Structural insights into the hot spot amino acid residues of mushroom tyrosinase for the bindings of thujaplicins

Computational studies of tyrosinase inhibitors

Computational studies have significantly advanced the understanding of tyrosinase (TYR) function, mechanism, and inhibition, accelerating the development of more effective and selective inhibitors. This chapter provides an overview of in silico studies on TYR inhibitors, emphasizing key inhibitory chemotypes and the main residues involved in ligand-target interactions. The chapter discusses tools applied in the context of TYR inhibitor development, e.g., structure-based virtual screening, molecular docking, artificial intelligence, and machine learning algorithms.

Catechol-mimicking transition-state analogues as non-oxidizable inhibitors of tyrosinases

Tyrosinases are copper-containing metalloenzymes involved in several processes in both mammals, insects, bacteria, fungi and plants. Their phenol oxidation properties are especially responsible for human melanogenesis, potentially leading to abnormal pigmentation, and for postharvest vegetable tissue browning. Thus, targeting tyrosinases attracts interest for applications both in dermocosmetic and agrofood fields. However, a large part of the literature about tyrosinase inhibitors is dedicated to the report of copper-interacting phenolic compounds, that are more likely alternative substrates leading to undesirable toxic quinones production. To circumvent this issue, the use of catechol-mimicking copper-chelating groups that are analogues of the tyrosinase oxidation transition state appears as a valuable strategy. Relying on several non-oxidizable pyridinone, pyrone or tropolone moieties, innovative inhibitors were developed, especially within the past decade, and the best reported analogues reached IC50 values in the nanomolar range. Herein, we review the design, the activity against several tyrosinases, and the proposed binding modes of reported catechol-mimicking, non-oxidizable molecules, in light of recent structural data.

Pharmacoinformatics exploration of polyphenol oxidases leading to novel inhibitors by virtual screening and molecular dynamic simulation study

Polyphenol oxidases (PPOs)/tyrosinases are metal-dependent enzymes and known as important targets for melanogenesis. Although considerable attempts have been conducted to control the melanin-associated diseases by using various inhibitors. However, the exploration of the best anti-melanin inhibitor without side effect still remains a challenge in drug discovery. In present study, protein structure prediction, ligand-based pharmacophore modeling, virtual screening, molecular docking and dynamic simulation study were used to screen the strong novel inhibitor to cure melanogenesis. The 3D structures of PPO1 and PPO2 were built through homology modeling, while the 3D crystal structures of PPO3 and PPO4 were retrieved from PDB. Pharmacophore modeling was performed using LigandScout 3.1 software and top five models were selected to screen the libraries (2601 of Aurora and 727, 842 of ZINC). Top 10 hit compounds (C1-10) were short-listed having strong binding affinities for PPO1-4. Drug and synthetic accessibility (SA) scores along with absorption, distribution, metabolism, excretion and toxicity (ADMET) assessment were employed to scrutinize the best lead hit. C4 (name) hit showed the best predicted SA score (5.75), ADMET properties and drug-likeness behavior among the short-listed compounds. Furthermore, docking simulations were performed to check the binding affinity of C1-C10 compounds against target proteins (PPOs). The binding affinity values of complex between C4 and PPOs were higher than those of other complexes (-11.70, -12.1, -9.90 and -11.20kcal/mol with PPO1, PPO2, PPO3, or PPO4, respectively). From comparative docking energy and binding analyses, PPO2 may be considered as better target for melanogenesis than others. The potential binding modes of C4, C8 and C10 against PPO2 were explored using molecular dynamics simulations. The root mean square deviation and fluctuation (RMSD/RMSF) graphs results depict the significance of C4 over the other compounds. Overall, bioactivity and ligand efficiency profiles suggested that the proposed hit may be more effective inhibitors for melanogenesis.

Computational approaches to predict binding interactions between mammalian tyrosinases and (S)-(+)-decursin and its analogues as potent inhibitors

Using computational methods, we elucidated the binding modes of (S)-(+)-decursin with mouse and human tyrosinases, and thereby provided insight to new tyrosinase inhibitors.

Exploring the interaction of N/S compounds with a dicopper center: tyrosinase inhibition and model studies

Tyrosinase (Ty) is a copper-containing enzyme widely present in plants, bacteria, and humans, where it is involved in biosynthesis of melanin-type pigments. Development of Ty inhibitors is an important approach to control the production and the accumulation of pigments in living systems. In this paper, we focused our interest in phenylthiourea (PTU) and phenylmethylene thiosemicarbazone (PTSC) recognized as inhibitors of tyrosinase by combining enzymatic studies and coordination chemistry methods. Both are efficient inhibitors of mushroom tyrosinase and they can be considered mainly as competitive inhibitors. Computational studies verify that PTSC and PTU inhibitors interact with the metal center of the active site. The KIC value of 0.93 μM confirms that PTSC is a much more efficient inhibitor than PTU, for which a KIC value of 58 μM was determined. The estimation of the binding free energies inhibitors/Ty confirms the high inhibitor efficiency of PTSC. Binding studies of PTSC along with PTU to a dinuclear copper(II) complex ([Cu2(μ-BPMP)(μ-OH)](ClO4)2 (1); H-BPMP = 2,6-bis-[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol) known to be a structural and functional model for the tyrosinase catecholase activity, have been performed. Interactions of the compounds with the dicopper model complex 1 were followed by spectrophotometry and electrospray ionization (ESI). The molecular structure of 1-PTSC and 1-PTU adducts were determined by single-crystal X-ray diffraction analysis showing for both an unusual bridging binding mode on the dicopper center. These results reflect their adaptable binding mode in relation to the geometry and chelate size of the dicopper center.

Structure-activity relationships of the thujaplicins for inhibition of human tyrosinase

Tyrosinase inhibitors have become increasingly critical agents in cosmetic, agricultural, and medicinal products. Although a large number of tyrosinase inhibitors have been reported, almost all the inhibitors were unfortunately evaluated by using commercial available mushroom tyrosinase. Here, we examined the inhibitory effects of three isomers of thujaplicin (α, β, and γ) on human tyrosinase and analyzed their binding modes using homology model and docking studies. As the results, γ-thujaplicin was found to strongly inhibit human tyrosinase with the IC50 of 1.15 μM, extremely superior to a well-known tyrosinase inhibitor kojic acid (IC50 = 571.17 μM). MM-GB/SA binding free energy decomposition analyses suggested that the potent inhibitory activity of γ-thujaplicin may be due to the interactions with His367, Ile368, and Val377 (hot spot amino acid residues) in human tyrosinase. Furthermore, the binding mode of α-thujaplicin indicated that Val377 and Ser380 may cause van der Waals clashes with the isopropyl group of α-thujaplicin. These results provide a novel structural insight into the hot spot of human tyrosinase for the specific binding of γ-thujaplicin and a way to optimize not only thujaplicins but also other lead compounds as specific inhibitors for human tyrosinase in a rational manner.

Investigation of binding-site homology between mushroom and bacterial tyrosinases by using aurones as effectors

Tyrosinase is a copper-containing enzyme found in plants and bacteria, as well as in humans, where it is involved in the biosynthesis of melanin-type pigments. Tyrosinase inhibitors have attracted remarkable research interest as whitening agents in cosmetology, antibrowning agents in food chemistry, and as therapeutics. In this context, commercially available tyrosinase from mushroom (TyM) is frequently used for the identification of inhibitors. This and bacterial tyrosinase (TyB) have been the subjects of intense biochemical and structural studies, including X-ray diffraction analysis, and this has led to the identification of structural homology and divergence among enzymes from different sources. To better understand the behavior of potential inhibitors of TyM and TyB, we selected the aurone family-previously identified as potential inhibitors of melanin biosynthesis in human melanocytes. In this study, a series of 24 aurones with different hydroxylation patterns at the A- and B-rings were evaluated on TyM and TyB. The results show that, depending on the hydroxylation pattern of A- and B-rings, aurones can behave as inhibitors, substrates, and activators of both enzymes. Computational analysis was performed to identify residues surrounding the aurones in the active sites of both enzymes and to rationalize the interactions. Our results highlight similarities and divergence in the behavior of TyM and TyB toward the same set of molecules.

Oxidation of cyanobenzocycloheptatrienes: Synthesis, photooxygenation reaction and carbonic anhydrase isoenzymes inhibition properties of some new benzotropone derivatives

The oxidation of some cyanocycloheptatrienes with CrO3 and pyridine was investigated and a few new nitrile functionalised benzotropone derivatives were obtained. Photooxygenation reaction of these products was also studied. The structures of the formed products were determined on the basis of NMR spectroscopy and the formation mechanism of unusual products was discussed. Human carbonic anhydrase isoenzymes I, and II (hCA I and hCA II) inhibition properties of nitrile functionalized new benzotropone derivatives were also studied. Both CA isozymes were inhibited in the low micromolar range by these nitrile functionalized benzotropone analogues. The newly synthesized benzotropone derivatives showed inhibition constants in the sub-micromolar range (2.51-4.06μM). The best hCA I inhibition was observed in 5H-benzocycloheptene-7-carbonitrile (Ki: 2.88±0.86μM). On the other hand, 5-oxo-5H-benzocycloheptatriene-7-carbonitrile showed the powerful inhibitory effect against hCA II (Ki: 2.51±0.34μM).

Condensed tannins from Ficus virens as tyrosinase inhibitors: structure, inhibitory activity and molecular mechanism

Condensed tannins from Ficus virens leaves, fruit, and stem bark were isolated and their structures characterized by 13C nuclear magnetic resonance spectrometry, high performance liquid chromatography electrospray ionization mass spectrometry, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The results showed that the leaves, fruit, and stem bark condensed tannins were complex mixtures of homo- and heteropolymers of B-type procyanidins and prodelphinidins with degrees of polymerization up to hexamer, dodecamer, and pentadecamer, respectively. Antityrosinase activities of the condensed tannins were studied. The results indicated that the condensed tannins were potent tyrosinase inhibitors. The concentrations for the leaves, fruit, and stem bark condensed tannins leading to 50% enzyme activity were determined to be 131.67, 99.89, and 106.22 μg/ml on monophenolase activity, and 128.42, 43.07, and 74.27 μg/ml on diphenolase activity. The inhibition mechanism, type, and constants of the condensed tannins on the diphenolase activity were further investigated. The results indicated that the condensed tannins were reversible and mixed type inhibitors. Fluorescence quenching, copper interacting, and molecular docking techniques were utilized to unravel the molecular mechanisms of the inhibition. The results showed that the hydroxyl group on the B ring of the condensed tannins could chelate the dicopper irons of the enzyme. Moreover, the condensed tannins could reduce the enzyme product o-quinones into colourless compounds. These results would contribute to the development and design of antityrosinase agents.

Structure characterization of proanthocyanidins from Caryota ochlandra Hance and their bioactivities

Proanthocyanidins (PAs) from Caryota ochlandra fruit pericarp and fruit flesh were characterized by (13)C nuclear magnetic resonance, high performance liquid chromatography-electrospray ionization mass spectrometry, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry techniques. The fruit pericarp and flesh PAs were complex mixtures of homo- and heteropolymers of B-type procyanidins and prodelphinidins both with degrees of polymerization up to dodecamer. Their antioxidant and antityrosinase activities were investigated. The fruit pericarp PAs exhibited potent antioxidant activity with IC50 values of 142.86 ± 1.53 and 80.51 ± 0.4 μg/ml for DPPH and ABTS free-radical scavenging assays; with FRAP value of 373.09 ± 5.02 mg ascorbic acid equivalent/g dry weight. Furthermore, the fruit pericarp PAs had antityrosinase activity while the fruit flesh PAs could be oxidized by tyrosinase. The structure and antioxidant activities of the C. ochlandra fruit PAs together with their effects on tyrosinase activity would lay scientific foundation for their utilization in food and nutrition industry.

Versatile effects of aurone structure on mushroom tyrosinase activity

Elucidation of the binding modes of Ty inhibitors is an important step for in-depth studies on how to regulate tyrosinase activity. In this paper we highlight the extraordinarily versatile effects of the aurone structure on mushroom Ty activity. Depending on the position of the OH group on the B-ring, aurones can behave either as substrates or as hyperbolic activators. The synthesis of a hybrid aurone through combination of an aurone moiety with HOPNO (2-hydroxypyridine N-oxide), a good metal chelate, led us to a new, efficient, mixed inhibitor for mushroom tyrosinase. Another important feature pointed out by our study is the presence of more than one site for aurone compounds on mushroom tyrosinase. Because study of the binding of the hybrid aurone was difficult to perform with the enzyme, we undertook binding studies with tyrosinase functional models in order to elucidate the binding mode (chelating vs. bridging) on a dicopper(II) center. Use of EPR combined with theoretical DFT calculations allowed us to propose a preferred chelating mode for the interaction of the hybrid aurone with a dicopper(II) center.