Enzymatic reactions inhibition

Example 13.14 A model for an enzyme reaction inhibited by the substrate and product Consider the enzymatic reaction inhibited by the substrate and product... 

Consider the enzymatic reaction inhibited by the substrate and product... 

ESI (LC-MS) Kinetic study of enzymatic reaction, inhibition Simithy eta/. [337]... 

Enzymatic reactions frequently undergo a phenomenon referred to as substrate inhibition. Here, the reaction rate reaches a maximum and subsequently falls as shown in Eigure 11-lb. Enzymatic reactions can also exhibit substrate activation as depicted by the sigmoidal type rate dependence in Eigure 11-lc. Biochemical reactions are limited by mass transfer where a substrate has to cross cell walls. Enzymatic reactions that depend on temperature are modeled with the Arrhenius equation. Most enzymes deactivate rapidly at temperatures of 50°C-100°C, and deactivation is an irreversible process. 

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... 

If the velocity of an enzymatic reaction is decreased or inhibited, the kinetics of the reaction obviously have been perturbed. Systematic perturbations are a basic tool of experimental scientists much can be learned about the normal workings of any system by inducing changes in it and then observing the effects of the change. The study of enzyme inhibition has contributed significantly to our understanding of enzymes. 

Dihydropteroic acid (85) is an intermediate to the formation of the folic acid necessary for intermediary metabolism in both bacteria and man. In bacteria this intermediate is produced by enzymatic condensation of the pteridine, 86, with para-amino-benzoic acid (87). It has been shown convincingly that sulfanilamide and its various derivatives act as a false substrate in place of the enzymatic reaction that is, the sulfonamide blocks the reaction by occupying the site intended for the benzoic acid. The lack of folic acid then results in the death of the microorganism. Mammals, on the other hand, cannot synthesize folic acid instead, this compound must be ingested preformed in the form of a vitamin. Inhibition of the reaction to form folic acid Ls thus without effect on these higher organisms. 

Cheng, Y. C., andPrasoff, W. H. (1973). Relationship between the inhibition constant (Ki) and the concentration of inhibitor which causes 50 percent inhibition (150) of an enzymatic reaction. Biochem. Pharmacol. 22 3099—3108. 

The values determined from Figure 5.23 agree well with the values calculated from the equations (Table 5.5), with an error of 3.81% for the slope and 4.65% for the intersect, respectively. The obtained experimental data were consistent with the proposed enzymatic reaction and the reaction mechanisms with uncompetitive substrate inhibition and the noncompetitive product inhibition model. 

There are other substrates for the E. coli Met(0) peptide reductase, one of which is Met(0)-a-l-PI. The native protein is the major serum elastase inhibitor that functions by forming a binary complex with elastase which inhibits its activity. Met(0)-a-l-PI, on the other hand, which can be formed by treatment of the protein with TV-chlorosuccinimide, cannot form a complex with elastase and therefore is not able to inhibit elastase activity117,118. Table 6 shows, however, that when Met(0)-a-l-PI is incubated in the presence of Met(0)-peptide reductase and dithiothreitol the protein regains its ability to form a complex with elastase and inhibit elastase activity119. Similar to results found with Met(0)-L12 reduced thioredoxin could replace the dithiothreitol as reductant in the enzymatic reaction. 

FIGURE 12.1 Effects of substrate (reactant) concentration on the rate of enzymatic reactions (a) simple Michaelis-Menten kinetics (b) substrate inhibition (c) substrate activation. 

If more than one substrate participates in an enzymatic reaction, the kinetic effects of an inhibitor can be quite complex. In this case, rules formulated by Cleland (36) are useful in gaining a qualitative picture of the inhibition patterns to be expected of a given mechanism. 

The discovery of ABTS as a laccase substrate mediating or enhancing the enzyme action was essential to increase the range of molecules that can be converted by laccases (Fig. 4.5). Such a mediator requires several conditions (1) it must be a good laccase substrate (2) its oxidized and reduced forms must be stable (3) it must not inhibit the enzymatic reaction and (4) its redox conversion must be cyclic. 

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