6,7-Bis-(2-methoxyethoxy)-4(3H)-quinazolinone as a novel inhibitor of tyrosinase and potential anti-browning agent of fresh-cut apples

6,7-Bis-(2-methoxyethoxy)-4(3H)-quinazolinone (BMEQ) was selected from quinazolinones for its strong tyrosinase inhibitory activity (IC50 = 160 ± 6 μM). It suppressed tyrosinase activity in a competitive way and quenched the fluorescence of the enzyme through a static mechanism. The binding of BMEQ to tyrosinase increased the hydrophobicity of the latter and facilitated non-radiative energy transfer between them. The formation of BMEQ-tyrosinase complex was driven by hydrogen bonds and hydrophobic interactions, and it loosened the basic framework structure of tyrosinase, affecting the conformation of the enzyme, and leading to a decrease in tyrosinase activity. In addition, the BMEQ postponed the oxidation of phenolics and flavonoids by inhibiting polyphenol oxidase (PPO) and peroxidase (POD), which resulted in the inhibition of the browning of fresh-cut apples. This study identified a novel tyrosinase inhibitor BMEQ and verified its potential application for improving the preservation of postharvest fruits.

Biomolecular interactions and binding dynamics of inhibitor arachidonic acid, with tyrosinase enzyme

Hyperpigmentation is a disorder caused by increased melanin deposition and changes in skin pigmentation. Inhibition of tyrosinase activity contributes to the control of food browning and skin pigmentation diseases. The effects of arachidonic acid (AA) on tyrosinase activity were examined using different spectroscopy methods including UV-VIS spectrophotometry, fluorescence spectroscopy, circular dichroism (CD) differential scanning calorimetry, and molecular dynamics (MD) simulations. Based on the kinetic results, arachidonic acid showed mixed-type of inhibition with Ki = 4.7 µM. Fluorescence and CD studies showed changes of secondary and tertiary structures of enzyme and a reduction of α-helix* amino acids after its incubation with different concentrations of AA, which is also confirmed by DSSP analysis. In addition, differential scanning calorimetry (DSC) studies showed a decrease in thermodynamic stability of enzyme from Tm = 338.65k for sole enzyme after incubation with AA in comparison with complex enzyme with Tm= 334.26k, ΔH =7.52 kJ/mol, and ΔS = 0.15 kJ/mol k. Based on the theoretical methods, it was found that the interaction between enzyme and AA follows an electrostatic manner with ΔG = -8.314 kJ/mol and ΔH = -12.9 kJ/mol. The MD results showed the lowest flexibility in the complex amino acids and minimal fluctuations in AA interaction with tyrosinase in Residue 240 to 260 and 66 to 80. Thus, AA inhibitory and structural and thermodynamic instability of tyrosinase supported advantages of this fatty acid for prevention of medical hyperpigmentation. Therefore, it is a good candidate for cosmetic applications. Communicated by Ramaswamy H. Sarma.

Natural allosteric modulators and their biological targets: molecular signatures and mechanisms

Covering: 2008 to 2018Over the last decade more than two hundred single natural products were confirmed as natural allosteric modulators (alloNPs) of proteins. The compounds are presented and discussed with the support of a chemical space, constructed using a principal component analysis (PCA) of molecular descriptors from chemical compounds of distinct databases. This analysis showed that alloNPs are dispersed throughout the majority of the chemical space defined by natural products in general. Moreover, a cluster of alloNPs was shown to occupy a region almost devoid of allosteric modulators retrieved from a dataset composed mainly of synthetic compounds, further highlighting the importance to explore the entire natural chemical space for probing allosteric mechanisms. The protein targets which alloNPs bind to comprised 81 different proteins, which were classified into 5 major groups, with enzymes, in particular hydrolases, being the main representative group. The review also brings a critical interpretation on the mechanisms by which alloNPs display their molecular action on proteins. In the latter analysis, alloNPs were classified according to their final effect on the target protein, resulting in 3 major categories: (i) local alteration of the orthosteric site; (ii) global alteration in protein dynamics that change function; and (iii) oligomer stabilisation or protein complex destabilisation via protein-protein interaction in sites distant from the orthosteric site. G-protein coupled receptors (GPCRs), which use a combination of the three types of allosteric regulation found, were also probed by natural products. In summary, the natural allosteric modulators reviewed herein emphasise their importance for exploring alternative chemotherapeutic strategies, potentially pushing the boundaries of the druggable space of pharmacologically relevant drug targets.

Five individual polyphenols as tyrosinase inhibitors: Inhibitory activity, synergistic effect, action mechanism, and molecular docking

Polyphenols can inhibit the enzymatic browning in food, but their indistinct synergistic effect and conformational change have limited their applications. In this paper, the mixture of quercetin, cinnamic acid and ferulic acid (Group 11, K_I = 0.239 mM) possessed a higher inhibition ability than quercetin (K_I = 0.361 mM), which could promote the spontaneous binding process. The final Group 11-tyrosinase complex is more stable, and the hydrophobic effect is the major driving force during the binding process. Moreover, there is not a direct relationship between the destruction of secondary structures and catalytic activity of tyrosinase. The interaction between ferulic acid and tyrosinase could destroy the secondary structures of enzyme but it had little impact on the tyrosinase activity. Molecular docking suggested that three polyphenols from Group 11 have synergistic effect on tyrosinase. This study provides new perspectives about the development of tyrosinase inhibitors in food products.

Effect of chrysin omega-3 and 6 fatty acid esters on mushroom tyrosinase activity, stability, and structure

The estreification of chrysin with α-Linolenic acid (complex I) and linoleic acid (complex II) poly unsaturated fatty acids resulted to design of new mushroom tyrosinase (MT) inhibitors. Thermodynamic parameters of enzymes, including the melting point (T_m ) and ∆G values, were obtained from thermal and chemical denaturation curves. Complexes I and II showed a competitive inhibitory effect on MT with K_i values of 0.45 and 0.29 mM, respectively. The T_m values were calculated as 328.6, 322.4, and 318 K and the ∆G values as 62.8, 52.9, and 47.1 KJ mol^-1 for the enzyme alone and its interaction with complexes I and II, respectively. Intrinsic and extrinsic fluorescence techniques showed structural instability of the enzyme in concomitance with a decrease in the regular secondary structure acquired using CD spectrometry. This data clearly prove that the new derivatives show a stronger inhibitory effect than the separate compounds. Molecular docking analysis showed that the best possible interaction condition was achieved for chrysin with n-6. PRACTICAL APPLICATIONS: MT is a suitable model in medicine for the investigation of melanogenesis, skin disorders, and hyperpigmentation because of its accessibility and close structural similarity to mammalian tyrosinase. In recent years, the designing of tyrosinase inhibitors from natural substances for prevention of hyperpigmentation in medicine, skin cosmetics, and undesired browning in agriculture and food industry has risen sharply. Many of the pharmaceutical products based on the use of flavonoids and poly unsaturated acids as natural compounds or on their semi-synthetic derivatives have been interested for investigations because of their usefulness in many pathological conditions such as inflammation, cancer, and skin disorders. The limitation of the flavonoids applications are low bioavailability, permeability, and solubility for the cells. In this study, conjugation of chrysin with n-3 and n-6 fatty acids resulted in a stronger inhibitors of MT with a synergic inhibitory effect on its activity.

Modifying effects of carboxyl group on the interaction of recombinant S100A8/A9 complex with tyrosinase

Tyrosinase is a determinant enzyme for modulating melanin production as its abnormal activity can result in an increased amount of melanin. Reduction of tyrosinase activity has been targeted for preventing and healing hyperpigmentation of skin, such as melanoma and age related spots. The aim of this systematic study is to investigate whether recombinant S100A8/A9 and its modified form reduce the activity of mushroom tyrosinase (MT) through changing its structure. Recombinant His-Tagged S100A8 and S100A9 are expressed in Escherichia coli BL21 (DE3) and modified using Woodward's reagent K which is a carboxyl group modifier. The structures of S100A8/A9 and its modified form are studied using fluorescence and circular dichroism spectroscopy, and the activity of MT is measured using UV-visible spectrophotometry in the presence of its substrate, L-3,4-dihydroxyphenylalanine (L-DOPA). The results show a lower stability of the modified protein when compared with its unmodified form. The interaction of S100A8/A9 with MT changes the structure and successfully reduces the activity of mushroom tyrosinase. Recombinant S100A8/A9 complex decreases MT activity which can control malignant melanoma, the most dangerous type of skin cancer.