Enzymatic reactions, competitive

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... 

It is revealing to compare the equation for the uninhibited case. Equation (14.23) (the Michaelis-Menten equation) with Equation (14.43) for the rate of the enzymatic reaction in the presence of a fixed concentration of the competitive inhibitor, [I]... 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

In Chapter 3 we saw that inhibitors of different modalities respond differently to the concentration of substrate used in an enzymatic reaction. Recall that the apparent affinity of the free enzyme for substrate was diminished in the presence of a competitive inhibitor, and vice versa, the apparent affinity of a competitive inhibitor could be abrogated at high substrate concentrations. On the other hand, the appar-... 

Because mechanism-based inactivators behave as alternative substrates for the enzyme, they must bind in the enzyme active site. Binding of a mechanism-based inactivator is therefore mutually exclusive with binding of the cognate substrate of the normal enzymatic reaction (we say cognate substrate here because for bisubstrate reactions, the mechanism-based inactivator could be competitive with one substrate and noncompetitive or uncompetitive with the other substrate of the reaction, depending on the details of the reaction mechanism). Thus, as the substrate concentration is increased, the observed rate of inactivation should decrease (Figure 8.10) as... 

In the processes that require regeneration of cofactors such as nicotinamide adenine dinucleotide phosphate (NAD(P)H) and adenosine triphosphate (ATP), whole-cell biotransformations are more advantageous than enzymatic systems [12,15]. Whole cells also have a competitive edge over the isolated enzymes in complex conversions involving multiple enzymatic reactions [14]. 

There exists a wide variety in the setup of ELISA assays (direct binding or competition setups) and the enzymatic reaction utilized [148]. A similar principle to enhance sensitivity by enzymatic coupling is realized after gel electrophoretic separation of proteins. Here proteins are transferred to nitrocellulose ( western blot ) and detected by antibody-coupled enzymes. 

As in the homogeneous case, expression of the plateau current in equation (5.20) gives a simple representation of the competition between substrate and cosubstrate in the kinetic control of the enzymatic reaction. Equation (5.19) suggests the construction of primary and secondary plots allowing the derivation of the kinetic constants, as will be shown in the next section. 

In spite of the limitations of direct non-competitive measurements of KIE, their use is sometimes unavoidable. For example, when information on the KIE of the VmaX parameter for an enzymatic reaction is required, non-competitive kinetic runs using... 

Enzymatic reactions can be impeded by the addition of exogenous molecules. This is how drugs are used to control biochemical reactions, and most drugs are used for inhibitory functions. Drugs may function as competitive inhibitors or as noncompetitive inhibitors. Competitive inhibitors compete with the substrates for binding to the active sites, whereas noncompetitive inhibitors bind to another location (allosteric site) but affect the active site and its consequential interactions with the substrates. Some drugs used as enzyme inhibitors are the following ... 

General aspects of enzymatic reactions cateuLyzed by kinases are briefly mentioned. Many alternate substrates, competitive inhibitors and affinity labels based either on the structure of ATP or on the structure of the non-ATP kinase substrates are described. Several examples are presented that should be of particular interest to the medicinal chemist. Finally, the design of an affinity label for creatine kinase is reviewed as an example of how such information can be used in the search for agents directed at an enzyme s active site. 

A requirement for determination of optical purity (P) is that the specific rotation of the pure enantiomer, [a]max, is known with certainty. This maximum rotation can be established by calculation, e.g., via competitive reaction methods41,42, or by direct determination employing an enantiomerically pure sample. Enantiomerically pure samples are generally believed to arise from enzymatic reactions performed on biogenic substrates, an assumption which in some instances is incorrect31, or are obtained in most cases by crystallization2. As many direct, nonchiroptical methods are available for determining enantiomeric purities, maximum rotations can also be extrapolated from the specific rotation of a sample of known ee. 

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. 

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]).

Nucleotides in the form of metal ion complexes are involved in a variety of enzymatic reactions either as substrates or as cofactors. These may also be viewed as monomers of DNA and RNA. Lanthanide complexes of nucleotides have been extensively studied because (i) the conformation of nucleotides in solution can be elucidated from lanthanide induced NMR chemical shifts and line-broadenings [40] and (ii) lanthanide nucleotide complexes may act as competitive inhibitors in enzymatic reactions [88] and hence can be used as paramagnetic probes in the mapping of their binding site on the enzyme [89]. 

In the preparation of dynamic nitroaldol systems, different aldehydes and nitroalkanes were first evaluated for reversible nitroaldol reactions in the presence of base to avoid any side- or competitive reactions, and to investigate the rate of the reactions. 1H-NMR spectroscopy was used to follow the reactions by comparison of the ratios of aldehyde and the nitroalcohols. Among various bases, triethylamine was chosen as catalyst because its reactions provided the fastest exchange reaction and proved compatible with the enzymatic reactions. Then, five benzaldehydes 18A-E and 2-nitropropane 19 (Scheme 9) were chosen to study dynamic nitroaldol system (CDS-2) generation, because of their similar individual reactivity and product stabilities in the nitroaldol reaction. Ten nitroaldol adducts ( )-20A-E were generated under basic conditions under thermodynamic control, showing... 

Transition-state inhibitors stably mimic the transition state of the enzymatic reaction, and thereby interact with the substrate-bin-ding and catalytic machinery of the enzyme in a low-energy conformation. Transition-state analogs are competitive, reversible inhibitors, although some have extremely low Kj s and very slow off-rates. All proteases activate a nucleophile to attack a carbonyl, which leads to the formation of a tetrahedral intermediate that then collapses to form the enzyme products—two peptides. Thus, synthetic small molecules that mimic the tetrahedral intermediate of the protease reaction are attractive transition-state analogs. A classic class of protease transition-state inhibitors uses a boronic acid scaffold (4, 10). Boronic acid adopts a stable tetrahedral conformation in the protease active site that is resistant to nucleophilic attack. Boronic acid inhibitors, which are derivatized with different specificity elements, have been developed against every class of protease... 

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. 

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