Substrate concentration, effect response

Acetylcholine sensors construction, 115 schematic representation, 120f substrate concentration, effect on response, 121/... 

The operational model allows simulation of cellular response from receptor activation. In some cases, there may be cooperative effects in the stimulus-response cascades translating activation of receptor to tissue response. This can cause the resulting concentration-response curve to have a Hill coefficient different from unity. In general, there is a standard method for doing this namely, reexpressing the receptor occupancy and/or activation expression (defined by the particular molecular model of receptor function) in terms of the operational model with Hill coefficient not equal to unity. The operational model utilizes the concentration of response-producing receptor as the substrate for a Michaelis-Menten type of reaction, given as... 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

The detection of many pesticides at extremely low levels can be best achieved not by direct detection of the pesticide itself but rather by detection of its inhibitory effects on enzyme reactions. An enzyme-electrode is first constructed and its response when exposed to a suitable concentration of its substrate determined. When an electrode is then exposed to a dilute pesticide solution, the pesticide interacts with the enzyme and diminishes (or completely destroys) its activity. This inhibition can then be easily quantified by further exposure to the initial substrate concentration and comparison with the response prior to pesticide exposure. 

The metabolic control analysis determines quantitatively the effects of various metabolic pathway reactions on flows and on metabolic concentrations. The analysis defines two coefficients (i) the control coefficients, which characterize the response of the system flows, concentrations, and other variables after parameter perturbations and (ii) the elasticity coefficients, which quantify the changes of reaction rates after perturbations of substrate concentrations or kinetic parameters under specified conditions. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

To directly assess the effect of Zn partitioning in sediments on uptake, we have assumed partitioning, and, in response, availability will change with changes in the concentrations of substrates which bind Zn. Despite this obvious oversimplification we have found ratios of sedimentary substrate concentrations that explain a very large proportion of the variance in either the bioavailability or the concentration of Zn in two species of bivalves. We have reported similar results for lead (33) Although the ratios of substrates which predicted Zn uptake differed between southwest England and San Francisco Bay, most of those differences were consistent with chemical... 

Note that a different termination step from that in the 2-propanol oxidation is responsible for the rate being independent of the substrate concentration. The absence of in equation (41) predicts the same rate for CD3OH. The observed isotope effect ( h/ d) is 1 3 which can reasonably be ascribed to a secondary effect on or an inverse effect on k (Edwards et aV ). 

Figure 13A. Effect of on response to a step change in influent substrate concentration for a continuous flow reactor...

The effects of increasing concentrations of 8-OHQ-5SA, OP, and aa D on the activity of carboxypeptidase at a constant substrate concentration of 0.02 M CGP are shown in Fig. 2. (Vallee and Neurath, 1955.) Activity of the inhibited reaction was expressed as per cent of the proteolytic coefficient observed at zero inhibitor concentration. The conditions of preincubation are indicated. Recent and unpublished data indicate the time course of the inhibitory effects of these agents OP in concentrations of 1 X 10" M causes 90 % inhibition of the reaction in 60 minutes. 80 % of the inhibition occurs in the first 15 minutes (Fig. 3), Addition of 1 X 10" M zinc ions to the enzyme thus inhibited restores enzymatic activity, demonstrating the reversibility of inhibition (unpublished results). Since inhibition did not occur when chelating agents were first incubated with zinc, cupric, or ferrous ions to form the respective metal chelate, it appeared that the sites of chelation of these compounds are responsible for the observed inhibition. Inhibition is therefore not caused by any structural similarity between the inhibitors and the substrate. 

White, C.. and Gadd, G. M. (1996a). Mixed sulphate-reducing bacterial cultures for bioprecipitation of toxic metals factorial and response-surface analysis of the effects of dilution rate, sulphate and substrate concentration. Microbiology. 142, 2197-2205. 

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