Catalysis competitive inhibition

Catalyst inhibition is traditionally associated with biocatalytic processes, but can also apply to homogeneous and heterogeneous catalysis. Competitive inhibition is analogous to competitive adsorption in gas/solid heterogeneous catalysis, where two molecules from the gas phase compete for the same active site on the catalyst surface. A competitive inhibitor is any chemical species I which can bind to the same site as the substrate, or to another site on the enzyme (an allosteric site). The overall reaction scheme is then given by Eqs. (2.58)-(2.60), where El indicates an enzyme-inhibitor complex. 

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). 

Reversible Inhibition One common type of reversible inhibition is called competitive (Fig. 6-15a). A competitive inhibitor competes with the substrate for the active site of an enzyme. While the inhibitor (I) occupies the active site it prevents binding of the substrate to the enzyme. Many competitive inhibitors are compounds that resemble the substrate and combine with the enzyme to form an El complex, but without leading to catalysis. Even fleeting combinations of this type will reduce the efficiency of the enzyme. By taking into account the molecular geometry of inhibitors that resemble the substrate, we can reach conclusions about which parts of the normal substrate bind to the enzyme. Competitive inhibition can be analyzed quantitatively by steady-state kinetics. In the presence of a competitive inhibitor, the Michaelis-Menten equation (Eqn 6-9) becomes... 

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. 

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. 

When benzyl alccdiol, 2,4-dinitrophenol, dioxane, etc. were added to the reaction system, the competitive inhibition was observed as inferred from the chai of the lineweaver-Burk plot (70). The fact that mutral molecules sudi as benzyl alccdiol competitively inhibit the catalysis, indicates that the hydrophobic nature of the catalytic site makes a major contributicxi to substrate binding. 

This article describes various approaches to inhibition of enzyme catalysis. Reversible inhibition includes competitive, uncompetitive, mixed inhibition, noncompetitive inhibition, transition state, and slow tight-binding inhibition. Irreversible inhibition approaches include affinity labeling and mechanism-based enzyme inhibition. The kinetics of the various inhibition approaches are summarized, and examples of each type of Inhibition are presented. 

ATP hydrolysis is non-linear with respect to time. This is not due to enzyme inactivation since progress curves, where the product of variable enzyme concentration and time is plotted against the amount of product, yield points that fall on a single line. This is a response expected if a competitive inhibition occurs during catalysis [80] and suggests an explanation for non-linearity since ADP is a competitive inhibitor with respect to ATP [19]. However, the presence of an ATP-generating system, which increases the rate of phosphate production by a factor of three, does not result in a linear rate [81]. 

AMP is a competitive inhibitor (see "Enzymes Catalysis and Kinetics" Lecture) of Adenylosuccinate Synthetase, GMP competitively inhibits IMP Dehydrogenase. Note GTP is required for AMP syndesis and ATP is required for GMP synthesis, hence there is coordinated regulation of these nucleotides. 

Ret er.stble inhibition, in contrast with irreversible inhibition, is acterized by a rapid dissociation of the enzyme-inhibitor complex. In the type of reversible inhibition called competitive inhibition, an enzyme can bind substrate (forming an ES complex) or inhibitor 1) but not both (ESI). The competitive inhibitor often resembles the substrate and binds to the active site of the enzyme (Figure 8.15). The substrate is thereby prevented from binding to the same active site. A competitive inhibitor dimmishes the rate oj catalysis by reducing the pro-por/ion of enzyme molecules bound to a substrate. At any given inhibitor concentration, competitive inhibition can be relieved by increasing... 

How can we determine whether a reversible inhibitor acts by competitive or noncompetitive inhibition Let us consider only enzymes that exhibit Michaelis- Menten kinetics. Measurements of the rates of catalysis at different concentrations of substrate and inhibitor serve to distinguish the three types of inhibition. In competitive inhibition, the inhibitor competes with the substrate for the active site. The dissociation constant for the inhibitor is given by... 

This is a case of competitive inhibition, as the inhibitor and reactant compete directly for the active site. Ligand-deficient catalysis is then a special case of competitive inhibition when the ligand itself acts as an inhibition although it is a necessary ingredient of the catalyst system. 

The Michaelis-Menten equation (8.8) and the irreversible Uni Uni kinetic scheme (Scheme 8.1) are only really applicable to an irreversible biocatalytic process involving a single substrate interacting with a biocatalyst that comprises a single catalytic site. Hence with reference to the biocatalyst examples given in Section 8.1, Equation (8.8), the Uni Uni kinetic scheme is only really directly applicable to the steady state kinetic analysis of TIM biocatalysis (Figure 8.1, Table 8.1). Furthermore, even this statement is only valid with the proviso that all biocatalytic initial rate values are determined in the absence of product. Similarly, the Uni Uni kinetic schemes for competitive, uncompetitive and non-competitive inhibition are only really applicable directly for the steady state kinetic analysis for the inhibition of TIM (Table 8.1). Therefore, why are Equation (8.8) and the irreversible Uni Uni kinetic scheme apparently used so widely for the steady state analysis of many different biocatalytic processes A main reason for this is that Equation (8.8) is simple to use and measured k t and Km parameters can be easily interpreted. There is only a necessity to adapt catalysis conditions such that... 

CyDs accelerate or decelerate various reactions, ediibiting many kinetic features shown by enzyme reactions, i.e. catalyst-substrate complex formation, competitive inhibition, saturation, and stereospecific catalysis [67]. CyD-catalyzed reactions can generally be classified in the following three categories according to the type of stimulation (a) partidpation of the hydroxyl groups of CyDs (b) the microsolvent effect of the hydrophobic CyD cavity and (c) the conformational or steric effect of CyDs [67]. 

The properties of the carriers involved in both active and passive transport suggest that they are proteins. Apart from the fact that no other type of molecule has the necessary ability for specific recognition of the substance to be carried, carrier-mediated transport mechanisms show features that are reminiscent of enzyme catalysis, i.e. they are pH dependent, can be competitively inhibited by structurally similar compounds and poisoned by other compounds. Moreover, they show a relationship to the concentration of transported material that is essentially similar to that of substrate concentration on enzyme activity. It appears therefore that passive carrier-mediated transport may be a catalysed permeability in which the equilibrium of the reaction is not affected although the rate of attainment of equilibrium is greatly enhanced while in the case of active transport, there is a modification of the diffusion equilibrium as a result of the coupling of the transport process to an energy-yielding reaction. 

Specific detail on Michaelis-Menten kinetics, quasi steady-state approximations, competitive and non-competitive inhibitions, substrate inhibition, rate expressions for enzyme catalysis and deactivations, Monod growth kinetics, etc. are not presented in an extensive manner although additional information is available in the work of Vasudevan for the interested leader. " Also note that the notation adopted by Vasudevan is employed throughout this chapter. 

P Amylases.—The thiol groups of the j8-amylase from wheat reacted instantly with 4-chloromercuribenzoate and A -ethylmaleimide in the absence of a substrate. Derivatization almost completely abolished the enzymic activity, and the kinetics indicated that inhibition by the mercury compound is non-competitive. Inhibition by these reagents was considerably less in the presence of a substrate, since the enzyme undergoes an induced conformational change that renders the internal thiol groups inaccessible. The free thiol groups of the enzyme were divided into two types in these reactions two groups on the molecular surface that play no part in the catalysis, and two in the interior that do. 

An important method for investigating enzymes is to study the effect of substances that are structurally similar to the substrate on the rate of catalysis. In general, the rate is decreased by such substances, and this phenomenon is called inhibition. One type of inhibition occurs when the inhibitor binds to the same site in the free enzyme as the substrate, and because the substrate and inhibitor compete for the same binding site, this is called competitive inhibition. This can be accommodated into the simple Michaelis-Menten mechanism described by Eq. [42] by addition of the equilibrium... 

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