An approximate analytical solution to the lag period of monophenolase activity of tyrosinase

Molecular Docking Studies of Ortho-Substituted Phenols to Tyrosinase Helps Discern If a Molecule Can Be an Enzyme Substrate

Phenolic compounds with a position ortho to the free phenolic hydroxyl group occupied can be tyrosinase substrates. However, ortho-substituted compounds are usually described as inhibitors. The mechanism of action of tyrosinase on monophenols is complex, and if they are ortho-substituted, it is more complicated. It can be shown that many of these molecules can become substrates of the enzyme in the presence of catalytic o-diphenol, MBTH, or in the presence of hydrogen peroxide. Docking studies can help discern whether a molecule can behave as a substrate or inhibitor of the enzyme. Specifically, phenols such as thymol, carvacrol, guaiacol, eugenol, isoeugenol, and ferulic acid are substrates of tyrosinase, and docking simulations to the active center of the enzyme predict this since the distance of the peroxide oxygen from the oxy-tyrosinase form to the ortho position of the phenolic hydroxyl is adequate for the electrophilic attack reaction that gives rise to hydroxylation occurring.

Considerations about the inhibition of monophenolase and diphenolase activities of tyrosinase. Characterization of the inhibitor concentration which generates 50 % of inhibition, type and inhibition constants. A review

Tyrosinase is a copper oxidase enzyme which catalyzes the first two steps in the melanogenesis pathway, L-tyrosine to L-dopa conversion and, then, to o-dopaquinone and dopachrome. Hypopigmentation and, above all, hyperpigmentation issues can be originated depending on their activity. This enzyme also promotes the browning of fruits and vegetables. Therefore, control of their activity by regulators is research topic of great relevance. In this work, we consider the use of inhibitors of monophenolase and diphenolase activities of the enzyme in order to accomplish such control. An experimental design and data analysis which allow the accurate calculation of the degree of inhibition of monophenolase activity (iM) and diphenolase activity (iD) are proposed. The IC50 values (amount of inhibitor that causes 50 % inhibition at a fixed substrate concentration) can be calculated for the two activities and from the values of IC50M (monophenolase) and IC50D(diphenolase). Additionally, the strength and type of inhibition can be deduced from these values. The data analysis from these IC50D values allows to obtain the values of [Formula: see text] or [Formula: see text] , or and [Formula: see text] from the values of IC50M. In all cases, the values of the different must satisfy their relationship with IC50M and IC50D.

The Relationship between the IC50 Values and the Apparent Inhibition Constant in the Study of Inhibitors of Tyrosinase Diphenolase Activity Helps Confirm the Mechanism of Inhibition

Tyrosinase is the enzyme involved in melanization and is also responsible for the browning of fruits and vegetables. Control of its activity can be carried out using inhibitors, which is interesting in terms of quantitatively understanding the action of these regulators. In the study of the inhibition of the diphenolase activity of tyrosinase, it is intriguing to know the strength and type of inhibition. The strength is indicated by the value of the inhibition constant(s), and the type can be, in a first approximation: competitive, non-competitive, uncompetitive and mixed. In this work, it is proposed to calculate the degree of inhibition (iD), varying the concentration of inhibitor to a fixed concentration of substrate, L-dopa (D). The non-linear regression adjustment of iD with respect to the initial inhibitor concentration [I]0 allows for the calculation of the inhibitor concentration necessary to inhibit the activity by 50%, at a given substrate concentration (IC₅₀), thus avoiding making interpolations between different values of iD. The analytical expression of the IC₅₀, for the different types of inhibition, are related to the apparent inhibition constant (KIapp). Therefore, this parameter can be used: (a) To classify a series of inhibitors of an enzyme by their power. Determining these values at a fixed substrate concentration, the lower IC₅₀, the more potent the inhibitor. (b) Checking an inhibitor for which the type and the inhibition constant have been determined (using the usual methods), must confirm the IC₅₀ value according to the corresponding analytical expression. (c) The type and strength of an inhibitor can be analysed from the study of the variation in iD and IC₅₀ with substrate concentration. The dependence of IC₅₀ on the substrate concentration allows us to distinguish between non-competitive inhibition (iD does not depend on [D]0) and the rest. In the case of competitive inhibition, this dependence of iD on [D]0 leads to an ambiguity between competitive inhibition and type 1 mixed inhibition. This is solved by adjusting the data to the possible equations; in the case of a competitive inhibitor, the calculation of KI1app is carried out from the IC₅₀ expression. The same occurs with uncompetitive inhibition and type 2 mixed inhibition. The representation of iD vs. n, with n=[D]0/KmD, allows us to distinguish between them. A hyperbolic iD vs. n representation that passes through the origin of coordinates is a characteristic of uncompetitive inhibition; the calculation of KI2app is immediate from the IC₅₀ value. In the case of mixed inhibitors, the values of the apparent inhibition constant of meta-tyrosinase (Em) and oxy-tyrosinase (Eox), KI1app and the apparent inhibition constant of metatyrosinase/Dopa complexes (EmD) and oxytyrosinase/Dopa (EoxD), KI2app are obtained from the dependence of iD vs. n, and the results obtained must comply with the IC₅₀ value.

Tyrosinase inhibitory mechanism and the anti-browning properties of piceid and its ester

Different mechanisms for inhibiting tyrosinase can be exploited to avoid quality losses caused by the enzymatic browning of fruits and vegetables. Piceid (PI) and piceid 6″-O- azelaic acid ester (PIA) are oxidized by tyrosinase; however, their oxidation products may have inhibitory effects on tyrosinase. This notion is because l-DOPA oxidation was inhibited after the pre-incubation of PI/PIA with tyrosinase, however, l-DOPA oxidation was not affected if this pre-incubation was not performed. Circular dichroism analysis indicated a conformational change in the secondary structure of tyrosinase after pre-incubation. Further, molecular docking and enzyme reaction kinetics assays were employed to reveal the mechanism underlying the effects of PI/PIA on tyrosinase in the absence of pre-incubation with tyrosinase. PI/PIA had anti-browning effects in the potato models. The increased rate of A₄₂₀ in PI/PIA groups at 24 h were 281% and 279%, which were approximately 2.4- and 2.5-fold lower than that of control (668%).

Study of tyrosine and dopa enantiomers as tyrosinase substrates initiating l- and d-melanogenesis pathways

Tyrosinase starts melanogenesis and determines its course, catalyzing the oxidation by molecular oxygen of tyrosine to dopa, and that of dopa to dopaquinone. Then, nonenzymatic coupling reactions lead to dopachrome, which evolves toward melanin. Recently, it has been reported that d-tyrosine acts as tyrosinase inhibitor and depigmenting agent. The action of tyrosinase on the enantiomers of tyrosine (l-tyrosine and d-tyrosine) and dopa (l-dopa and d-dopa) was studied for the first time focusing on quantitative transient phase kinetics. Post-steady-state transient phase studies revealed that l-dopachrome is formed more rapidly than d-dopachrome. This is due to the lower values of Michaelis constants for l-enantiomers than for d-enantiomers, although the maximum rates are equal for both enantiomers. A deeper analysis of the inter-steady-state transient phase of monophenols demonstrated that the enantiomer d-tyrosine causes a longer lag period and a lower steady-state rate, than l-tyrosine at the same concentration. Therefore, d-melanogenesis from d-tyrosine occurs more slowly than does l-melanogenesis from l-tyrosine, which suggests the apparent inhibition of melanin biosynthesis by d-tyrosine. As conclusion, d-tyrosine acts as a real substrate of tyrosinase, with low catalytic efficiency and, therefore, delays the formation of d-melanin.

A family AA5_2 carbohydrate oxidase from Penicillium rubens displays functional overlap across the AA5 family

Copper radical alcohol oxidases belonging to auxiliary activity family 5, subfamily 2 (AA5_2) catalyze the oxidation of galactose and galactosides, as well as aliphatic alcohols. Despite their broad applied potential, so far very few AA5_2 members have been biochemically characterized. We report the recombinant production and biochemical characterization of an AA5_2 oxidase from Penicillium rubens Wisconsin 54–1255 (PruAA5_2A), which groups within an unmapped clade phylogenetically distant from those comprising AA5_2 members characterized to date. PruAA5_2 preferentially oxidized raffinose over galactose; however, its catalytic efficiency was 6.5 times higher on glycolaldehyde dimer compared to raffinose. Deep sequence analysis of characterized AA5_2 members highlighted amino acid pairs correlated to substrate range and conserved within the family. Moreover, PruAA5_2 activity spans substrate preferences previously reported for AA5 subfamily 1 and 2 members, identifying possible functional overlap across the AA5 family.

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.

Spectrophotometric Characterization of the Action of Tyrosinase on p-Coumaric and Caffeic Acids: Characteristics of o-Caffeoquinone

New methods are proposed to determine the activity of tyrosinase on caffeic and p-coumaric acids. Because o-quinone from caffeic acid is unstable in its presence, it has been characterized through spectrophotometric measurements of the disappearance of coupled reducing agents, such as nicotinamide adenine dinucleotide reduced form. It has also been characterized by a chronometric method, measuring the time that a known concentration of ascorbic acid takes to be consumed. The activity on p-coumaric acid has been followed by measuring the formation of o-quinone of caffeic acid at the isosbestic point originated between caffeic acid and o-caffeoquinone and measuring the formation of o-quinone at 410 nm, which is stable in the presence of p-coumaric acid (both of them in the presence of catalytic amounts of caffeic acid, maintaining the ratio between p-coumaric acid and caffeic acid constant; R = 0.025). The k_cat value of tyrosinase obtained for caffeic acid was higher than that obtained for p-coumaric acid, while the affinity was higher for p-coumaric acid. These values agree with those obtained in docking studies involving these substrates and oxytyrosinase.

Kinetic characterization of substrate-analogous inhibitors of tyrosinase

The development of effective tyrosinase inhibitors has become increasingly important in the cosmetic, medicinal, and agricultural industries for application as antibrowning and depigmenting agents. The kinetic mechanisms of action of tyrosinase on monophenols and o-diphenols are complex, particularly in the case of monophenols because of the lag period that occurs at the beginning of the reaction. When enzyme inhibitors are studied, the problem becomes more complicated because the lag period increases, which has led to erroneous identification of the type of inhibition that many compounds exert on the monophenolase activity and the inaccurate determination of their inhibition constants. When the degrees of inhibition of an inhibitor which is analogous to tyrosinase substrates are the same for both monophenolase and diphenolase activities, this means that the inhibitor binds to the same enzymatic species and so the inhibition constants should be similar for both activities. In this study, we demonstrate this typical behavior of substrate-analogous inhibitors and propose a methodology for determining the type of inhibition and the inhibition constants for the monophenolase and diphenolase activities of the enzyme. Benzoic acid and cinnamic acid were used as inhibitors and the monophenol/o-diphenol pairs l-tyrosine/l-dopa and α-methyl-L-tyrosine/α-methyl-L-dopa as substrates.

Mechanistic studies of the tyrosinase-catalyzed oxidative cyclocondensation of 2-aminophenol to 2-aminophenoxazin-3-one

Tyrosinase (EC 1.14.18.1) catalyzes the monophenolase and diphenolase reaction associated with vertebrate pigmentation and fruit/vegetable browning. Tyrosinase is an oxygen-dependent, dicopper enzyme that has three states: Emet, Eoxy, and Edeoxy. The diphenolase activity can be carried out by both the met and the oxy states of the enzyme while neither mono- nor diphenolase activity results from the deoxy state. In this study, the oxidative cyclocondensation of 2-aminophenol (OAP) to the corresponding 2-aminophenoxazin-3-one (APX) by mushroom tyrosinase was investigated. Using a combination of various steady- and pre-steady state methodologies, we have investigated the kinetic and chemical mechanism of this reaction. The kcat for OAP is 75 ± 2s(-1), K(OAP)M = 1.8 ± 0.2mM, K(O2)M =25 ± 4 μM with substrates binding in a steady-state preferred fashion. Stopped flow and global analysis support a model where OAP preferentially binds to the oxy form over the met (k7 ≫ k1). For the met form, His269 and His61 are the proposed bases, while the oxy form uses the copper-peroxide and His61 for the sequential deprotonation of anilinic and phenolic hydrogens. Solvent KIEs show proton transfer to be increasingly rate limiting for kcat/K(OAP)M as [O2] → 0 μM (1.38 ± 0.06) decreasing to 0.83 ± 0.03 as [O2] → ∞ reflecting a partially rate limiting μ-OH bond cleavage (E met) and formation (E oxy) following protonation in the transition state. The coupling and cyclization reactions of o-quinone imine and OAP pass through a phenyliminocyclohexadione intermediate to APX, forming at a rate of 6.91 ± 0.03 μM(-1)s(-1) and 2.59E-2 ± 5.31E-4s(-1). Differences in reactivity attributed to the anilinic moiety of OAP with o-diphenols are discussed.

Discrimination between alternative substrates and inhibitors of tyrosinase

Many phenolic compounds have been described in the scientific literature as inhibitors of tyrosinase. In this work a test is proposed that allows us to distinguish whether a molecule is an enzyme inhibitor or substrate. The test has several stages. First, the degree of inhibition of the studied molecule is determined on the monophenolase activity (i(M)) and on the diphenolase activity (i(D)). If i(M) = i(D), it is an inhibitor. If i(M) ≠ i(D), the molecule could be substrate or inhibitor. Several additional stages are proposed to solve this ambiguity. The study described herein was carried out using the following molecules: benzoic acid, cinnamic acid, guaiacol, isoeugenol, carvacrol, 4-tert-butylphenol, eugenol, and arbutin.

Bacterial tyrosinases: old enzymes with new relevance to biotechnology

Tyrosinases are copper-containing dioxygen activating enzymes found in many species of bacteria and are usually associated with melanin production. These proteins have a strong preference for phenolic and diphenolic substrates and are somewhat limited in their reaction scope, always producing an activated quinone as product. Despite this fact they have potential in several biotechnological applications, including the production of novel mixed melanins, protein cross-linking, phenolic biosensors, production of l-DOPA, phenol and dye removal and biocatalysis. Although most studies have used Streptomyces sp. enzymes, there are several other examples of these proteins that are also of potential interest. For instance a solvent tolerant enzyme has been described, as well as an enzyme with both tyrosinase and laccase activities, enzymes with altered substrate preferences, an enzyme produced as an inactive zymogen as well as examples which do not require auxiliary proteins for copper insertion (unlike the Streptomyces sp. enzymes which do require such a protein). This article will summarise the reports on the biotechnological applications of bacterial tyrosinases as well as the current information available on the different types of this enzyme.

Generation of hydrogen peroxide in the melanin biosynthesis pathway

The generation of H(2)O(2) in the melanin biosynthesis pathway is of great importance because of its great cytotoxic capacity. However, there is controversy concerning the way in which H(2)O(2) is generated in this pathway. In this work we demonstrate that it is generated in a series of chemical reactions coupled to the enzymatic formation of o-quinones by tyrosinase acting on monophenols and o-diphenols and during the auto-oxidation of the o-diphenols and other intermediates in the pathway. The use of the enzymes such as catalase, superoxide dismutase and peroxidase helps reveal the H(2)O(2) generated. Based on the results obtained, we propose a scheme of enzymatic and non-enzymatic reactions that lead to the biosynthesis of melanins, which explains the formation of H(2)O(2).

Phenolic substrates and suicide inactivation of tyrosinase: kinetics and mechanism

The suicide inactivation mechanism of tyrosinase acting on its substrates has been studied. The kinetic analysis of the proposed mechanism during the transition phase provides explicit analytical expressions for the concentrations of o-quinone against time. The electronic, steric and hydrophobic effects of the substrates influence the enzymatic reaction, increasing the catalytic speed by three orders of magnitude and the inactivation by one order of magnitude. To explain the suicide inactivation, we propose a mechanism in which the enzymatic form Eox (oxy-tyrosinase) is responsible for such inactivation. A key step might be the transfer of the C-1 hydroxyl group proton to the peroxide, which would act as a general base. Another essential step might be the axial attack of the o-diphenol on the copper atom. The rate constant of this reaction would be directly related to the strength of the nucleophilic attack of the C-1 hydroxyl group, which depends on the chemical shift of the carbon C-1 (δ1) obtained by 13C-NMR. Protonation of the peroxide would bring the copper atoms together and encourage the diaxial nucleophilic attack of the C-2 hydroxyl group, facilitating the co-planarity with the ring of the copper atoms and the concerted oxidation/reduction reaction, and giving rise to an o-quinone. The suicide inactivation would occur if the C-2 hydroxyl group transferred the proton to the protonated peroxide, which would again act as a general base. In this case, the co-planarity between the copper atom, the oxygen of the C-1 and the ring would only permit the oxidation/reduction reaction on one copper atom, giving rise to copper(0), hydrogen peroxide and an o-quinone, which would be released, thus inactivating the enzyme.