Enzymatic reactions environmental factors

The rate of an enzymatic reaction is affected by a number of environmental factors, such as solvent, ionic strength, temperature, pH, and presence of inhibitor/activator. Some of these effects are described below. 

We will consider in this chapter the general processes by which enzymes achieve enhancement of reaction rates, basic chemical and enzymatic kinetics and inhibition, the roles of cofactors and coenzymes, the effects of environmental factors, the regulation of enzyme activity, and some clinical applications of enzymology. 

The occurrence of polychlorinated aromatic sulfur compounds in the environment has been reported during the last decade. PCDDs and PCDFs are formed by different chemical,photochemical and enzymatic reactions [10], It is possible that PCDTs and PCTAs are formed by chemical processes similar to those of the formation of PCDFs and PCDDs. Until now, PCDTs have been analyzed in stack gas and fly ash samples, in sediments, pulp mill effluents, and in some aquatic organisms. Some PCTA congeners have been observed in pulp mill effluents, stack gas, and soil and compost samples. The compound 2468-TeCDT has been found to accumulate from environmentally contaminated sediments to sandworms, clams, and grass shrimp. Accumulation factors have been calculated [11]. 

The denitrification process could be described as a modular organization in which every biochemical reaction is catalyzed by specific reductase enzymes (Cuervo-Lopez et al., 2009). Four enzymatic reactions take place in the cell as follows (l) nitrate is reduced to nitrite by nitrate reductase (Nar) (ii) a subsequent reduction of nitrite to nitric oxide is carried out by nitrite reductase (Mr) (iu) afterwards, nitric oxide is reduced to nitrous oxide by the enzyme nitric oxide reductase (Nor) (iz ) finally, nitrous oxide is reduced to N2 by the enzyme nitrous oxide reductase (Nos) (Lalucat et al., 2006) (Table 9). These reactions take place when environmental conditions become anaerobic (Berks et al., 1995 Hochstein Tomlinson, 1988). The enzymatic reactions, which are thermodynamically favored, are carried out in the cell membrane and periplasmic space. Small half saturation constant values (Km) have been reported for different nitrogen substrates for some denitrifying bacteria, indicating that denitrifying enzymes have a high affinity for their substrate. However, several factors have to be considered, as the presence of small quantities of molybdenum, cooper and hem to ensure the successful enzymatic activity, as they are known cofactors for denitrifying enzymes. 

The course of an enzyme reaction is affected by various environmental factors (pH, temperature, ionic strength, presence of different substances, etc.). The substances that affect the course of an enzyme reaction positively are called activators (or positive effectors) and those affecting it negatively are called inhibitors (or negative effectors). These effectors may be assessed enzymatically as well as substrates. In the case of an activator, the degree of reaction rate increase is related to the activator concentration and, in contrast, the degree of the reaction rate decrease corresponds to the concentration of an inhibitor [22]. 

If at low [ADP] the reaction rate is limited by a relatively slow translocation of nucleotides near the enzyme compared with the rate of catalysis in situ, this limitation should also be relieved by reducing the catalytic ability of the enzyme independently of substrate concentration. This is illustrated in Fig. 6, assum ing that the limiitation is proportional to the stimiulation observed after hypotonic treatment. These results show that the rate of phosphorylation may be limited by either enzymatic catalysis per se, or by extrinsic environmental factors. 

The toxieity of triehloroethylene is dependent upon metabolism and induction of cytochrome P450. Triehloroethylene is metabolized through chloral hydrate to compounds including trichloroacetic acid and dichloroacetic acid which alter intercellular communication, induce peroxisome proliferation and may promote tumor production. Significant variability in trichloroethylene metabolism in 23 human haptic microsomal samples was reported by Lipscomb et al. It was also demonstrated that the trichloroethylene metabolism is dependent on enzymatic activities of the cytochrome system, and they conclude that their data indicates that humans are not uniform in their capacity for CPY dependent metabolism of trichloroethylene and increased activity may increase susceptibility to trichloroethylene induced toxicity in humans. These observations are compatible with the variability reaction which is depending on nutritional factors, enzyme induction factors, hormonal factors and interaction with other environmental chemicals, prescription medications and general health conditions, and explains the variable reports as far as trichloroethylene and level of liver toxicity in the various individuals studied. 

SCFs are an environmentally friendly alternative to organic solvents as media for biocatalysis. A key feature of biocatalysis in SCFs is the tunability of the medium [75]. Enzymatic activity in SCFs has been proven and well documented [76]. Limiting factors, which may affect enzymatic activity in supercritical solvent systems, have been identified and are well characterized. A major limitation to the broader use of SCFs is their inability to dissolve a wide range of hydrophilic and ionic compounds, which greatly impedes their ability to carry out biolransformation with polar substrates. The interest in water-in-SCF microemulsion as reaction media stems from the fact that in such systems high concentrations of both polar and apolar molecules can be dissolved within the dispersed aqueous and continuous SCF phases, respectively. 

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