Phenolic substrate

Fluorinated Muconates Formed from Fluorophenols by Phenol Hydroxylase and Catechol 1,2-Dioxygenase from Exophilia jeanselmei Fluoromuconate Metabolite Phenolic Substrate(s)... 

Gauvin, R. M. Rozenberg, H. Shimon, L. J. W. Ben-David, Y. Milstein, D. Osmium-mediated C—H and C—C bond cleavage of a phenolic substrate p-quinone methide and methylene arenium pincer complexes. Chem. Eur. J. 2007, 13, 1382-1393. 

Parry, A. D. and Edwards, R. (1994). Characterization of o-glucosyltransferases with activities toward phenolic substrates in alfalfa. Phytochemistry 37 655-661. Phamdelegue, M. H., Loublier, Y., Ducruet, V., Douault, P., Marilleau, R. and Etievant, P. (1994). Characterization of chemicals involved in honeybee-plant interactions. Grana 33 184-190. 

Baiocco P, Barreca AM, Fabbrini M, Galli C and Gentili P. 2003. Promoting laccase activity towards non-phenolic substrates a mechanistic investigation with some laccase—mediator systems. Org Biomol Chem 1(1) 191—197. 

The classic oxidizing systems of human myeloperoxidase and horseradish peroxidase were exploited for their well-known abilities to oxidize phenolic substrates. Under conditions of incubations, the following oxidation pathway was defined (155). Peroxidases are first converted to the oxidized... 

What are the main error sources in PAC experiments One of them may result from the calibration procedure. As happens with any comparative technique, the conditions of the calibration and experiment must be exactly the same or, more realistically, as similar as possible. As mentioned before, the calibration constant depends on the design of the calorimeter (its geometry and the operational parameters of its instruments) and on the thermoelastic properties of the solution, as shown by equation 13.5. The design of the calorimeter will normally remain constant between experiments. Regarding the adiabatic expansion coefficient (/), in most cases the solutions used are very dilute, so the thermoelastic properties of the solution will barely be affected by the small amount of solute present in both the calibration and experiment. The relevant thermoelastic properties will thus be those of the solvent. There are, however, a number of important applications where higher concentrations of one or more solutes have to be used. This happens, for instance, in studies of substituted phenol compounds, where one solute is a photoreactive radical precursor and the other is the phenolic substrate [297]. To meet the time constraint imposed by the transducer, the phenolic... 

The occurrence of an optimum frequency at 200 kHz was explained through a two step reaction pathway. In the first step water sonolysis produces radicals within the bubble. In step two the radicals must migrate to the bubble interface or into the bulk aqueous medium to form peroxide or react with the phenolic substrate. The authors suggest that the lower frequencies are the most efficient for the decomposition of molecules inside the bubble but a proportion of the radicals recombine inside the bubble at high temperature to form water thereby reducing the overall yield of H2O2 (Eqs.4.1 and 4.2). 

To elucidate some enzymatic characteristics of the isolated laccases I, II, and III, substrate specificities for several simple phenols, electrophoresis patterns, ultraviolet spectra, electron spin resonance spectra, copper content, and immunological similarities were investigated. Tyrosine, tannic acid, g c acid, hydroquinone, catechol, pyrogallol, p-cresol, homocatechol, a-naphthol, -naphthol, p-phenylenediamine, and p-benzoquinone as substrates. No differences in the specificities of these substrates was found. The UV spectra for the laccases under stucfy are shown in Figure 4. Laccase III displays three adsorption bands (280, 405, and 600nm), laccase II shows one band 280nm), and laccase I shows two bands (280 and 405 nm). These data appear to indicate differences in chemical structure. The results of the copper content analysis (10) and two-dimensional electrophoresis also indicate that these fractions are completely different proteins (10), Therefore, we may expect differences in substrate specificities between the three laccase fractions for more lignin-like substrates, yet no difference for some simple phenolic substrates. 

Mechanisms for other reactions of non-phenolic substrates catalyzed by the enzyme are also understood on the basis of cation radical intermediates (15,52). 

Thus, almost all the reactions of lignin substructure model dimers by the enzyme are explained on the basis of cation radical intermediates and their subsequent reactions with nucleophiles such as H2O and intramolecular hydroxyl groups, and with radicals such as di oxygen (for non-phenolic substrates), or on the basis of phenoxy radical intermediates (for phenolic substrates). 

The veiy different reactivities of hemocyanin and tyrosinase toward oxidizable substrates se n to be due to the presence of a substrate binding site in the latter. It appears, therefore, that the oxygenation of the phenol substrate occurs either by reaction of copper-bound pooxide or hydrc ieroxide with the ortho position of the copper-bound phenol (16) or that the 0-0 bond of the peroxide ligand is cleaved... 

A commonly used strategy for the higher nitration of phenolic substrates is to sulfonate the electron-rich aromatic ring before nitration. Sulfonic acid groups are electron withdrawing and... 

Nitrosation of phenolic substrates usually uses nitrous acid prepared in situ from a dilute mineral acid and an alkali metal nitrite. In general, for every phenolic group present in a substrate an equal number of nitroso groups can be introduced into the aromatic ring phenol, resorcinol and phloroglucinol react with nitrous acid to form 4-nitrosophenol, 2,4-dinitrosoresorcinol and 2,4,6-trinitrosophloroglucinol respectively. 

Some phenolic substrates are readily nitrated with nitric acid of 5 % concentration or lower. Such reactions are catalyzed by nitrous acid either already present in the nitric acid or from initial oxidation of the phenolic substrate. Reaction of the substrate with nitrous acid... 

Scheme 6.17 Product range of the 9-catalyzed tetrahydropyranylation of sterically hindered and phenolic substrates.

Sulfotransferases (SULTs) are important for the metabolism of a number of drugs, neurotransmitters, and hormones, especially the steroid hormones. The cosubstrate for these reactions is 3 -phosphoadenosine 5 -phosphosulfate (PAPS) (Fig. 4.1). Like the aforementioned enzymes, sulfate conjugation typically renders the compound inactive and more water soluble. However, this process can also result in the activation of certain compounds, such as the antihypertensive minoxidil and several of the steroid hormones. Seven SULT isoforms identified in humans, including SULTs lAl to 1A3, possess activity toward phenolic substrates such as dopamine, estradiol, and acetaminophen. SULTIBI possesses activity toward such endogenous substrates as dopamine and triiodothyronine. SULTIEI has substantial activity toward steroid hormones, especially estradiol and dehydroepiandrosterone, and toward the anti-... 

Herrmann, K. Oxidative enzymes and phenolic substrate in vegetables and fruit. I. Hydroxycinnamic acids. Z Lebensm-Unters Forsch 1957 106 341-348. 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

Two substrates are required in the tyrosinase-catalyzed reaction, phenolic substrate (dopa) and dioxygen. The conditions described in the experiment are such that the reaction mixtures are saturated with dissolved dioxygen. Therefore, when measurements are made for Ku, only the concentration of dopa is limiting, so the rate of the reaction depends on dopa concentration. The dopachrome assay is extremely flexible, as it can be applied to a variety of studies of tyrosinase. 

Two principal methods are widely used for the assay of DPOs. For enzyme kinetic studies, the most appropriate methods are polarographic and use an 02 electrode (Basic Protocol), which allows the direct measurement of the rate of utilization of oxygen and a true comparison of different phenolic substrates. Minor disadvantages of this method are that it requires more specialized equipment and that assays can only be carried out one at a time. Nevertheless it has proven to be the basis of some excellent undeigraduate biochemical laboratory experiments. 

The simplest, but less accurate, method of assaying DPO activity is to record the final color yield when the enzyme is incubated with a suitable phenolic substrate such as catechol, 4-methylcatechol, terr-butylcatechol, or 3,4-dihydroxyphenylalanine (Mayer et al., 1966). As with the polarographic assay system described in the Basic Protocol, it is advisable to establish an optimum working concentration of enzyme (range-finding Figure C4.1.3). 

In order to overcome the above limitations, several workers developed a chronometric assay that involves measuring the rate of loss of ascorbate in an o-DPO/phenolic substrate/ascorbate coupled system however, this is a cumbersome procedure. Other coupled assay systems have also been developed, some using quinone complexing agents such as Besthom s hydrazone (Pifferi and Baldassari, 1973 Espin et al., 1995). 

For a cleaner preparation, and to remove any residual phenolic substrates, the filtrate can be centrifuged at high speed (20,000 x g) at 4°C for 5 to 20 min. The supernatant is then decanted and tested for DPO activity. The precipitate is washed twice by resuspending in cold 1% KC1 and recentrifuging. Finally, the washed precipitate is resuspended in 1% KC1 or a suitable buffer and is stored at 4°C or deep-frozen. 

It should also be noted that peroxidases and mixed-function oxidases can also bring about the oxidation of phenolic substrates this can be checked by appropriate controls (e.g., addition of H202 to the assay system will stimulate peroxidase activity). 

---