Enzymatic reactions, competitive inhibition

Complications arise when a product of enzymatic reaction or the substrate, 22,23 itself, inhibits the enzyme reaction. In cases where inhibition is effected by the products of the reaction it is necessary to select as short a reaction time as possible in which to calculate the velocity. This type of effect by inorganic phosphate on the kinetics of prostatic acid phosphatase has been rigorously presented by Schpnheyder. Neurath and Schwert have analyzed the kinetics of reactions competitively inhibited by reaction products. 

The three most common types of inhibitors in enzymatic reactions are competitive, non-competitive, and uncompetitive. Competitive inliibition occurs when tlie substrate and inhibitor have similar molecules that compete for the identical site on the enzyme. Non-competitive inhibition results in enzymes containing at least two different types of sites. The inhibitor attaches to only one type of site and the substrate only to the other. Uncompetitive inhibition occurs when the inhibitor deactivates the enzyme substrate complex. The effect of an inhibitor is determined by measuring the enzyme velocity at various... 

The enzymatic activity of the L-19 IVS ribozyme results from a cycle of transesterification reactions mechanistically similar to self-splicing. Each ribozyme molecule can process about 100 substrate molecules per hour and is not altered in the reaction therefore the intron acts as a catalyst. It follows Michaelis-Menten kinetics, is specific for RNA oligonucleotide substrates, and can be competitively inhibited. The kcat/Km (specificity constant) is 10s m- 1 s lower than that of many enzymes, but the ribozyme accelerates hydrolysis by a factor of 1010 relative to the uncatalyzed reaction. It makes use of substrate orientation, covalent catalysis, and metalion catalysis—strategies used by protein enzymes. 

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. 

A classic example of competitive inhibition is the inhibition of succinate dehydrogenase by malonate, a structural analogue of succinate. Competitive inhibitors are usually structural analogues of the substrate, the molecule with which they are competing. They bind to the active site but either do not have a structure that is conducive to enzymatic modification or do not induce the proper orientation of catalytic amino acyl residues required to affect catalysis. Consequently, they displace the substrate from the active site and thereby depress the velocity of the reaction. Increasing [S] will displace the inhibitor. 

Figure 2.5 Interactions between n-hexane and its metabolites. Arrows (— ) represent enzymatic reaction, and lines terminating in -Y depict a competitive inhibition. (After Andersen and Clewell [1983]).

The effects of macromolecules other than surfactants on the rates of organic reactions have been investigated extensively (Morawetz, 1965). In many cases, substrate specificity, bifunctional catalysis, competitive inhibition, and saturation (Michaelis-Menten) kinetics have been observed, and therefore these systems also serve as models for enzyme-catalyzed reactions and, in these and other respects, resemble micellar systems. Indeed, in some macromolecular systems micelle formation is very probable or is known to occur, and in others mixed micellar systems are likely. Recent books and reviews should be consulted for a more detailed description of macromolecular systems and for their applicability as models for enzymatic catalysis and other complex interactions (Morawetz, 1965 Bruice and Benkovic, 1966 Davydova et al., 1968 Winsor, 1968 Jencks, 1969 Overberger and Salamone, 1969). 

Figure 17.7. (a)Lineweaver-Burk, (b) Eadie-Hofstee, and (c) Hanes-Woolf plots exhibiting competitive inhibition patterns. The dashed line indicates the reaction in the absence of inhibitor, whereas the solid lines represent enzymatic reactions in the presence of increasing concentrations cf inhibitor. 

Thus, the product P of enzymatic reactions is often a competitive inhibitor of the enzyme leading to product inhibition (compare Eqs. (27) and (37)). The influence of product in the case of reversible reactions will be discussed later. 

The first term in the denominator of Eq. (66) represents a non-competitive inhibition (compare to Eq. (31)) of the enzyme by the sum of concentrations of A, B and P. This non-specific inhibition could be correlated with an increased viscosity of the reaction medium in the presence of A, B and P147- mi. As the mutarotation of the carbohydrates is fast compared to the enzymatic reaction there was no need for a discrimination between the a,P-anomers or the open-chain form of the monosaccharides. A complete set of kinetic parameters was determined, summarized in Table 7-4. 

The three most common types of reversible inhibition occurring in enzymatic reaction.s are cotnpeiitive, uncompetitive, and noncompetitive. The enzyme molecule is analogous to a heterogeneous catalytic surface in that it contains active sites. When competitive inhibition occurs, the substrate and... 

The influence on the enzymatic reaction has to be interpreted in terms of either competitive inhibition (ACB R)31), uncompetitive inhibition (ACB R S), mixed inhibition (ACB R + ACB R S), or substrate capture by the conjugates. 

Competitive inhibitors decrease the velocity of an enzymatic reaction by increasing the amount of substrate required to saturate the enzyme therefore, they increase the apparent Km but do not affect Vmax. A Lineweaver-Burk plot of a competitively inhibited enzyme reaction has an increased slope, but its intercept is unchanged. 

FIGURE 8.3 Lineweaver-Burk plots for competitive (2), noncompetitive (3), and mixed (4) inhibition, relative to the enzymatic reaction in the absence of inhibitors (1). 

Inhibition of enzymes can basically be divided into reversible or irreversible. According to inhibition kinetics, it can be divided into three types— competitive, non competitive, and allosteric. Competitive inhibition can be characterized by binding of the inhibitor to the active site of the enzyme (they are structurally similar to substrate) and inhibition can be reversed by substrate access (reversible inhibition). The reaction rate is dependent on the substrate and inhibitor concentrations and their affinity to the enzyme. Noncompetitive inhibition cannot be reversed by substrate access and the inhibitor reacts with other parts of the enzyme rather than the active site, and it is not structurally similar to the substrate. The enzymatic reaction can be irreversible when the affinity of the inhibitor to the enzyme is relatively high. Allosteric ligands (inhibitors or activators) are bound to quite another... 

Another factor that greatly influences the rate of enzyme-catalyzed reactions in addition to pH and temperature is the presence of an inhibitor. As follows, we need to mention the effect that different inhibitors have on the rate. The three most common types of reversible inhibition occuring in enzymatic reactions are competitive, uncompetitive, and noncompetitive [3]. The enzyme molecule is analogous to the heterogeneous catalytic surface in that it contains active sites. When competitive inhibition occurs, the substrate and inhibitor are usually similar molecules that compete for the same site on the enzyme. The resulting inhibitor-enzyme complex is inactive. The reactions can be developed as follows (Eq. 4-1 and Eqs. 4-12 to 4-3). 

Second, the Lineweaver-Burk plot of a analytical method of the enzymatic reaction speed revealed the inhibitor constant (competitive inhibition. Ki. The inhibition action is strong so that a Ki value is low) value 21 p.M for (-)-epicatechin (1). From the above results, it suggests that (-)-epicatechin (1) could protect against the nemodegeneration in vitro and this finding might also increase the interest in the health benefits of Hypericum patulum [4]. 

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