Sigmoidal plot

The ALIS-based off-rate measurement method was applied to a proprietary series of Zap-70 Kinase inhibitors. First, an ACE50 experiment was conducted to demonstrate that the compounds bind the same site as the quench reagent staurospor-ine. As shown in Fig. 3.15, sigmoidal plots indicate that, with the exception of one compound, the ACE50 values were all very similar to one-another. Linear ratio plots of the same ACE50 data confirm that the compounds all bind isosteri-cally with respect to the quench reagent, a necessary prerequisite for effective competition. 

For example, Bachelard used [Mgtotai]/[ATPtotai ] = 1 in his rate studies, and he obtained a slightly sigmoidal plot of initial velocity versus substrate ATP concentration. This culminated in the erroneous proposal that brain hexokinase was allosterically activated by magnesium ions and by magnesium ion-adenosine triphosphate complex. Purich and Fromm demonstrated that failure to achieve adequate experimental control over the free magnesium ion concentration can wreak havoc on the examination of enzyme kinetic behavior. Indeed, these investigators were able to account fully for the effects obtained in the previous hexokinase study. ... 

There are exceptions to Michaelis-Menten behaviour. For example allosteric enzymes which instead of a hyperbolic curve in a Lversus [S] graph yield a sigmoidal plot (the behaviour is rather like non-catalytic allosteric proteins, such as haemoglobin, Section 2.5. This type of curve can indicate cooperative binding of the substrate to the enzyme. We have discussed cooperativity in Section 1.5 (see also Section 10.4.3). In addition, regulatory molecules can further alter the activity of allosteric enzymes. 

Plotting the normalized fluorescence intensities of excitation and emission spectra vs temperature yields a sigmoidal plot (Figure 12.16) with an inflection point equal to that observed when thermal DNA denaturation is observed by following the ODs at 260 nm (Figure 12.14). 

The solubility of metal-hydroxide precipitates in water varies depending on ionic strength and number of pairs and/or complexes (Chapter 2). A practical approach to determining the pH of minimum metal-hydroxide solubility, in simple or complex solutions, is potentiometric titration, as demonstrated in Figure 12.3. The data show that potentiometric titration of a solution with a given heavy metal is represented by a sigmoidal plot. The long pH plateau represents pH values at which metals precipitate the equivalence point, or titration end point, indicates the pH at the lowest metal-... 

Typical sigmoidal plots of retention of a basic analyte as a function of pH can be observed in Figure 9.1. Since HA is cationic and A is neutral for a base, knA kA and retention decreases with decreasing pH. The curves are translated toward higher pH for higher of the analyte. The inflection point of the curve corresponds to... 

During methods evaluation, standard curves should be plotted in several ways, since different plots reveal different reaction characteristics. For example, the bound/free versus concentration plot is very steep at the low concentration end, and may be used to calculate the detection limit of the assay. The sigmoidal plots of bound/total against log concentration clearly show regions of insensitivity (low and high concentration ends) that might not be clearly apparent in the logit-log curve. 

Figure 3. 2-D lattice tetramer toy model. Black circles represent solvophobic repeat units (H) and open circles polar repeat units (P). The sigmoidal plot shows the fraction of folded molecules as a function of temperature.

I 56. In the study of enzymes, a sigmoidal plot of substrate concentration ([S]) versus reaction velocity (V) may indicate... 

The answer is e. (Murray, pp 48-73. Scrivci, pp 4571-4636. Sack, pp 3-17. Wilson, pp 287-317.) Unlike Michaelis-Menten enzymes, allosteric enzymes exhibit sigmoidal plots when reaction velocity is plotted against substrate concentrations. The enzyme contains both a catalytic site and a regulatory site. The binding of regulatory molecules to the regulatory site... 

Figure 1. Sigmoid plot of —Av from nmr shifts vs —Hq to determine protonation of methyl isopropyl sulfide.

Plot of initial rate vs. substrate cortcentration, sometimes called a characteristic curve. 1 is a Michaelis-Menten hyperbola 2 is a sigmoidal plot characteristic of a cooperative enzyme. For linear transformations see Kinetic data evaluation. 

This relationship results in a sigmoidal plot of absorbance versus pH centered on the pKa value. In comparison to fluorescence, the absorbance method is simple... 

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