A new continuous spectrophotometric assay method for DOPA oxidase activity of tyrosinase

Bacillus aryabhattai TFG5-mediated synthesis of humic substances from coir pith wastes

Background

Humic substances (HS) form the largest proportion among all the constituents of soil organic matter and are a key component of the terrestrial ecosystem. HS plays a multifunctional role in the environment by controlling the biogeochemical carbon cycle, providing nutrients and bio-stimulants for plant growth, and interacting with inorganic and organic pollutants. The rate of formation of HS in soils determines its productivity and carbon sequestration capacity. Enhancement of HS synthesis in the soil through the microbial route not only increases CO₂ sequestration but also mitigates the greenhouse gas emissions in the environment.

Result

In this study, we attempted to understand the mechanism of formation and enhancement of HS from coir pith wastes using the tyrosinase produced by Bacillus aryabhattai TFG5. The bacterium TFG5 isolated from the termite garden produced the tyrosinase (1.34 U mL⁻¹) and laccase (2.1 U mL⁻¹) at 48 h and 60 h of fermentation, respectively. The extracellular tyrosinase from B. aryabhattai TFG5 was designated as TyrB. Homology modeling of TyrB revealed a structure with a predicted molecular mass of 35.23 kDa and two copper ions in the active center with its conserved residues required for the tyrosinase activity. TyrB efficiently transformed and polymerized standard phenols, such as p-cresol, p-hydroxyl benzoic acid, Levo DOPA, and 2,6 DMP, besides transforming free phenols in coir pith wash water (CWW). Additionally, UV–Vis and FT-IR spectra of the degradation products of the coir pith treated with TyrB revealed the formation of HS within 3 days of incubation. Furthermore, the E472/664 ratio of the degradation products revealed a higher degree of condensation of the aromatic carbons and the presence of more aliphatic structures in the HS.

Conclusion

The results confirmed the influence of TyrB for the effective synthesis of HS from coir pith wastes. The results of the present study also confirm the recently accepted theory of humification proposed by the International Humic Substances Society.

Functionalized Carbon Quantum Dots with Dopamine for Tyrosinase Activity Monitoring and Inhibitor Screening: In Vitro and Intracellular Investigation

Sensitive assay of tyrosinase (TYR) activity is in urgent demand for both fundamental research and practical application, but the exploration of functional materials with good biocompatibility for its activity evaluation at the intracellular level is still challenging until now. In this work, we develop a convenient and real-time assay with high sensitivity for TYR activity/level monitoring and its inhibitor screening based on biocompatible dopamine functionalized carbon quantum dots (Dopa-CQDs). Dopamine with redox property was functionalized on the surface of carbon quantum dots to construct a Dopa-CQDs conjugate with strong bluish green fluorescence. When the dopamine moiety in Dopa-CQDs conjugate was oxidized to a dopaquinone derivative under specific catalysis of TYR, an intraparticle photoinduced electron transfer (PET) process between CQDs and dopaquinone moiety took place, and then the fluorescence of the conjugate could be quenched simultaneously. Quantitative evaluation of TYR activity was established in terms of the relationship between fluorescence quenching efficiency and TYR activity. The assay covered a broad linear range of up to 800 U/L with a low detection limit of 7.0 U/L. Arbutin, a typical inhibitor of TYR, was chosen as an example to assess its function of inhibitor screening, and positive results were observed that fluorescence quenching extent of the probe was reduced in the presence of arbutin. It is also demonstrated that Dopa-CQD conjugate possesses excellent biocompatibility, and can sensitively monitor intracellular tyrosinase level in melanoma cells and intracellular pH changes in living cells, which provides great potential in application of TYR/pH-associated disease monitoring and medical diagnostics.

On the interpretation of tyrosinase inhibition kinetics

Abstract Tyrosinases (monophenol monooxygenases, EC 1.14.18.1) utilize a reaction mechanism that involves two intertwined catalytic cycles and at least three different ligand-binding enzyme forms. Therefore a variety of different inhibition types may arise in inhibition experiments, depending on the binding mode of the compound studied. Here we discuss a steady-state equation that describes inhibition of the diphenolase cycle of tyrosinase catalysis in a general way. In addition, we employ numerical simulations to explore the kinetic outcome of various binding schemes. As the full equation is far too complex to be applicable for data evaluation by curve fitting, we propose to use the general modifier scheme of Botts-Morales for fitting and demonstrate that especially the value of parameter α of the equation allows conclusions about the binding mode of the inhibitor. The approach is exemplified by selected data describing the inhibition of human tyrosinase by typical inhibitors.

Biotransformation of sucrose into 5-hydroxy-2-hydroxymethyl-γ-pirone by Aspergillus flavus

The sucrose hydrolysis and the preference of consumption of glucose instead of fructose were investigated for the production of 5-hydroxy-2-hydroxymethyl-γ-pyrone (HHMP) in the presence of Aspergillus flavus IOC 3974 cultivated in liquid Czapeck medium. Standardized 0.5g of pellets were transferred as inoculum into twelve conical flasks of 250 ml containing 100 ml of medium with different sucrose concentration, which was kept at 120 rpm and 28"C for 16 days without pH adjustment. Aliquots of 500 μl of the broth culture were withdrawn at 24 h intervals and analyzed. The major yield of HHMP was 26g l(-1) in 120g l(-1) of sucrose. At these conditions, A. flavus produced an invertase capable of hydrolyzing 65% of total sucrose concentration in 24h, and an isomerase capable of converting fructose into glucose. In this work, it focused the preference for glucose and, then, of fructose by A. flavus and the strategy used to produce HHMP.

Biocatalytic formation of synthetic melanin: the role of vanadium haloperoxidases, L-DOPA and iodide

The vanadium haloperoxidase (V-HPO) enzyme, extracted from the brown alga Laminaria saccharina, is able to catalyze the formation of a black precipitate, using as precursor the amino acid L-dopa in the presence of hydrogen peroxide and iodide, in one-pot synthesis. The L-dopa oxidation is a multistep reaction with a crucial role played by the iodide in the enzyme catalyzed peroxidative production of dopachrome, a well known intermediate in the synthesis of melanin. Dopachrome is then converted to a synthetic form of melanin through a polymerization reaction. Factors, such as buffer composition and pH, influence significantly the reaction first steps, but further steps of melanin production are hardly influenced. The biosynthetic melanin produced through the combination V-HPO/I/H(2)O(2), was characterized by several spectroscopic techniques (UV-vis and FT-IR) as well as XRD. Moreover, this biopolymer is light sensitive, decomposing into oligo- and monomeric units. Scanning electron microscopy (SEM) imaging showed different morphologies when compared with commercial available melanin. The biosynthetic production of melanin can have a wide range of applications from photosensitive cells to biomedicine with the advantage of being produced under eco-friendly and mild conditions.

A review on spectrophotometric methods for measuring the monophenolase and diphenolase activities of tyrosinase

Tyrosinase is a copper enzyme with broad substrate specifity toward a lot of phenols with different biotechnological applications. The availability of quick and reliable measurement methods of the enzymatic activity of tyrosinase is of outstanding interest. A series of spectrophotometric methods for determining the monophenolase and diphenolase activities of tyrosinase are discussed. The product of both reactions is the o-quinone of the corresponding monophenol/diphenol. According to the stability and properties of the o-quinone, the substrate is classified as four substrate types. For each of these substrate types, we indicate the best method for measuring diphenolase activity (among eight methods) and, when applicable, for measuring monophenolase activity (among four methods). The analytical and numerical solutions to the system of differential equations corresponding to the reaction mechanism of each case confirm the underlying validity of the different spectrophotometric methods proposed for the kinetic characterization of tyrosinase in its action on different substrates.

The inhibition kinetics and thermodynamic changes of tyrosinase via the zinc ion

We found that Zn(2+) conspicuously inactivated tyrosinase in a mixed-type inhibition manner: the final level of residual activity was abolished at the equilibrium state with concentration of 0.25 mM Zn(2+). Changes of both K(m) and V(max) by various concentrations of Zn(2+) in Lineweaver-Burk plot were observed. To see whether Zn(2+) also induced conformational change of tyrosinase and how thermodynamical changes by ligand binding were occurred, the intrinsic fluorescence studies as well as calorimetric measurements were conducted. The results showed that the Zn(2+) binding to tyrosinase directly induced conformational change of tyrosinase, and the changes of thermodynamic parameters such as enthalpy (DeltaH), Gibbs free-energy (DeltaG), and entropy (DeltaS) were obtained as 60+/-7.0 kJ/mol, -14.54 kJ/mol and 248.53 J/(K mol), respectively. The inactivating effect of Zn(2+) on tyrosinase was completely prevented by incubation with bovine serum albumin, which has a Zn(2+) binding motif in its structure. We suggested that Zn(2+) ligand-binding affected the substrate's accessibility due to the conformational changes and thus, the complex type of inhibition has occurred with the calorimetric changes.

A new type of uncompetitive inhibition of tyrosinase induced by Cl– binding

A new type of Cl- induced inhibition of mushroom derived tyrosinase has been detected in this study, and it is defined as the reversible partial hyperbolic uncompetitive inhibition. The Cl- binding site was only induced at the state of the enzyme-substrate complex, and this was confirmed with the intrinsic fluorescence changes. As the oxygen bridge is broken by L-DOPA binding, Cl- simultaneously binds to the ES state to form the ESI complex. It is worth noticing that tyrosinase reacts sensitively to Cl- in the manner of a complex interaction, and this indicates that Cl- might be physiologically involved in the regulation of tyrosinase activity.