Enzymes of phenolic metabolism

With regard to the first point, most of the final enzymes of phenolic metabolism, that is, those that give rise to the formation of lignin, have not been characterized nor are their biochemical properties known, and therefore in the future it would be useful to elucidate these stages of phenylpropanoid metabolism. 

Fig. (1). Phenolic metabolism. PEP (phospho enol pyruvate) E4P (erythrose 4-phosphate) DAHP (3-Deoxy-D-arabino-heptulosonate 7-phosphate) PAL (phenylalanine ammonia lyase). Stress transcriptionally activates the key enzymes of phenolic metabolism (PAL and DAHP synthase).

Table III. Effect of 0.5 mM glyphosate on extractable activity of four enzymes of phenolic metabolism from wheat root tips. Adapted from Cole et al. (72).

Brackmann R, G Fuchs (1993) Enzymes of anaerobic metabolism of phenolic compounds. 4-hydroxybenzoyl-CoA reductase (dehydroxylating) from a denitrifying Pseudomonas sp. Eur J Biochem 213 563-571. 

The metabolism of phenolics is regulated by the activity of various enzymes. As indicated above, the main and determining enzyme of phenolic synthesis is PAL, while in oxidation processes, the enzymes involved are peroxidase (POD) and primarily polyphenoloxidase (PPO). 

Xenoblotics can alter various metabolic pathways In plants. Chemical alteration of phenolic compounds for agricultural purposes has been suggested to have potential (130. 131). Since several major plant enzymes of secondary metabolism are Involved In resistance to pathogens, alteration of secondary metabolism could Influence disease development. Herbicides, plant growth regulators (synthetic and natural), and enzyme Inhibitors such as those presented for PAL (Fig. 

Ibrahim, R. K., Immunolocalization of flavonoid conjugates and their enzymes in Phenolic Metabolism in Plants (H. A. Stafford and R. K. Ibrahim, eds.), Recent Advances in Phytochemistry Vol. 26, 25-61, Plenum Press, New York, 1992. 

Nonfreezing low temperature stress on phenolics metabolism has been considered in several papers. These studies have shown that the phenolic metabolism is enhanced under chill stress and that the behavior of the same metabolism is further dependent on the storage temperature. There is a low critical temperature below which an increase of phenolic metabolism is stimulated, and this temperature is related to the threshold temperature at which chilling injury is also induced. It has been also observed that the low temperarnre effect involves a cold-induced stimulatimi of the phenylalanine ammonia-lyase activity (PAL, EC 4.3.1.5) as well as other enzymes important in the... 

In some cases, microorganisms can transform a contaminant, but they are not able to use this compound as a source of energy or carbon. This biotransformation is often called co-metabolism. In co-metabolism, the transformation of the compound is an incidental reaction catalyzed by enzymes, which are involved in the normal microbial metabolism.33 A well-known example of co-metabolism is the degradation of (TCE) by methanotrophic bacteria, a group of bacteria that use methane as their source of carbon and energy. When metabolizing methane, methanotrophs produce the enzyme methane monooxygenase, which catalyzes the oxidation of TCE and other chlorinated aliphatics under aerobic conditions.34 In addition to methane, toluene and phenol have been used as primary substrates to stimulate the aerobic co-metabolism of chlorinated solvents. 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

Metabolism of BP mediated by the cytochrome P-450 monooxygenase system forms three classes of products phenols, dihydrodiols and quinones. Formation of phenols and dihydrodiols is obtained by an initial electrophilic attack of an enzyme-generated oxygen atom. 

Polyphenol oxidase occurs within certain mammalian tissues as well as both lower (46,47) and higher (48-55) plants. In mammalian systems, the enzyme as tyrosinase (56) plays a significant role in melanin synthesis. The PPO complex of higher plants consists of a cresolase, a cate-cholase and a laccase. These copper metalloproteins catalyze the one and two electron oxidations of phenols to quinones at the expense of 02. Polyphenol oxidase also occurs in certain fungi where it is involved in the metabolism of certain tree-synthesized phenolic compounds that have been implicated in disease resistance, wound healing, and anti-nutrative modification of plant proteins to discourage herbivory (53,55). This protocol presents the Triton X-114-mediated solubilization of Vida faba chloroplast polyphenol oxidase as performed by Hutcheson and Buchanan (57). 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

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