Tyrosinase kinetics: a semi-quantitative model of the mechanism of oxidation of monohydric and dihydric phenolic substrates--reply

Biosynthesis of betalains: yellow and violet plant pigments

Betalains are the yellow and violet pigments that substitute anthocyanins in plants belonging to the order Caryophyllales. These pigments have attracted much attention because of their bioactivities, which range from an antioxidant capacity to the chemoprevention of cancer. However, the biosynthetic pathway of betalains remains under discussion; the main steps have been characterized in recent years, but multiple side reactions are possible. The key enzymes involved have only recently been described, providing clues about the regulation of betalain biosynthesis. In this review, we provide a comprehensive view of the biosynthetic scheme of betalains and discuss the different reactions that have been demonstrated experimentally or proposed in the literature.

Redox biocatalysis and metabolism: molecular mechanisms and metabolic network analysis

Whole-cell biocatalysis utilizes native or recombinant enzymes produced by cellular metabolism to perform synthetically interesting reactions. Besides hydrolases, oxidoreductases represent the most applied enzyme class in industry. Oxidoreductases are attributed a high future potential, especially for applications in the chemical and pharmaceutical industries, as they enable highly interesting chemistry (e.g., the selective oxyfunctionalization of unactivated C-H bonds). Redox reactions are characterized by electron transfer steps that often depend on redox cofactors as additional substrates. Their regeneration typically is accomplished via the metabolism of whole-cell catalysts. Traditionally, studies towards productive redox biocatalysis focused on the biocatalytic enzyme, its activity, selectivity, and specificity, and several successful examples of such processes are running commercially. However, redox cofactor regeneration by host metabolism was hardly considered for the optimization of biocatalytic rate, yield, and/or titer. This article reviews molecular mechanisms of oxidoreductases with synthetic potential and the host redox metabolism that fuels biocatalytic reactions with redox equivalents. The tools discussed in this review for investigating redox metabolism provide the basis for studies aiming at a deeper understanding of the interplay between synthetically active enzymes and metabolic networks. The ultimate goal of rational whole-cell biocatalyst engineering and use for fine chemical production is discussed.

Fungal tyrosinases: new prospects in molecular characteristics, bioengineering and biotechnological applications

Tyrosinases are type-3 copper proteins involved in the initial step of melanin synthesis. These enzymes catalyse both the o-hydroxylation of monophenols and the subsequent oxidation of the resulting o-diphenols into reactive o-quinones, which evolve spontaneously to produce intermediates, which associate in dark brown pigments. In fungi, tyrosinases are generally associated with the formation and stability of spores, in defence and virulence mechanisms, and in browning and pigmentation. First characterized from the edible mushroom Agaricus bisporus because of undesirable enzymatic browning problems during postharvest storage, tyrosinases were found, more recently, in several other fungi with relevant insights into molecular and genetic characteristics and into reaction mechanisms, highlighting their very promising properties for biotechnological applications. The limit of these applications remains in the fact that native fungal tyrosinases are generally intracellular and produced in low quantity. This review compiles the recent data on biochemical and molecular properties of fungal tyrosinases, underlining their importance in the biotechnological use of these enzymes. Next, their most promising applications in food, pharmaceutical and environmental fields are presented and the bioengineering approaches used for the development of tyrosinase-overproducing fungal strains are discussed.

Indirect oxidation of 6-tetrahydrobiopterin by tyrosinase

6-Tetrahydrobiopterin is known to bind to an allosteric site of tyrosinase to directly inhibit the enzyme. However, simultaneous measurements of ultraviolet-visible absorption spectra and oxygen consumption led us to conclude that the inhibition was due to oxidation of 6-tetrahydrobiopterin by dopaquinone. Immediately after addition of 6-tetrahydrobiopterin, tyrosinase stopped producing dopachrome from either tyrosine or dopa. Duration of inhibition was proportional to the concentration of added 6-tetrahydrobiopterin and the enzyme activity was fully restored after the inhibition. Surprisingly, there was a rapid consumption of oxygen during the inhibition period. In addition, absorption spectra indicated that the only reaction that occurred during the inhibition was oxidation of 6-tetrahydrobiopterin to 7,8-dihydrobiopterin. In the absence of tyrosine or dopa, tyrosinase did not oxidize 6-tetrahydrobiopterin, suggesting that a reaction intermediate between dopa and dopachrome was a target for the inhibition. We propose a new mechanism in which dopa is oxidized to dopaquinone and the latter, instead of producing dopachrome, is reduced back to dopa by 6-tetrahydrobiopterin.

Tyrosinase kinetics: discrimination between two models to explain the oxidation mechanism of monophenol and diphenol substrates

The kinetic behaviour of tyrosinase is very complex because the enzymatic oxidation of monophenol and o-diphenol to o-quinones occurs simultaneously with the coupled non-enzymatic reactions of the latter. Both reaction types are included in the kinetic mechanism proposed for tyrosinase (Mechanism I [J. Biol. Chem. 267 (1992) 3801-3810]). We previously confirmed the validity of the rate equations by the oxidation of numerous monophenols and o-diphenols catalysed by tyrosinase from different fruits and vegetables. Other authors have proposed a simplified reaction mechanism for tyrosinase (Mechanism II [Theor. Biol. 203 (2000) 1-12]), although without deducing the rate equations. In this paper, we report new experimental work that provides the lag period value, the steady-state rate, o-diphenol concentration released to the reaction medium. The contrast between these experimental data and the respective numerical simulations of both mechanisms demonstrates the feasibility of Mechanism I. The need for the steps omitted from Mechanism II to interpret the experimental data for tyrosinase, based on the rate equations previously deduced for Mechanism I is explained.

Catalytic oxidation of acetaminophen by tyrosinase in the presence of L-proline: a kinetic study

A kinetic study of acetaminophen oxidation by tyrosinase in the presence of a physiological nucleophilic agent such as the amino acid L-proline is performed in the present paper. The o-quinone product of the catalytic activity, 4-acetamido-o-benzoquinone, becomes unstable through the chemical addition of L-proline, in competition with the nucleophilic addition of hydroxide ion from water. In both cases, the catechol intermediate, 3(')-hydroxyacetaminophen, is generated, as can be demonstrated by liquid chromatography. When the effect of the presence of the nucleophilic agent on the time course of the enzymatic reaction was kinetically analyzed, it was seen to decrease the duration of the lag period and increase the steady-state rate. Rate constants for the reaction of 4-acetamido-o-benzoquinone with water and L-proline were also determined. The results obtained in this paper open a new possibility to acetaminophen toxicity, that has been attributed hitherto to its corresponding p-quinone, N-acetyl-p-benzoquinone imine.