Inhibitor constant

Substrate and product inhibitions analyses involved considerations of competitive, uncompetitive, non-competitive and mixed inhibition models. The kinetic studies of the enantiomeric hydrolysis reaction in the membrane reactor included inhibition effects by substrate (ibuprofen ester) and product (2-ethoxyethanol) while varying substrate concentration (5-50 mmol-I ). The initial reaction rate obtained from experimental data was used in the primary (Hanes-Woolf plot) and secondary plots (1/Vmax versus inhibitor concentration), which gave estimates of substrate inhibition (K[s) and product inhibition constants (A jp). The inhibitor constant (K[s or K[v) is a measure of enzyme-inhibitor affinity. It is the dissociation constant of the enzyme-inhibitor complex. 

In the presence of sucrose alone as the single substrate, initial reaction rates follow Michaelis-Menten kinetics up to 200 mM sucrose concentration, but the enzyme is inhibited by higher concentrations of substrate.30 The inhibitor constant for sucrose is 730 mM. This inhibition can be overcome by the addition of acceptors.31,32 The enzyme activity is significantly enhanced, and stabilized, by the presence of dextran, and by calcium ions. 

As discussed above, the degree of inhibition is indicated by the ratio of k3/k and defines an inhibitor constant (Kj) [Eq. (3.19)], whose value reports the dissociation of the enzyme-inhibitor complex (El) [Eq. (3.20)]. Deriving the equation for competitive inhibition under steady-state conditions leads to Eq. (3.21). Reciprocal plots of 1/v versus 1/5 (Lineweaver-Burk plots) as a function of various inhibitor concentrations readily reveal competitive inhibition and define their characteristic properties (Fig. 3.5). Notice that Vmax does not change. Irrespective of how much competitive inhibitor is present, its effect can be overcome by adding a sufficient amount of substrate, i.e., substrate can be added until Vmax is reached. Also notice that K i does change with inhibitor concentration therefore the Km that is measured in the presence of inhibitor is an apparent Km- The true KM can only be obtained in the absence of inhibitor. 

Enzyme kinetics Michaelis constant, symbol iCm maximum velocity of an enzyme catalysed reaction, Vm DC inhibitor constant, symbol X Michaelis-Menten equation and graph in the absence and the presence of inhibitors. Lineweaver-Burke and Eadie-Hofstee plots. 

In just the way that Km defines the dissociation of an enzyme from its substrate, K (the inhibitor constant) defines the strength of binding of a reversible inhibitor to the enzyme. 

On the other hand, combinations between Eqs. (44) and (45) show how, via modification of the electrostatic potential, ionic strength affects Km (and Ky, inhibitor constants), and thus the enzyme activity. 

Dixon M, The determination of en2yme inhibitor constant, Biochem/ 55 170-171, 1953. 

To determine inhibitor constants (K ), repeat step 17 in the presence of one or two different concentrations of inhibitor, [I]. Alternatively, for a Dixon Plot, test a range of inhibitor concentrations at two different substrate concentrations. Then plot 1/v against [I] for each value of [S]. 

Michaelis Constants, Inhibitor Constants, and Maximum Velocity... 

In this case, Kt is known as the inhibitor constant and the equilibrium between enzyme and inhibitor is almost instantaneous on mixing as 9t0 = k2 ES (the rate or reaction) is directly dependent on the concentration of the enzyme-substrate complex. The total enzyme present in the present of the inhibitor will be ... 

ESI is the dead-end complex the inhibitor constant K, = ES -J— Under steady state conditions ES ... 

Figure S.14 shows a plot of such an inhibition pattern. There are few clear-cut examples of non-competitive inhibition of a single-substrate reaction, as might be expected from this special case. Normally the inhibitor constants in Scheme S.AS.3 are different.

The inhibition effect of poly (vinyl alcohol) on the amylose hydrolysis was investigated. Figure 7 shows Lineweaver-Burk plots of the amylose hydrolysis rates catalyzed by the random copolymer in the presence of poly (vinyl alcohol). The reaction rate is found to decrease with increasing the concentration of poly (vinyl alcohol), and all of the straight lines obtained in the plots cross with each other at a point on the ordinate. This is a feature of the competitive inhibition in the enzymatic reactions. In the present reaction system, however, it is inferred to suggest that the copolymer and poly (vinyl alcohol) molecules competitively absorb the substrate molecules. The elementary reaction can be described in the most simplified form as in Equation 3 where Z, SI, and Kj[ are inhibitor, nonproductive complex, and inhibitor constant, respectively. Then the reaction rate is expressed with Equation 4. 

In noncompetitive inhibition, the inhibitor binds to the enzyme at a site other than the substrate-binding site and binds in the same manner to the ES complex, producing two inactive forms, El and ESI E + I El and ES + I ESI. There are two inhibitor constants ... 

Many substances interact with enzymes to lower their activity that is, to inhibit them. Valuable information about the mechanism of action of the inhibitor can frequently be obtained through a kinetic analysis of its effects. To illustrate, let us consider a case of competitive inhibition, in which an inhibitor molecule, I, combines only with the free enzyme, E, but cannot combine with the enzyme to which the substrate is attached, ES. Such a competitive inhibitor often has a chemical structure similar to the substrate, but is not acted on by the enzyme. For example, malonate (-OOCCH2COO-) is a competitive inhibitor of succinate (-OOCCH2CH2COO-) dehydrogenase. If we use the same approach that was used in deriving the Michaelis-Menten equation together with the additional equilibrium that defines a new constant, an inhibitor constant, A),... 

Biochemists observe other kinds of enzyme inhibition. Noncompetitive inhibition consists of cases in which an inhibitor combines with either the E or the ES form of the enzyme. This requires definition of two new inhibitor constants ... 

In uncompetitive inhibition, the inhibitor combines only with the ES form of the enzyme. This pattern of inhibition is not seen frequently except in studies of inhibition by the products of enzyme reactions. If you define an inhibitor constant... 

The determination of binding constants, K, and inhibitor constants, Ki, for micelle-catalyzed reactions permits at least qualitative interpretations of the effect of substrate structure on the extent and nature of micellar complexation and allows a comparison of the magnitude of the binding constants for substrates in micellar systems with those in enzymatic systems. The hmited quantity of such data available at... 

The electrostatic model for the micellar effect on the hydrolysis of phosphate monoesters is also consistent with the results of inhibition studies (Bunton et al., 1968, 1970). The CTAB catalyzed hydrolysis of the dinitrophenyl phosphate dianions was found to be inhibited by low concentrations of a number of salts (Fig. 9). Simple electrolytes such as sodium chloride, sodium phosphate, and disodium tetraborate had little effect on the micellar catalysis, but salts with bulky organic anions such as sodium p-toluenesulfonate and sodium salts of aryl carboxylic and phosphoric acids dramatically inhibited the micelle catalysis by CTAB. From equation 14 and Fig. 10, the inhibitor constants, K, were calculated (Bunton et al., 1968) and are given in Table 9. The linearity of the plots in Fig. 10 justifies the assumption that the inhibition is competitive and that incorporation of an inhibitor molecule in a micelle prevents incorporation of the substrate (see Section III). Comparison of the value of for phenyl phosphate and the values of K for 2,4-and 2,6-dinitrophenyl phosphates suggests that nitro groups assist the... 

The inhibitor constants, Kj, may be determined by measuring the variation of v with [S] at different concentrations of the inhibitor. Lineweaver—Burk plots can then be used to find K/ from measurements made in the presence, and absence, of inhibitor (see Fig. 2 and Table I). The kinetics of the inhibition of such enzymes as alpha-amylase, hefo-amylase, and phosphorylase have been studied, and inhibitor constants evaluated. 

Dixon M. The determination of enzyme inhibitor constants. Biochem. J. 1953 55(1) 170-171. 

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