Isolation and Characterization of the Tyrosinase Gene from Neurospora crassa

Silencing of Amylomyces rouxii aspartic II protease by siRNA to increase tyrosinase activity

Amylomyces rouxii is a zygomycete that produces extracellular protease and tyrosinase. The tyrosinase activity is negatively regulated by the proteases and, which attempts to purify the tyrosinase (tyr) enzyme that has been hampered by the presence of a protease that co-purified with it. In this work we identified genes encoding aspartic protease II (aspII) and VI of A. rouxii. Using an RNAi strategy based on the generation of a siRNA by transcription from two opposite-orientated promoters, the expression of these two proteases was silenced, showing that this molecular tool is suitable for gene silencing in Amylomyces. The transformant strains showed a significant attenuation of the transcripts (determined by RT-qPCR), with respective inhibition of the protease activity. In the case of aspII, inhibition was in the range of 43-90 % in different transformants, which correlated well with up to a five-fold increase in tyr activity with respect to the wild type and control strains. In contrast, silencing of aspVI caused a 43-65 % decrease in protease activity but had no significant effect on the tyr activity. The results show that aspII has a negative effect on tyr activity, and that the silencing of this protease is important to obtain strains with high levels of tyr activity.

Inhibitory Activities of Samples on Tyrosinases Were Affected by Enzyme Species and Sample Addition Methods

The inhibition of tyrosinase (TYR) activity is an effective measure to inhibit melanin synthesis. At present, there are many methods with discrepant details that study the TYR inhibitory activity of samples. Under the same experimental conditions, this paper systematically studies whether enzyme species and sample addition methods are the key factors that determine the TYR inhibitory activity of samples. TYRs extracted from B16F10 cells, apple and mushroom, called BTYR, ATYR and MTYR, respectively, were selected to implement this study. Results showed that TYR inhibitory activities of samples were obviously affected by the above two factors. It was necessary to select the appropriate enzyme according to the problems to be explained. It was speculated that indirectly inhibitory activity reflected the comprehensive effects of samples on TYR catalytic activity and intracellular TYR synthesis pathway, while directly inhibitory activity reflected the effects of samples on TYR catalytic activity. Additionally, kojic acid could be used as a positive control for both B16F10 cells and MTYR models. The TYR inhibitory activity of β-arbutin was complicated and fickle, while that of epigallocatechin gallate (EGCG) was universal and stable, which is to say, EGCG always inhibited TYR activity in a dose-dependent manner. In conclusion, the TYR inhibitory activities of samples were affected by enzyme species and sample addition methods. Compared with the unstable β-arbutin, EGCG was more valuable for clinical research.

Biochemical Properties of Tyrosinase from Aspergillus terreus and Penicillium copticola; Undecanoic Acid from Aspergillus flavus, an Endophyte of Moringa oleifera, Is a Novel Potent Tyrosinase Inhibitor

Tyrosinase is a copper-containing monooxygenase catalyzing the O-hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine then to dopaquinone that is profoundly involved in melanin synthesis in eukaryotes. Overactivation of tyrosinase is correlated with hyperpigmentation that is metabolically correlated with severe pathological disorders, so, inhibition of this enzyme is the most effective approach in controlling the overproduction of melanin and its hazardous effects. Thus, searching for a powerful, selective inhibitor of human tyrosinase to limit the hyper-synthesis of melanin is a challenge. Unlike the difficulty of overexpression of human tyrosinase, using fungal tyrosinase as a model enzyme to the human one to evaluate the mechanistics of enzyme inhibition in response to various compounds is the most feasible strategy. Thus, the purification of highly catalytic-efficient fungal tyrosinase, exploring a novel inhibitor, and evaluating the mechanistics of enzyme inhibition are the main objectives of this work. Aspergillus terreus and Penicillium copticola were reported as the most potential tyrosinase producers. The biochemical properties suggest that this enzyme displays a higher structural and catalytic proximity to human tyrosinase. Upon nutritional bioprocessing by Plackett–Burman design, the yield of tyrosinase was increased by about 7.5-folds, compared to the control. The purified tyrosinase was strongly inhibited by kojic acid and A. flavus DCM extracts with IC₅₀ values of 15.1 and 12.6 µg/mL, respectively. From the spectroscopic analysis, the main anti-tyrosinase compounds of A. flavus extract was resolved, and verified as undecanoic acid. Further studies are ongoing to unravel the in vivo effect and cytotoxicity of this compound in fungi and human, that could be a novel drug to various diseases associated with hyperpigmentation by melanin.

Selective inhibition of Rhizopus eumelanin biosynthesis by novel natural product scaffold-based designs caused significant inhibition of fungal pathogenesis

Melanin is a dark color pigment biosynthesized naturally in most living organisms. Fungal melanin is a major putative virulence factor of Mucorales fungi that allows intracellular persistence by inducing phagosome maturation arrest. Recently, it has been shown that the black pigments of Rhizopus delemar is of eumelanin type, that requires the involvement of tyrosinase (a copper-dependent enzyme) in its biosynthesis. Herein, we have developed a series of compounds (UOSC-1-14) to selectively target Rhizopus melanin and explored this mechanism therapeutically. The compounds were designed based on the scaffold of the natural product, cuminaldehyde, identified from plant sources and has been shown to develop non-selective inhibition of melanin production. While all synthesized compounds showed significant inhibition of Rhizopus melanin production and limited toxicity to mammalian cells, only four compounds (UOSC-1, 2, 13, and 14) were selected as promising candidates based on their selective inhibition to fungal melanin. The activity of compound UOSC-2 was comparable to the positive control kojic acid. The selected candidates showed significant inhibition of Rhizopus melanin but not human melanin by targeting the fungal tyrosinase, and with an IC50 that are 9 times lower than the reference standard, kojic acid. Furthermore, the produced white spores were phagocytized easily and cleared faster from the lungs of infected immunocompetent mice and from the human macrophages when compared with wild-type spores. Collectively, the results suggested that the newly designed derivatives, particularly UOSC-2 can serve as promising candidate to overcome persistence mechanisms of fungal melanin production and hence make them accessible to host defenses.

The Genetics and Biochemistry of Cell Wall Structure and Synthesis in Neurospora crassa, a Model Filamentous Fungus

This review discusses the wealth of information available for the N. crassa cell wall. The basic organization and structure of the cell wall is presented and how the wall changes during the N. crassa life cycle is discussed. Over forty cell wall glycoproteins have been identified by proteomic analyses. Genetic and biochemical studies have identified many of the key enzymes needed for cell wall biogenesis, and the roles these enzymes play in cell wall biogenesis are discussed. The review includes a discussion of how the major cell wall components (chitin, β-1,3-glucan, mixed β-1,3-/ β-1,4- glucans, glycoproteins, and melanin) are synthesized and incorporated into the cell wall. We present a four-step model for how cell wall glycoproteins are covalently incorporated into the cell wall. In N. crassa, the covalent incorporation of cell wall glycoproteins into the wall occurs through a glycosidic linkage between lichenin (a mixed β-1,3-/β-1,4- glucan) and a "processed" galactomannan that has been attached to the glycoprotein N-linked oligosaccharides. The first step is the addition of the galactomannan to the N-linked oligosaccharide. Mutants affected in galactomannan formation are unable to incorporate glycoproteins into their cell walls. The second step is carried out by the enzymes from the GH76 family of α-1,6-mannanases, which cleave the galactomannan to generate a processed galactomannan. The model suggests that the third and fourth steps are carried out by members of the GH72 family of glucanosyltransferases. In the third step the glucanosyltransferases cleave lichenin and generate enzyme/substrate intermediates in which the lichenin is covalently attached to the active site of the glucanosyltransferases. In the final step, the glucanosyltransferases attach the lichenin onto the processed galactomannans, which creates new glycosidic bonds and effectively incorporates the glycoproteins into the cross-linked cell wall glucan/chitin matrix.

Involvement of a G Protein Regulatory Circuit in Alternative Oxidase Production in Neurospora crassa

The Neurospora crassa nuclear aod-1 gene encodes an alternative oxidase that functions in mitochondria. The enzyme provides a branch from the standard electron transport chain by transferring electrons directly from ubiquinol to oxygen. In standard laboratory strains, aod-1 is transcribed at very low levels under normal growth conditions. However, if the standard electron transport chain is disrupted, aod-1 mRNA expression is induced and the AOD1 protein is produced. We previously identified a strain of N. crassa, that produces high levels of aod-1 transcript under non-inducing conditions. Here we have crossed this strain to a standard lab strain and determined the genomic sequences of the parents and several progeny. Analysis of the sequence data and the levels of aod-1 mRNA in uninduced cultures revealed that a frameshift mutation in the flbA gene results in the high uninduced expression of aod-1 The flbA gene encodes a regulator of G protein signaling that decreases the activity of the Gα subunit of heterotrimeric G proteins. Our data suggest that strains with a functional flbA gene prevent uninduced expression of aod-1 by inactivating a G protein signaling pathway, and that this pathway is activated in cells grown under conditions that induce aod-1 Induced cells with a deletion of the gene encoding the Gα protein still have a partial increase in aod-1 mRNA levels, suggesting a second pathway for inducing transcription of the gene in N. crassa We also present evidence that a translational control mechanism prevents production of AOD1 protein in uninduced cultures.

Recombinant Tyrosinase from Polyporus arcularius: Overproduction in Escherichia coli, Characterization, and Use in a Study of Aurones as Tyrosinase Effectors

Tyrosinases act in the development of organoleptic properties of tea, raisins, etc., but also cause unwanted browning of fruits, vegetables, and mushrooms. The tyrosinase from Agaricus bisporus has been used as a model to study tyrosinase inhibitors, which are also indispensable in the treatment of skin pigmentation disorders. However, this model has disadvantages such as side enzyme activities and the presence of multiple isoenzymes. Therefore, we aimed to introduce a new tyrosinase model. The pro-tyrosinase from Polyporus arcularius was overproduced in Escherichia coli. Trypsin digestion led to a cleavage after R388 and hence enzyme activation. The tyrosinase was a homodimer and transformed L-DOPA and tert-butylcatechol preferentially. Various aurons were examined as effectors of this enzyme. 2'- and 3'-hydroxyaurones acted as its activators and 2',4'-dihydroxyaurone as an inhibitor, whereas 4'-hydroxyaurones were its substrates. The enzyme is a promising model for tyrosinase effector studies, being a single isoenzyme and void of side enzyme activities.

The pro-enzyme C-terminal processing domain of Pholiota nameko tyrosinase is responsible for folding of the N-terminal catalytic domain

Pholiota nameko (Pholiota microspore) tyrosinase is expressed as a latent 67-kDa pro-tyrosinase, comprising a 42-kDa N-terminal catalytic domain with a binuclear copper centre and a 25-kDa C-terminal domain and is activated by proteolytic digestion of the C-terminal domain. To investigate the role of the C-terminal processing domain of pro-tyrosinase, we constructed a recombinant tyrosinase lacking the C-terminal domain and four recombinant pro-tyrosinase mutants (F515G, H539N, L540G and Y543G) carrying substituted amino acid residues on the C-terminal domain. The recombinant tyrosinase lacking the C-terminal domain had no catalytic activity; whereas the mutant L540G was copper depleted, the other mutants had copper contents similar to that of the wild-type pro-tyrosinase. Proteolytic digestion activated the mutants H539N and Y543G following release of the C-terminal domain, and the resulting tyrosinases had higher K m values for t-butyl catechol than the wild-type pro-tyrosinase. The mutants F515G and L540G were degraded by proteolytic digestion and yielded smaller proteins with no activity. These data suggest that the C-terminal processing domain of P. nameko pro-tyrosinase is essential for correct folding of the N-terminal catalytic domain and acts as an intramolecular chaperone during assembly of the active-site conformation.

Site-directed mutagenesis around the CuA site of a polyphenol oxidase from Coreopsis grandiflora (cgAUS1)

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Site-directed mutations of AUS1 around the CuA site were generated and verified.

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All mutations led to loss of diphenolase activity with butein as substrate.

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Exchange of histidines in the CuA resulted in enzymes containing only one Cu.

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F273 mutation to alanine did not increase the monophenolase activity.

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C97 mutation eliminated the diphenolase activity, but 2 Cu atoms were incorporated.

Aurone synthase from Coreopsis grandiflora (cgAUS1), catalyzing conversion of butein to sulfuretin in a type-3 copper center, is a rare example of a polyphenol oxidase involved in anabolism. Site-directed mutagenesis around the CuA site of AUS1 was performed, and recombinant enzymes were analyzed by mass spectrometry. Replacement of the coordinating CuA histidines with alanine resulted in the presence of a single copper and loss of diphenolase activity. The thioether bridge-building cysteine and a phenylalanine over the CuA site, exchanged to alanine, have no influence on copper content but appear to play an important role in substrate binding.

pH-induced structural changes of tyrosinase from Agaricus bisporus using fluorescence and in silico methods

BACKGROUND

Tyrosinases are involved in enzymatic browning reactions in damaged fruits during post-harvest handling and processing. The overall structure of tyrosinase from Agaricus bisporus mushrooms at different pH values was monitored using fluorescence spectroscopy and molecular dynamics simulations.

RESULTS

When the pH value was increased from 6.0 to 9.0, the protein passed through several structural intermediates, including the tetramer, trimer and dimer stages. Changes in the secondary and tertiary structure of tyrosinase at neutral pH were outlined after running molecular dynamics simulations. A detailed check at the single-molecule level by means of molecular modeling tools suggested that the most important contribution to the fluorescence intensity is given by the H subunits with seven Trp and nine Tyr residues exposed to the solvent, whereas the lectin-like folded L subunits have only six Trp and three Tyr residues, of which only Trp(15) , Trp(59) and Trp(93) are partially exposed to the solvent.

CONCLUSIONS

The results indicated that the enzyme was sensitive to pH. The experimental results revealed the unfolding of the native tetrameric enzyme in acidic pH range, causing exposure of the hydrophobic residues.

Crystal structures of copper-depleted and copper-bound fungal pro-tyrosinase: insights into endogenous cysteine-dependent copper incorporation

Tyrosinase, a dinuclear copper monooxygenase/oxidase, plays a crucial role in the melanin pigment biosynthesis. The structure and functions of tyrosinase have so far been studied extensively, but the post-translational maturation process from the pro-form to the active form has been less explored. In this study, we provide the crystal structures of Aspergillus oryzae full-length pro-tyrosinase in the holo- and the apo-forms at 1.39 and 2.05 Å resolution, respectively, revealing that Phe(513) on the C-terminal domain is accommodated in the substrate-binding site as a substrate analog to protect the dicopper active site from substrate access (proteolytic cleavage of the C-terminal domain or deformation of the C-terminal domain by acid treatment transforms the pro-tyrosinase to the active enzyme (Fujieda, N., Murata, M., Yabuta, S., Ikeda, T., Shimokawa, C., Nakamura, Y., Hata, Y., and Itoh, S. (2012) ChemBioChem. 13, 193-201 and Fujieda, N., Murata, M., Yabuta, S., Ikeda, T., Shimokawa, C., Nakamura, Y., Hata, Yl, and Itoh, S. (2013) J. Biol. Inorg. Chem. 18, 19-26). Detailed crystallographic analysis and structure-based mutational studies have shown that the copper incorporation into the active site is governed by three cysteines as follows: Cys(92), which is covalently bound to His(94) via an unusual thioether linkage in the holo-form, and Cys(522) and Cys(525) of the CXXC motif located on the C-terminal domain. Molecular mechanisms of the maturation processes of fungal tyrosinase involving the accommodation of the dinuclear copper unit, the post-translational His-Cys thioether cross-linkage formation, and the proteolytic C-terminal cleavage to produce the active tyrosinase have been discussed on the basis of the detailed structural information.

Experimental and bioinformatic investigation of the proteolytic degradation of the C-terminal domain of a fungal tyrosinase

Proteolytic processing is a key step in the production of polyphenol oxidases such as tyrosinases, converting the inactive proenzyme to an active form. In general, the fungal tyrosinase gene codes for a ~60 kDa protein that is, however, isolated as an active enzyme of ~40 kDa, lacking the C-terminal domain. Using the secreted tyrosinase 2 from Trichoderma reesei as a model protein, we performed a mutagenesis study of the residues in proximity of the experimentally determined cleavage site which are possibly involved in the proteolytic process. However, the mutant forms of tyrosinase 2 were not secreted in a full-length form retaining the C-terminal domain, but they were processed to give a ~45 kDa active form. Aiming at explaining this phenomenon, we analysed in silico the properties of the C-terminal domain of tyrosinase 2, of 23 previously retrieved homologous tyrosinase sequences from fungi (C. Gasparetti, G. Faccio, M. Arvas, J. Buchert, M. Saloheimo, K. Kruus, Appl. Microbiol. Biotechnol. 86 (2010) 213-226) and of nine well-characterised polyphenol oxidases. Based on the results of our study, we exclude the key role of specific amino acids at the cleavage site in the proteolytic process and report an overall higher sensitivity to proteolysis of the linker region and of the whole C-terminal domain of fungal tyrosinases.

Multifunctions of MelB, a fungal tyrosinase from Aspergillus oryzae

The pro form of melB tyrosinase from the melB gene of Aspergillus oryzae was over-produced from E. coli and formed a homodimer that exhibited the spectral features of met-tyrosinase. In the presence of NH(2)OH (reductant), the proenzyme bound dioxygen to give a stable (μ-η(2):η(2) -peroxo)dicopper(II) species (oxy form), thus indicating that the pro form tyrosinase can function as an oxygen carrier or storage protein like hemocyanin. The pro form tyrosinase itself showed no catalytic activity toward external substrates, but proteolytic digestion with trypsin activated it to induce tyrosinase activity. Mass spectroscopy analyses, mutagenesis experiments, and colorimetry assays have demonstrated that the tryptic digestion induced cleavage of the C-terminal domain (Glu458-Ala616), although the dimeric structure of the enzyme was retained. The structural changes induced by proteolytic digestion might open the entrance to the enzyme active site for substrate incorporation.

Tyrosinase expression during black truffle development: from free living mycelium to ripe fruit body

The present work studies the expression of tyrosinase (monophenol:diphenol oxygen oxidoreductase, EC 1.14.18.1) during the development of the black truffle Tuber melanosporum Vittad., an ectomycorrhizal fungus of great biological and economic interest. As widely reported in the literature, melanins and the enzymes that synthesize them, are of paramount importance in fungal development and sexual differentiation. Tyrosinase and laccase are the enzymes that produce melanins from monophenols and diphenols. We have detected tyrosinase expression from the stage of free living mycelium, through the mychorrizal stage and the six fruit body developmental stages by measuring the levels of tyrosinase mRNA by quantitative PCR (q-PCR), spectrophotometry, histochemistry, immunohistochemistry and electrophoresis. Tyrosinase is always expressed, from the free living mycelium to the ripe fruit body developmental stages, when it is very low. The switching off of the tyrosinase gene during T. melanosporum development when the fruit body is ripe and no more cell walls are to be built is discussed in relation of thioflavour production. Specific primers, prepared from the cloned T. melanosporum tyrosinase cDNA were used for the q-PCR and the deduced aminoacid sequences of the CuA and CuB binding sites were compared to those of various ascomycetes and basidiomycetes.

Cloning, characterization and expression of two new polyphenol oxidase cDNAs from Agaricus bisporus

Two new polyphenol oxidase (PPO) cDNAs (PPO3 and PPO4 cDNAs, accession numbers GQ354801 and GQ354802, respectively) were obtained by RACE-PCR from Agaricus bisporus. PPO3 cDNA was 1844 bp in length with an open reading frame of 1731 bp, while PPO4 cDNA was 2042 bp with an open reading frame of 1836 bp. PPO3 and PPO4 cDNAs, with 52% identity at the nucleic acid level, encoded a 576-amino acid protein of 66.3 kDa and 611-amino acid protein of 68.3 kDa, respectively. Mature forms of PPO3 and PPO4 were characterized after removing the specific C-terminal region and expressed in Escherichia coli BL21 (DE3) RIPL using pGEX-4T-1 vector. The expressed proteins were probed by the anti-A. bisporus PPO antibody but without PPO activity. This indicated that the recombinant mature PPO3 and mature PPO4 could not form an active center in prokaryotic expression system.

The tyrosinase-encoding gene of Lentinula edodes, Letyr, is abundantly expressed in the gills of the fruit-body during post-harvest preservation

The gill browning of Lentinula edodes fruit-bodies during preservation is thought to be due to melanin biosynthesis catalyzed by tyrosinase. We isolated a genomic DNA sequence and cDNA encoding a putative tyrosinase from the white rot basidiomycete Lentinula edodes (shiitake mushroom). The gene, named Letyr, consists of a 1,854-bp open reading frame interrupted by eight introns, and encodes a putative protein of 618 amino acid residues with an estimated molecular mass of 68 kDa. Amino acid residues known to be involved in copper-binding domains were conserved in the deduced amino acid residues of LeTyr. Transcriptional and translational expression of Letyr in the gills of the fruit-body increased during preservation after harvest. This correlation between Letyr expression and fruit-body preservation suggests that tyrosinase gene expression contributes to gill browning.

Mitogen-activated protein kinase cascade required for regulation of development and secondary metabolism in Neurospora crassa

Mitogen-activated protein kinase (MAPK) signaling cascades are composed of MAPK kinase kinases (MAPKKKs), MAPK kinases (MAPKKs), and MAPKs. In this study, we characterize components of a MAPK cascade in Neurospora crassa (mik-1, MAPKKK; mek-1, MAPKK; and mak-1, MAPK) homologous to that controlling cell wall integrity in Saccharomyces cerevisiae. Growth of basal hyphae is significantly reduced in mik-1, mek-1, and mak-1 deletion mutants on solid medium. All three mutants formed short aerial hyphae and the formation of asexual macroconidia was reduced in Deltamik-1 mutants and almost abolished in Deltamek-1 and Deltamak-1 strains. In contrast, the normally rare asexual spores, arthroconidia, were abundant in cultures of the three mutants. Deltamik-1, Deltamek-1, and Deltamak-1 mutants were unable to form protoperithecia or perithecia when used as females in a sexual cross. The MAK-1 MAPK was not phosphorylated in Deltamik-1 and Deltamek-1 mutants, consistent with the involvement of MIK-1, MEK-1, and MAK-1 in the same signaling cascade. Interestingly, we observed increased levels of mRNA and protein for tyrosinase in the mutants under nitrogen starvation, a condition favoring sexual differentiation. Tyrosinase is an enzyme that catalyzes production of the secondary metabolite l-DOPA melanin. These results implicate the MAK-1 pathway in regulation of development and secondary metabolism in filamentous fungi.

Expression of the Trichoderma reesei tyrosinase 2 in Pichia pastoris: isotopic labeling and physicochemical characterization

Trichoderma reesei tyrosinase TYR2 has been demonstrated to be able to oxidize various phenolic compounds and also peptide and protein bound tyrosine, and thus is of great interest for different biotechnological applications. In order to understand the reaction mechanism of the enzyme it would be essential to solve its three dimensional structure. Pichia pastoris is a suitable expression system for the production of recombinant enzymes for NMR studies and therefore we expressed TYR2 in this host. As a result of extensive optimization, the production yield of active histidine tagged tyrosinase purified from P. pastoris shake flask cultures was increased from 2.5 to 24 mg/L. Correct copper concentration in the growth medium was critical for the expression of this copper containing enzyme. Our analysis showed that TYR2 expressed in P. pastoris is post-translationally modified; the C-terminal domain of 153 amino acids of the protein is proteolytically cleaved off from the catalytic domain and the only potential N-glycosylation site is glycosylated. The activities of TYR2 expressed in P. pastoris and T. reesei on diphenolic L-dopa and monophenolic L-tyrosine were rather similar. The TYR2 expressed in P. pastoris showed the same physicochemical properties in CD and unfolding assays as the native TYR2 enzyme. Uniform isotopic (15)N-labeling of TYR2 was carried out with (15)NH(4)SO(4) in minimal medium to assess the suitability of the expression system for investigation by NMR spectroscopy.

Naturally occurring tyrosinase inhibitors: mechanism and applications in skin health, cosmetics and agriculture industries

Tyrosinase is a copper-containing enzyme, which is widely distributed in microorganisms, animals and plants and is a key enzyme in melanin biosynthesis, involved in determining the color of mammalian skin and hair. In addition, unfavorable enzymatic browning of plant-derived foods by tyrosinase causes a decrease in nutritional quality and economic loss of food products. The inadequacy of current conventional methods to prevent tyrosinase action encourages researchers to seek new potent tyrosinase inhibitors for food and cosmetics. This article presents a study on the importance of tyrosinase, biochemical characteristics, type of inhibitions, activators from various natural sources with its clinical and industrial importance in recent prospects is discussed in this paper.

Production and characterization of a secreted, C-terminally processed tyrosinase from the filamentous fungus Trichoderma reesei

A homology search of the genome database of the filamentous fungus Trichoderma reesei identified a new T. reesei tyrosinase gene tyr2, encoding a protein with a putative signal sequence. The gene was overexpressed in the native host under the strong cbh1 promoter, and the tyrosinase enzyme was secreted into the culture supernatant. This is the first report on a secreted fungal tyrosinase. Expression of TYR2 in T. reesei resulted in good yields, corresponding to approximately 0.3 and 1 g.L(-1) tyrosinase in shake flask cultures and laboratory-scale batch fermentation, respectively. T. reesei TYR2 was purified with a three-step purification procedure, consisting of desalting by gel filtration, cation exchange chromatography and size exclusion chromatography. The purified TYR2 protein had a significantly lower molecular mass (43.2 kDa) than that calculated from the putative amino acid sequence (61.151 kDa). According to N-terminal and C-terminal structural analyses by fragmentation, chromatography, MS and peptide sequencing, the mature protein is processed from the C-terminus by a cleavage of a peptide fragment of about 20 kDa. The T. reesei TYR2 polypeptide chain was found to be glycosylated at its only potential N-glycosylation site, with a glycan consisting of two N-acetylglucosamines and five mannoses. Also, low amounts of shorter glycan forms were detected at this site. T. reesei TYR2 showed the highest activity and stability within a neutral and alkaline pH range, having an optimum at pH 9. T. reesei tyrosinase retained its activity well at 30 degrees C, whereas at higher temperatures the enzyme started to lose its activity relatively quickly. T. reesei TYR2 was active on both l-tyrosine and l-dopa, and it showed broad substrate specificity.

Fungal tyrosinases: new prospects in molecular characteristics, bioengineering and biotechnological applications

Tyrosinases are type-3 copper proteins involved in the initial step of melanin synthesis. These enzymes catalyse both the o-hydroxylation of monophenols and the subsequent oxidation of the resulting o-diphenols into reactive o-quinones, which evolve spontaneously to produce intermediates, which associate in dark brown pigments. In fungi, tyrosinases are generally associated with the formation and stability of spores, in defence and virulence mechanisms, and in browning and pigmentation. First characterized from the edible mushroom Agaricus bisporus because of undesirable enzymatic browning problems during postharvest storage, tyrosinases were found, more recently, in several other fungi with relevant insights into molecular and genetic characteristics and into reaction mechanisms, highlighting their very promising properties for biotechnological applications. The limit of these applications remains in the fact that native fungal tyrosinases are generally intracellular and produced in low quantity. This review compiles the recent data on biochemical and molecular properties of fungal tyrosinases, underlining their importance in the biotechnological use of these enzymes. Next, their most promising applications in food, pharmaceutical and environmental fields are presented and the bioengineering approaches used for the development of tyrosinase-overproducing fungal strains are discussed.

A novel putative tyrosinase involved in periostracum formation from the pearl oyster (Pinctada fucata)

Tyrosinase (monophenol, L-DOPA: oxygen oxidoreductase, EC 1.14.18.1), a kind of copper-containing phenoloxidase, arouses great interests of scientists for its important role in periostracum formation. A cDNA clone encoding a putative tyrosinase, termed OT47 because of its estimated molecular mass of 47kDa, was isolated from the pearl oyster, Pinctada fucata. This novel tyrosinase shares similarity with the cephalopod tyrosinases and other type 3 copper proteins within two conserved copper-binding sites. RT-PCR analysis showed that OT47 mRNA was expressed only in the mantle edge. Further in situ hybridization analysis and tyrosinase activity staining revealed that OT47 was expressed at the outer epithelial cells of the middle fold, different from early histological results in Mercenaria mercenaria, suggesting a different model of periostracum secretion in P. fucata. Taken together, these results suggest that OT47 is most likely involved in periostracum formation. The identification and characterization of oyster tyrosinase also help to further understand the structural and functional properties of molluscan tyrosinase.

Cloning of a novel tyrosinase-encoding gene (melB) from Aspergillus oryzae and its overexpression in solid-state culture (Rice Koji)

We have cloned a novel tyrosinase-encoding gene (melB) specifically expressed in solid-state culture of Aspergillus oryzae. A tyrosinase-encoding gene (melO) from A. oryzae was already cloned and the protein structures of its catalytic and copper binding domains were investigated. However, our recent results revealed that the melO gene was highly expressed in submerged culture but not in solid-state culture. Because tyrosinase activity was also detected in solid-state culture, we assumed that another tyrosinase gene other than melO is expressed in solid-state culture. Another tyrosinase gene was screened using the expressed sequence tag (EST) library. One redundant cDNA clone homologous with the tyrosinase gene was found in the collection of wheat bran culture. Northern blot analysis revealed that the gene corresponding to the cDNA clone was specifically expressed in solid-state culture (koji making), but not in submerged culture. Molecular cloning showed that the gene carried six exons interrupted by five introns and had an open reading frame encoding 616 amino acid residues. This gene was designated as melB. The deduced amino acid sequence of the gene had weak homology (24%-33%) with MelO and other fungal tyrosinases but the sequences of the copper binding domains were highly conserved. When the melB gene was expressed under the control of the glaB promoter in solid-state culture, tyrosinase activity was markedly enhanced and the culture mass was browned with the melanization by MelB tyrosinase. These results indicated that the melB gene encodes a novel tyrosinase associated with melanization in solid-state culture.

Cloning and characterization of a tyrosinase gene from the white-rot fungus Pycnoporus sanguineus, and overproduction of the recombinant protein in Aspergillus niger

A new tyrosinase-encoding gene (2,204 bp) and the corresponding cDNA (1,857 nucleotides) from the white-rot fungus Pycnoporus sanguineus BRFM49 were cloned. This gene consisted of seven exons and six introns and encoded a predicted protein of 68 kDa, exceeding the mature tyrosinase by 23 kDa. P. sanguineus tyrosinase cDNA was over-expressed in Aspergillus niger, a particularly suitable fungus for heterologous expression of proteins of biotechnological interest, under the control of the glyceraldehyde-3-phosphate-dehydrogenase promoter as strong and constitutive promoter. The glucoamylase preprosequence of A. niger was used to target the secretion. This construction enabled the production of recombinant tyrosinase in the extracellular medium of A. niger. The identity of the purified recombinant protein was confirmed by N-terminal amino acid sequencing. The maturation process was shown to be effective in A. niger, and the recombinant enzyme was fully active, with a molecular mass of 45 kDa. The best transformant obtained, A. niger D15#26-e, produced extracellular tyrosinase activities of 534 and 1,668 U l(-1) for monophenolase and diphenolase, respectively, which corresponded to a protein yield of ca. 20 mg l(-1).

Genetic and in silico comparative mapping of the polyphenol oxidase gene in bread wheat (Triticum aestivum L.)

Polyphenol oxidases (PPOs) are involved in the time-dependent darkening and discolouration of Asian noodles and other wheat end products. In this study, a doubled haploid (DH) population derived from Chara (moderately high PPO activity)/WW2449 (low PPO activity) was screened for PPO activity based on L-DOPA and L-tyrosine assays using whole seeds. Both these assays were significantly genetically correlated (r = 0.91) in measuring the PPO activity in this DH population. Quantitative trait loci (QTLs) analysis utilising a skeleton map enabled us to identify a major QTL controlling PPO activity based on L-DOPA and L-tyrosine on the long arm of chromosome 2A. The simple sequence repeat (SSR) marker GWM294b explained over 82% of the line mean phenotypic variation from samples collected in both 2000 and 2003. Four SSR markers were validated for PPO linkage in genetically diverse backgrounds and proven to correctly predict the PPO activity in more than 92% of wheat lines. Physical mapping using deletion lines of Chinese Spring has confirmed the location of the GWM294b, GWM312 and WMC170 on chromosome 2AL, between deletion breakpoints 2AL-C to 0.85. In order to identify functional gene markers, data searches for alignments between rice BAC/PAC clones assembled on chromosome 1 and 4, chromosome 7, and (1) the wheat expressed sequence tags mapped in deletion bin (2AL-C to 0.85) and (2) the coding sequence of a previously cloned wheat PPO gene were made and found significant sequence similarities with the PPO gene or common central domain of tyrosinase. Available PPO gene sequences in the National Centre for Biotechnology Information (NCBI) database have revealed that there is a significant molecular diversity at the nucleotide and amino acid level in the wheat PPO genes.

Characterization of a new tyrosinase from Pycnoporus species with high potential for food technological applications

AIMS

Tyrosinase production by Pycnoporus cinnabarinus and Pycnoporus sanguineus was screened among 20 strains originating from various geographical areas, particularly from tropical environments. The tyrosinase from the most efficient strain was purified and characterized and tested for food additive applications.

METHODS AND RESULTS

Monophenolase and diphenolase activities of tyrosinase were measured from cell lysate from the 20 Pycnoporus strains, for 8-10 days of cultivation. The strain P. sanguineus CBS 614.73 showed the highest productivity (45.4 and 163.6 U g(-1) protein per day for monophenolase and diphenolase respectively). P. sanguineus CBS 614.73 tyrosinase was purified from concentrated cell lysate, anion-exchange, size-exclusion and hydroxyapatite chromatography, with a final yield of 2% and a purification factor of 35-38. The pure enzyme was a monomere with a molecular mass of 45 kDa and it showed four isoforms or isoenzymes with pI between 4.5-5. No N-glycosylation was found. The N-terminal amino acid sequence was IVTGPVGGQTEGAPAPNR. The enzyme was shown to be almost fully active in a pH range of 6-7, in a large temperature range (30-70 degrees C), and was stable below 60 degrees C. The main kinetic constants were determined. The tyrosinase was able to convert p-tyrosol and p-coumaric acid into hydroxytyrosol and caffeic acid, respectively, and it could also catalyse the cross-linking formation of a model protein.

CONCLUSIONS

Among the genus Pycnoporus, known for the production of laccase, the strain P. sanguineus CBS 614.73 was shown to produce one other phenoloxidase, a new monomeric tyrosinase with a specific activity of 30 and 84 U mg(-1) protein for monophenolase and diphenolase respectively.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study identified P. sanguineus CBS 614.73 as a potential producer of a tyrosinase which demonstrated effectiveness in the synthesis of antioxidant molecules and in protein cross-linking.

Genetic evidence for a regulatory pathway controlling alternative oxidase production in Neurospora crassa

When the cytochrome-mediated mitochondrial electron transport chain of Neurospora crassa is disrupted, an alternative oxidase encoded by the nuclear aod-1 gene is induced. The alternative oxidase donates electrons directly to oxygen from the ubiquininol pool and is insensitive to chemicals such as antimycin A and KCN that affect the standard electron transport chain. To facilitate isolation of mutants affecting regulation of aod-1, a reporter system containing the region upstream of the aod-1 coding sequence fused to the coding sequence of the N. crassa tyrosinase gene (T) was transformed into a strain carrying a null allele of the endogenous T gene. In the resulting reporter strain, growth in the presence of chloramphenicol, an inhibitor of mitochondrial translation whose action decreases the level of mitochondrial translation products resulting in impaired cytochrome-mediated respiration, caused induction of both alternative oxidase and tyrosinase. Conidia from the reporter strain were mutagenized, plated on medium containing chloramphenicol, and colonies that did not express tyrosinase were identified as potential regulatory mutants. After further characterization, 15 strains were found that were unable to induce both the reporter and the alternative oxidase. Complementation analysis revealed that four novel loci involved in aod-1 regulation had been isolated. The discovery that several genes are required for regulation of aod-1 suggests the existence of a complex pathway for signaling from the mitochondria to the nucleus and/or for expression of the gene.

Purification, characterization and molecular cloning of tyrosinase from the cephalopod mollusk,Illex argentinus

Tyrosinase (monophenol, L-DOPA:oxygen oxidoreductase) was isolated from the ink of the squid, Illex argentinus. Squid tyrosinase, termed ST94, was found to occur as a covalently linked homodimeric protein with a molecular mass of 140.2 kDa containing two copper atoms per a subunit. The tyrosinase activity of ST94 was enhanced by proteolysis with trypsin to form a protein, termed ST94t, with a molecular mass of 127.6 kDa. The amino acid sequence of the subunit was deduced from N-terminal amino acid sequencing and cDNA cloning, indicating that the subunit of ST94 is synthesized as a premature protein with 625 amino acid residues and an 18-residue signal sequence region is eliminated to form the mature subunit comprised of 607 amino acid residues with a deduced molecular mass of 68,993 Da. ST94 was revealed to contain two putative copper-binding sites per a subunit, that showed sequence similarities with those of hemocyanins from mollusks, tyrosinases from microorganisms and vertebrates and the hypothetical tyrosinase-related protein of Caenorhabditis elegans. The squid tyrosinase was shown to catalyze the oxidation of monophenols as well as o-diphenols and to exhibit temperature-dependency of o-diphenolase activity like a psychrophilic enzyme.

Cloning, expression and characterisation of two tyrosinase cDNAs from Agaricus bisporus

Using primers designed on the basis of sequence homologies in the copper-binding domains for a number of plant and fungal tyrosinases, two tyrosinase encoding cDNAs were cloned from an Agaricus bisporus U1 cDNA-library. The sequences AbPPO1 and AbPPO2 were, respectively, 1.9 and 1.8 kb in size and encoded proteins of approximately 64 kDa. The cDNAs represent different loci. Both AbPPO1 and AbPPO2 occur as single copies on the genomes of the U1 parental strains H39 and H97. The genomic size of AbPPO1 and AbPPO2 is minimally 2.3 and 2.2 kb, respectively. Alignment and phylogenetic analysis of 35 tyrosinase and polyphenol oxidase sequences of animal, plant, fungal, and bacterial origin indicated conserved copper-binding domains, and stronger conservation within genera than between them. The translation products of AbPPO1 and AbPPO2 possess putative N-glycosylation and phosphorylation sites and are recognised by antibodies directed against a 43-kDa tyrosinase. The observations are consistent with previously proposed maturation and activation models for plant and fungal tyrosinases.

Biochemical and physiological aspects of brown blotch disease of Agaricus bisporus

Pseudomonas tolaasii is a bacterium endemic to the compost beds where common mushroom (Agaricus bisporus) is cultivated. Under some environmental conditions still not well-determined, but influenced by temperature and relative humidity, the bacterium can become pathogenic and provoke the brown blotch disease. This review describes the interaction between P. tolaasii and A. bisporus that results in the appearance of brown spots on the mushroom caps, typical symptoms of the disease. Firstly, P. tolaasii is studied, the changes in pathogenicity are explained, the compounds that provoke the damage are enumerated as well as various experimental methods to identify the pathogenic form of the bacteria. Secondly, mechanisms involved in the formation of the brown colour on the A. bisporus caps upon infection are briefly mentioned, taking into account the enzymes that catalyse the reaction, their mechanism, substrates and reaction products. Afterwards, a detailed description of the infection process is presented step by step, starting by the chemotactical attraction, fixation, secretion of the toxins, membrane breakdown, effect of the toxin on mushroom polyphenol oxidases and on the discolouration reaction. A possible mechanism of infection is hypothesised at the molecular level. Finally, the strategies tested until now to control the disease are discussed.

The enzymatic properties of Octopus vulgaris hemocyanin: o-diphenol oxidase activity

Hemocyanin and tyrosinase are dinuclear copper proteins capable of reversibly binding dioxygen. Despite the great similarity of structure and properties of their active site, the two proteins perform different biological functions (oxygen transport/storage versus monooxygenase and oxidase activity). In this paper, we show that Octopus vulgaris hemocyanin exhibits a tyrosinase-like activity; namely, it is capable of utilizing dioxygen for the oxidation of o-diphenol to quinone. The reaction is specific for this isomer of diphenol, the meta and para isomers being unreactive, and is strongly controlled by steric factors. Dioxygen represents a cosubstrate of the reaction, and it is involved in the catalytic turnover by binding to the dinuclear copper site of the protein to form, under steady-state conditions, oxy-Hc, which is the active species. The generation of semiquinone radicals, detected by EPR and by their reaction with N,N,N',N'-tetramethyl-1,4-phenylenediamine, strongly supports a reaction mechanism in which such radicals represent the reaction products of one-electron oxidation of the substrate, quinone being generated by dismutation of semiquinones. Met-Hc is regenerated by the substrate to the deoxy form. To close the catalytic cycle, the proposed reaction mechanism also involves the participation of two transient protein forms with the total oxidation state of the active site (V and IV) intermediate between that of oxy-Hcy, [CuIIO22-CuII]VI, and deoxy-Hc, [CuICuI]II. A mathematical model has been elaborated to describe the reaction kinetics. The differences in reaction mechanisms between hemocyanin and tyrosinase are discussed in terms of accessibility to exogenous molecules of their active sites.

Roles of copper ligands in the activation and secretion of Streptomyces tyrosinase

The expression of the melanin operon (melC) of Streptomyces antibioticus requires the chaperone-like protein MelC1 for the incorporation of two copper ions (designated as CuA and CuB) and the secretion of the apotyrosinase (MelC2) via a transient binary complex formation between these two proteins. To investigate whether the copper ligand of tyrosinase is involved in this MelC1.MelC2 binary complex function, six single substitution mutations were introduced into the CuA and CuB sites. These mutations led to differential effects on the stability, copper content, and export function of binary complexes but a complete abolishment of tyrosinase activity. The defects in the tyrosinase activity in mutants were not because of the impairment of the formation of MelC1. MelC2 complex but rather the failure of MelC2 to be discharged from the copper-activated binary complex. Moreover, the impairments on the discharge of the mutant MelC2 from all the mutant binary complexes appeared to result from the structural changes in their apoforms or copper-activated forms of the complexes, as evidenced by the fluorescence emission and circular dichroism spectral analysis. Therefore, each of six copper ligands in Streptomyces tyrosinase binuclear copper sites plays a pivotal role in the final maturation and the discharge of tyrosinase from the binary complex but has a less significant role in its secretion.

Calcineurin subunit B is required for normal vegetative growth in Neurospora crassa

A series of Neurospora crassa mutants affected in the ability to regulate entry into conidiation (an asexual developmental program) were isolated by using an insertional mutagenesis procedure followed by a screening protocol. One of the mutants isolated by this approach consisted entirely of cells with an abnormal morphology. The mutant produces chains of swollen septated cells. The developmentally regulated ccg-1 gene is constitutively expressed in these cells, suggesting that they have entered the conidial developmental program. The insertionally disrupted gene cnb-1 was isolated by plasmid rescue and found to encode calcineurin B, the regulatory subunit of the Ca2+ and calmodulin-dependent protein phosphatase calcineurin. The data demonstrate that calcineurin B is required for normal vegetative growth in N. crassa and suggest that the cnb-1 mutant is unable to repress entry into the asexual developmental program. The results suggest that Ca2+ may play an important role in regulating fungal morphology.

Sequence and structural features of plant and fungal tyrosinases

Tyrosinases from various organisms are compared with respect to enzymatic structure, primary, secondary and tertiary structure, domain structure, Cu binding sites, maturation mechanism and activation mechanism. On the basis of these comparisons, and by using hemocyanin structure as a template, a structure model for the active site of tyrosinases is proposed.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.

The o-diphenol oxidase activity of arthropod hemocyanin

Arthropod hemocyanin (isolated from the crab Carcinus maenas and the lobster Homarus americanus) is usually referred to as an oxygen transport-storage protein. The protein, however, also catalyses with low efficiency the oxidation of o-diphenol to quinone, similarly to tyrosinase (monophenol, o-diphenol:oxygen oxidoreductase). The enzymatic parameters of hemocyanin are affected by the aggregation state of the protein; namely V(max) exhibited by a dissociated subunit is one order of magnitude greater than that of aggregated species. The reaction velocity is increased by the presence of perchlorate, an anion of the Hofmeister series. The results are also discussed on the basis of active site accessibility in comparison with tyrosinase.

A cladistic analysis of the evolutionary relationships of the members of the tyrosinase gene family using sequence data

Recently, DNA sequence data have been published on tyrosinase and tyrosinase-related proteins (TRPs) in a wide variety of vertebrates ranging from Rana to Homo. These proteins are in turn members of a larger family of binuclear copper-binding proteins, which all contain two highly conserved copper-binding domains. This gene family also includes tyrosinases from fungi and bacteria as well as arthropodan and molluscan hemocyanins. Parsimony-based alignment and tree construction algorithms (Malign, v1.85 and PAUP, 3.1.1) were used to analyze the diversification of both the evolutionarily conserved copper-binding domains in copper-binding proteins in general as well as the diversification of the vertebrate tyrosinase gene family more specifically. These analyses show that the diversification of the vertebrate tyrosinase gene family minimally predates the diversification of vertebrates. Vertebrate tyrosinases proper first diverged from an ancestral tyrosinase-related protein (TRP) that then subsequently diverged to form tyrosinase-related protein-1s (TRP-1s) and tyrosinase-related protein-2s (TRP-2s).