag proteolytically without influencing the enzyme activity. MBP-rsLAAO1 and 9His-rsLAAO1 converted basic and large hydrophobic L-amino acids as well as methyl esters of these L-amino acids. The progress of the conversion of L-phenylalanine and L-leucine into the corresponding α-keto acids was determined by HPLC and ¹H-NMR analysis of reaction mixtures, respectively. Enzymatic activity was stimulated 50-100-fold by SDS treatment. K _m values ranging from 0.9-10 mM and v _max values from 3 to 10 U mg^-1 were determined after SDS activation of 9His-rsLAAO1 for the best substrates. The enzyme displayed a broad pH optimum between pH 7.0 and 9.5. In summary, a successful overexpression of recombinant L-AAO in E. coli was established that results in a promising enzymatic activity and a broad substrate spectrum for biotechnological application.

Heterologous expression of l-lysine α-oxidase from Scomber japonicus in Pichia pastoris and functional characterization of the recombinant enzyme

Fish have a complex self-defense mechanism against microbial invasion. Recently, l-lysine α-oxidases have been identified from a number of fish species as a novel type of antibacterial protein in the integument. These enzymes exhibit strict substrate specificity for l-lysine, but the underlying mechanisms and details of their catalytic properties remain unknown. In this study, a synthetic gene coding for Scomber japonicus l-lysine α-oxidase, originally termed AIP (for apoptosis-inducing protein), was expressed in Pichia pastoris, and the recombinant enzyme (rAIP) was purified and characterized. rAIP exhibited essentially the same substrate specificity as the native enzyme, catalyzing the oxidative deamination of l-lysine as an exclusive substrate. rAIP was N-glycosylated and remained active over a wide range of pH, with an optimal pH of 7.5. The enzyme was stable in the pH range from 4.5 to 10.0 and was thermally stable up to 60°C. A molecular modelling of rAIP and a comparative structure/sequence analysis with homologous enzymes indicate that Asp(220) and Asp(320) are the substrate-binding residues that are likely to confer exclusive substrate specificity for l-lysine on the fish enzymes.

L-Amino acid oxidases from microbial sources: types, properties, functions, and applications

L-Amino acid oxidases (LAAOs), which catalyze the stereospecific oxidative deamination of L-amino acids to α-keto acids and ammonia, are flavin adenine dinucleotide-containing homodimeric proteins. L-Amino acid oxidases are widely distributed in diverse organisms and have a range of properties. Because expressing LAAOs as recombinant proteins in heterologous hosts is difficult, their biotechnological applications have not been thoroughly advanced. LAAOs are thought to contribute to amino acid catabolism, enhance iron acquisition, display antimicrobial activity, and catalyze keto acid production, among other roles. Here, we review the types, properties, structures, biological functions, heterologous expression, and applications of LAAOs obtained from microbial sources. We expect this review to increase interest in LAAO studies.