Unravelling the suicide inactivation of tyrosinase: A discrimination between mechanisms

Catalytic mechanism of tyrosinases

Tyrosinases (TYR) play a key role in melanin biosynthesis by catalyzing two reactions: monophenolase and diphenolase activities. Despite low amino acid sequence homology, TYRs from various organisms (from bacteria to humans) have similar active site architectures and catalytic mechanisms. The active site of the TYRs contains two copper ions coordinated by histidine (His) residues. The catalytic mechanism of TYRs involves electron transfer between copper sites, leading to the hydroxylation of monophenolic compounds to diphenols and the subsequent oxidation of these to corresponding dopaquinones. Although extensive studies have been conducted on the structure, catalytic mechanism, and enzymatic capabilities of TYRs, some mechanistic aspects are still debated. This chapter will delve into the structure of the active site, catalytic function, and inhibition mechanism of TYRs. The goal is to improve our understanding of the molecular mechanisms underlying TYR activity. This knowledge can help in developing new strategies to modulate TYR function and potentially treat diseases linked to melanin dysregulation.

Targeting Tyrosinase in Hyperpigmentation: Current Status, Limitations and Future Promises

Hyperpigmentation is a common and distressing dermatologic condition. Since tyrosinase (TYR) plays an essential role in melanogenesis, its inhibition is considered a logical approach along with other therapeutic methods to prevent the accumulation of melanin in the skin. Thus, TYR inhibitors are a tempting target as the medicinal and cosmetic active agents of hyperpigmentation disorder. Among TYR inhibitors, hydroquinone is a traditional lightening agent that is commonly used in clinical practice. However, despite good efficacy, prolonged use of hydroquinone is associated with side effects. To overcome these shortcomings, new approaches in targeting TYR and treating hyperpigmentation are desperately requiredessentialneeded. In line with this purpose, several non-hydroquinone lightening agents have been developed and suggested as hydroquinone alternatives. In addition to traditional approaches, nanomedicine and nanotheranostic platforms have been recently proposed in the treatment of hyperpigmentation. In this review, we discuss the available strategies for the management of hyperpigmentation with a focus on TYR inhibition. In addition, alternative treatment options to hydroquinone are discussed. Finally, we present nano-based strategies to improve the therapeutic effect of drugs prescribed to patients with skin disorders.

The Effect of p-Coumaric Acid on Browning Inhibition in Potato Polyphenol Oxidase-Catalyzed Reaction Mixtures

There has been considerable interest in using natural polyphenol oxidase (PPO) inhibitors to control browning in fruit and vegetable products. p-Coumaric acid (pCA), a common secondary metabolite of plants, has been studied as an inhibitor of PPOs/tyrosinases from several foods (e.g., mushroom, apple, and potato). However, studies on the use of pCA for the inhibition of PPO-initiated browning in actual food systems are limited. Therefore, a study was carried out to ascertain the efficacy of using pCA to limit PPO-initiated browning in fresh potato juice. The extent of browning inhibition by pCA was shown to be reaction system-dependent. Browning in potato juice was unexpectedly enhanced by the addition of pCA. This was interpreted as pCA acting as an alternative substrate with significantly higher browning efficiency; extent of browning under this condition was higher than that observed in the native potato juice. The addition of pCA to any of the model reaction mixtures (i.e., those containing semi-purified enzymes and substrates) significantly inhibited browning. The discrepancy in pCA effects on browning inhibition in different reaction systems is postulated to be mainly due to non-enzyme and non-substrate components in potato juice that participate in the post-PPO reaction sequences, which ultimately lead to brown color formation.

Role of Ascorbic Acid in the Extraction and Quantification of Potato Polyphenol Oxidase Activity

The ability to accurately measure the activity of polyphenol oxidase (PPO) in complex matrices is essential. A problem encountered when using spectrophotometric methods is interference due to ascorbic acid (AA), often used as an enzyme “protecting agent” during PPO extraction. This study focuses on the nature of AA’s effect on spectrophotometric determinations of PPO activity as well as enzyme extraction. Potato extracts and semi-purified PPO were used as enzyme sources. The inactivation of PPO attributed to AA is substrate-mediated. The extent of AA-dependent inactivation of PPO in model systems varied between substrates. AA only slows mechanism-based inactivation of PPO induced by catechol, possibly owing to the prevention of quinone formation. AA minimally protects PPO activity during enzyme extraction. The problem associated with AA in PPO assay could be circumvented by using ascorbate oxidase to remove AA when catechol is the primary substrate or by using chlorogenic acid as the primary substrate.

Polyphenol-Hydroxylating Tyrosinase Activity under Acidic pH Enables Efficient Synthesis of Plant Catechols and Gallols

Tyrosinase is generally known as a melanin-forming enzyme, facilitating monooxygenation of phenols, oxidation of catechols into quinones, and finally generating biological melanin. As a homologous form of tyrosinase in plants, plant polyphenol oxidases perform the same oxidation reactions specifically toward plant polyphenols. Recent studies reported synthetic strategies for large scale preparation of hydroxylated plant polyphenols, using bacterial tyrosinases rather than plant polyphenol oxidase or other monooxygenases, by leveraging its robust monophenolase activity and broad substrate specificity. Herein, we report a novel synthesis of functional plant polyphenols, especially quercetin and myricetin from kaempferol, using screened bacterial tyrosinases. The critical bottleneck of the biocatalysis was identified as instability of the catechol and gallol under neutral and basic conditions. To overcome such instability of the products, the tyrosinase reaction proceeded under acidic conditions. Under mild acidic conditions supplemented with reducing agents, a bacterial tyrosinase from Bacillus megaterium (BmTy) displayed efficient consecutive two-step monophenolase activities producing quercetin and myricetin from kaempferol. Furthermore, the broad substrate specificity of BmTy toward diverse polyphenols enabled us to achieve the first biosynthesis of tricetin and 3'-hydroxyeriodictyol from apigenin and naringenin, respectively. These results suggest that microbial tyrosinase is a useful biocatalyst to prepare plant polyphenolic catechols and gallols with high productivity, which were hardly achieved by using other monooxygenases such as cytochrome P450s.

A comprehensive review on tyrosinase inhibitors

Tyrosinase is a multi-copper enzyme which is widely distributed in different organisms and plays an important role in the melanogenesis and enzymatic browning. Therefore, its inhibitors can be attractive in cosmetics and medicinal industries as depigmentation agents and also in food and agriculture industries as antibrowning compounds. For this purpose, many natural, semi-synthetic and synthetic inhibitors have been developed by different screening methods to date. This review has focused on the tyrosinase inhibitors discovered from all sources and biochemically characterised in the last four decades.

Conjugation with Dihydrolipoic Acid Imparts Caffeic Acid Ester Potent Inhibitory Effect on Dopa Oxidase Activity of Human Tyrosinase

Caffeic acid derivatives represent promising lead compounds in the search for tyrosinase inhibitors to be used in the treatment of skin local hyperpigmentation associated to an overproduction or accumulation of melanin. We recently reported the marked inhibitory activity of a conjugate of caffeic acid with dihydrolipoic acid, 2-S-lipoylcaffeic acid (LCA), on the tyrosine hydroxylase (TH) and dopa oxidase (DO) activities of mushroom tyrosinase. In the present study, we evaluated a more lipophilic derivative, 2-S-lipoyl caffeic acid methyl ester (LCAME), as an inhibitor of tyrosinase from human melanoma cells. Preliminary analysis of the effects of LCAME on mushroom tyrosinase indicated more potent inhibitory effects on either enzyme activities (IC50 = 0.05 ± 0.01 μM for DO and 0.83 ± 0.09 μM for TH) compared with LCA and the reference compound kojic acid. The inhibition of DO of human tyrosinase was effective (Ki = 34.7 ± 1.1 μM) as well, while the action on TH was weaker. Lineweaver⁻Burk analyses indicated a competitive inhibitor mechanism. LCAME was not substrate of tyrosinase and proved nontoxic at concentrations up to 50 μM. No alteration of basal tyrosinase expression was observed after 24 h treatment of human melanoma cells with the inhibitor, but preliminary evidence suggested LCAME might impair the induction of tyrosinase expression in cells stimulated with α-melanocyte-stimulating hormone. All these data point to this compound as a valuable candidate for further trials toward its use as a skin depigmenting agent. They also highlight the differential effects of tyrosinase inhibitors on the human and mushroom enzymes.

Action of tyrosinase on caffeic acid and its n-nonyl ester. Catalysis and suicide inactivation

Different mechanisms for inhibiting tyrosinase can be designed to avoid postharvest quality losses of fruits and vegetables. The action of tyrosinase on caffeic acid and its n-nonyl ester (n-nonyl caffeate) was characterized kinetically in this work. The results lead us to propose that both compounds are suicide substrates of tyrosinase, for which we establish the catalytic and inactivation efficiencies. The ester is more potent as inactivator than the caffeic acid and the number of turnovers made by one molecule of the enzyme before its inactivation (r) is lower for the ester. We proposed that the anti-browning and antibacterial properties may be due to suicide inactivation processes.

2-S-Lipoylcaffeic Acid, a Natural Product-Based Entry to Tyrosinase Inhibition via Catechol Manipulation

Conjugation of naturally occurring catecholic compounds with thiols is a versatile and facile entry to a broad range of bioinspired multifunctional compounds for diverse applications in biomedicine and materials science. We report herein the inhibition properties of the caffeic acid- dihydrolipoic acid S-conjugate, 2-S-lipoylcaffeic acid (LC), on mushroom tyrosinase. Half maximum inhibitory concentration (IC₅₀) values of 3.22 ± 0.02 and 2.0 ± 0.1 µM were determined for the catecholase and cresolase activity of the enzyme, respectively, indicating a greater efficiency of LC compared to the parent caffeic acid and the standard inhibitor kojic acid. Analysis of the Lineweaver–Burk plot suggested a mixed-type inhibition mechanism. LC proved to be non-toxic on human keratinocytes (HaCaT) at concentrations up to 30 µM. These results would point to LC as a novel prototype of melanogenesis regulators for the treatment of pigmentary disorders.

Heterologous expression and characterization of functional mushroom tyrosinase (AbPPO4)

Tyrosinases are an ubiquitous group of copper containing metalloenzymes that hydroxylate and oxidize phenolic molecules. In an application context the term 'tyrosinase' usually refers to 'mushroom tyrosinase' consisting of a mixture of isoenzymes and containing a number of enzymatic side-activities. We describe a protocol for the efficient heterologous production of tyrosinase 4 from Agaricus bisporus in Escherichia coli. Applying this procedure a pure preparation of a single isoform of latent tyrosinase can be achieved at a yield of 140 mg per liter of autoinducing culture medium. This recombinant protein possesses the same fold as the enzyme purified from the natural source as evidenced by single crystal X-ray diffraction. The latent enzyme can be activated by limited proteolysis with proteinase K which cleaves the polypeptide chain after K382, only one The latent enzyme can amino acid before the main in-vivo activation site. Latent tyrosinase can be used as obtained and enzymatic activity may be induced in the reaction mixture by the addition of an ionic detergent (e.g. 2 mM SDS). The proteolytically activated mushroom tyrosinase shows >50% of its maximal activity in the range of pH 5 to 10 and accepts a wide range of substrates including mono- and diphenols, flavonols and chalcones.

Latent and active aurone synthase from petals of C. grandiflora: a polyphenol oxidase with unique characteristics

Aurone synthase belongs to the novel group 2 polyphenol oxidases and the presented kinetic characterization suggests a differing aurone biosynthesis in Asteraceae species compared to snapdragon. Aurone synthases (AUS) are polyphenol oxidases (PPO) physiologically involved in the formation of yellow aurone pigments in petals of various Asteraceae species. They catalyze the oxidative conversion of chalcones into aurones. Latent (58.9 kDa) and active (41.6 kDa) aurone synthase from petals of C. grandiflora was purified by a quantitative removal of pigments using aqueous two-phase separation and several subsequent chromatographic steps. The purified enzymes were identified as cgAUS1 (A0A075DN54) and sequence analysis revealed that cgAUS1 is a member of a new group of plant PPOs. Mass determination experiments of intact cgAUS1 gave evidence that the C-terminal domain, usually shielding the active site of latent polyphenol oxidases, is linked to the main core by a disulfide bond. This is a novel and unique structural feature of plant PPOs. Proteolytic activation in vivo leads to active aurone synthase possessing a residual peptide of the C-terminal domain. Kinetic characterization of purified cgAUS1 strongly suggests a specific involvement in 4-deoxyaurone biosynthesis in Coreopsis grandiflora (Asteraceae) that differs in various aspects compared to the 4-hydroxyaurone formation in Antirrhinum majus (Plantaginaceae): cgAUS1 is predicted to be localized in the thylakoid lumen, it possesses exclusively diphenolase activity and the results suggest that aurone formation occurs at the level of chalcone aglycones. The latent enzyme exhibits allosteric activation which changes at a specific product concentration to a constant reaction rate. The presented novel structural and functional properties of aurone synthase provide further insights in the diversity and role of plant PPOs.

Determination of tyrosinase substrate-binding modes reveals mechanistic differences between type-3 copper proteins

Tyrosinase is responsible for the two initial enzymatic steps in the conversion of tyrosine to melanin. Many tyrosinase mutations are the leading cause of albinism in humans, and it is a prominent biotechnology and pharmaceutical industry target. Here we present crystal structures that show that both monophenol hydroxylation and diphenol oxidation occur at the same site. It is suggested that concurrent presence of a phenylalanine above the active site and a restricting thioether bond on the histidine coordinating CuA prevent hydroxylation of monophenols by catechol oxidases. Furthermore, a conserved water molecule activated by E195 and N205 is proposed to mediate deprotonation of the monophenol at the active site. Overall, the structures reveal precise steps in the enzymatic catalytic cycle as well as differences between tyrosinases and other type-3 copper enzymes.

Catalysis and inactivation of tyrosinase in its action on hydroxyhydroquinone

Hydroxyhydroquinone (HHQ) was characterized kinetically as a tyrosinase substrate. A kinetic mechanism is proposed, in which HHQ is considered as a monophenol or as an o-diphenol, depending on the part of the molecule that interacts with the enzyme. The kinetic parameters obtained from an analysis of the measurements of the initial steady state rate of 2-hydroxy p-benzoquinone formation were kcatapp=229.0±7.7 s(-1) and KMapp,HHQ=0.40±0.05 mM. Furthermore, the action of tyrosinase on HHQ led to the enzyme's inactivation through a suicide inactivation mechanism. This suicide inactivation process was characterized kinetically by λmaxapp (the apparent maximum inactivation constant) and r, the number of turnovers made by 1 mol of enzyme before being inactivated. The values of λmaxapp and r were (8.2±0.1)×10(-3) s(-1) and 35,740±2,548, respectively.

Deuterium isotope effect on the suicide inactivation of tyrosinase in its action on o-diphenols

A solvent deuterium isotope effect on the inactivation suicide of tyrosinase in its action on o-diphenols (catechol, 4-methylcatechol, and 4-tert-butylcatechol) was observed. This isotope effect, observed during kinetic studies in the transition phase, was higher than that described previously in the steady state, indicating that there is an additional slow step in the suicide inactivation mechanism, which we believe to be responsible for the inactivation. In a proton inventory study of oxidation of o-diphenols, the representation of λmax(D,fn)/λmax(D,f0) versus n (atom fractions of deuterium), where λmax(D,fn) is the maximum apparent inactivation constant for a molar fraction of deuterium (n) and λmax(D,f0) is the corresponding kinetic parameter in a water solution, was linear for all substrates. This suggests that only one of the protons transferred from the two hydroxyl groups of the substrate, which are oxidized in one turnover, is responsible for the isotope effects. We propose that this proton could be the proton transferred from the hydroxyl group of C-2 to the hydroperoxide of the oxytyrosinase form (Eox ) and that it probably causes enzyme inactivation through the reduction of the Cu(2+) A to Cu(0) and its subsequent release from the active site.

Indirect inactivation of tyrosinase in its action on 4-tert-butylphenol

Under anaerobic conditions, the o-diphenol 4-tert-butylcatechol (TBC) irreversibly inactivates met and deoxytyrosinase enzymatic forms of tyrosinase. However, the monophenol 4-tert-butylphenol (TBF) protects the enzyme from this inactivation. Under aerobic conditions, the enzyme suffers suicide inactivation when it acts on TBC. We suggest that TBF does not directly cause the suicide inactivation of the enzyme in the hydroxylase activity, but that the o-diphenol, which is necessary for the system to reach the steady state, is responsible for the process. Therefore, monophenols do not induce the suicide inactivation of tyrosinase in its hydroxylase activity, and there is a great difference between the monophenols that give rise to unstable o-quinones such as L-tyrosine, which rapidly accumulate L-dopa in the medium and those like TBF, after oxidation, give rise to a very stable o-quinone.

Kinetic characterisation of o-aminophenols and aromatic o-diamines as suicide substrates of tyrosinase

We study the suicide inactivation of tyrosinase acting on o-aminophenols and aromatic o-diamines and compare the results with those obtained for the corresponding o-diphenols. The catalytic constants follow the order aromatic o-diamines<o-aminophenols<o-diphenols, which agrees with the view that the transfer of the proton to the peroxide group of the oxy-tyrosinase form is the slowest step in the catalytic cycle. As regards the apparent inactivation constant, it remains within the same order of magnitude, although slightly lower in the case of the aromatic o-diamines and o-aminophenols than o-diphenols: o-diamines<o-aminophenols<o-diphenols. The efficiency of the second nucleophilic attack of substrate on CuA seems to be the determining factor in the bifurcation of the inactivation and catalytic pathways. This attack is more efficient in o-diamines (where it attacks a nitrogen atom) than in o-aminophenols and o-diphenols (where it attacks an oxygen atom), favouring the catalytic pathway and slowing down the inactivation pathway. The inactivation step is the slowest of the whole process. The values of r, the number of turnovers that 1mol of enzyme carries out before being inactivated, follows the order aromatic o-diamines<o-aminophenols<o-diphenols. As regards the Michaelis constants, that of the o-diamines is slightly lower than that of the o-diphenols, while that of the o-aminophenols is slightly greater than that observed for the o-diphenols. As a consequence of the above, the inactivation efficiency, λ(max)/K(m)(S), follows this order: o-diphenols>o-aminophenols>aromatic o-diamines.