Phenol, enzymatic reactions

Uriine (phenol) Enzymatic reaction HPLC/fluorometric detection 50 ppb 97.7 Jen and Tsai 1994... 

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Unfortunately, only a few studies have been done to investigate this matter mainly because of the difficulty of identifying the products of enzymatic reactions and the difficult task of assessing their toxicity. For example, it was demonstrated that solutions of phenols and chlorophenols... 

Waste-treatment processes commonly result in the production of solid wastes that must be disposed of safely. Enzymatic treatment is no exception. For example, although enzymatic treatment may not produce as large a quantity of solid products as does biological treatment, some solid residues may be formed, e.g., the polymer precipitates formed during the treatment of phenols with peroxidases, spent adsorbents such as talc, chitin, or activated carbon that are used to eliminate the soluble products of enzymatic reactions, or residues of plant materials such as raw soybean hulls when they are used in place of purified enzymes during treatment. Perhaps, the polymers and adsorbents could be incinerated to recover some energy if the emission of dangerous combustion by-products can be controlled or prevented. The residues of plant materials could potentially be composted and used as soil conditioners, provided that pollutants do not leach from them at substantial rates. To date, none of these disposal problems have been addressed adequately. 

Plant material has been successfully used to decontaminate water polluted with phenolic compounds through enzymatic reactions [24,104]. 

Brown BR, Bocks SM (1963) Some new enzymatic reaction of phenols. In Pridham JB (ed) Enzyme chemistry of phenolic compounds. Pergamon, Oxford... 

Using peroxidases in the presence of hydrogen peroxide, phenolic substrates are converted catalytically to phenoxy radicals (Fig. 8.1). The phenoxy radicals produced in the enzymatic reaction step can then, in postenzymatic reactions, couple with each other or with other reactive substances present in the system [28-32]. Self-coupling of phenol molecules with each other dominates in systems that lack appropriate substrates to participate in cross-coupling reactions with phenol, leading to formation of precipitated polymeric products [9, 33] that can be readily removed from water. 

In the particular case of phenolic compounds degradation, the polymeric products resulting from the enzymatic reaction cause the inactivation of the peroxidase. In that case, a strategy of drop wise addition of enzyme has been proposed, where the reactor receives fresh enzyme solution continuously. The rate of formation of free radicals is reduced and hence, their concentration falls and the amount of enzyme available grows, as a result of lower inactivation [9]. 

The rest of the amino acids have functional groups of various kinds and we shall deal with them by function. The simplest have hydroxyl groups and there are three of them—two alcohols and a phenol. Serine in particular is important as a reactive group in enzymatic reactions. It is a good nucleophile for carbonyl groups. 

The monoacylated derivative (6"-0-butyrate) was detected as the major product by H PLC analysis for all enzymatic reactions, while a diacylated derivative was also detected and confirmed by MS analysis [17]. As it can be seen in Table 9.3, higher regioselectivity of the process was observed in [bmim]BF4 than in [bmim]PF,5, while the selectivity was even lower in acetone. The lower regioselectivity of the enzymatic acylation of polyhydroxylated compounds in [bmim]PF,5 as well as in the organic solvent could be related to the lower solubility of unmodified phenolic substrates in these media, compared to that of their monoacylated derivatives [5, 16, 17]. On the other hand, the enhanced solubility of phenoHc substrates in [bmim]BF4 could explain the increased regioselectivity observed in this ionic liquid. 

Perhaps the most well-known peroxidase-catalyzed reactions are those involving electron transfer, in which an aromatic substrate is oxidized in a mono-electronic oxidation up to its mono-radical, Eq. (1), which is capable of participating further in a variety of non-enzymatic reactions such as disproportionation, polymerization and electron transfer. These types of reactions are very common during the peroxidase-catalyzed oxidation of phenols and, in some cases, during the oxidation of alkaloids. For example, peroxidase is capable of dimerizing jatrorrhizine (IV) to 4,4 -bis-jatrorrhizine (V) in the presence of H2O2 (Scheme III) [50]. 

Tyrosine can be part of low barrier hydrogen bonds in enzymatic reactions. This is snggested for ketosteroid isomerases . A fractionation factor of the COOH proton of Asp-99 (0.34) snpports this ". The phenol proton having a hydrogen bond to the steroid shows a fractionation factor of 0.97. The fractionation factors can be related to the O- O distance . 

Chi the basis of E. values we can make a conclusion about the difhision factors which are some of the most conqilicated points concerning catalysis with immobilized enzymes. The value for the activation energy on peroxidase oxidation of phenol with catalase immobilized on "NORIT" soot is E, =10.95 kJ.mof which is an indication that the process takes place under diSusion regime. The latter means that the enzymatic reaction rate is determined by the mass tranfer of substrate to the surfoce of the carrier particles and its diffiision into the carrier. 

In nature, the oxidative dimerization of phenols is controlled by enzymes, as is demonstrated by the axial chirality of the 6,8 -coupled juglone derivative isodiospyrin. In synthesis, however, phenol oxidation only proceeds in high yields when the enzymatic reaction control is replaced by substituent control, that is, if all but one of the positions with high spin density in the radical (ortho-and para positions) are blocked. 

Although the exact role of boron in plants is unknown, several physiological and biochemical activities associated with tissue boron content have been supported experimentally. This review covers some recent work on the role of boron in (1) organic translocation in plants, (2) enzymatic reactions, (3) plant growth regulator response, (4) cell division, (5) cell maturation, (6) nucleic acid metabolism, (7) phenolic acid biosynthesis, and (8) cell wall metabolism. 

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