Simple irreversible inhibition

The interaction of an enzyme (E) with an irreversible inhibitor (I), which results in the formation of an irreversible enzyme-inhibitor complex (El ), can be modeled as a second-order reaction between two dissimilar substrates  

Since the initial inhibitor concentration is known, the experimentally determined peudo-first-order inhibition rate constant, k (s ), can be used to 

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Figure 5.1. a) Increases in the concentration of inhibited enzyme as a function of time for simple irreversible enzyme inhibition, b) Semilogarithmic plot used in determination of the inhibition rate constant for the case of simple irreversible inhibition. 

TIME-DEPENDENT SIMPLE IRREVERSIBLE INHIBITION IN THE PRESENCE OF SUBSTRATE... 

Mutagenesis of known enzyme towards a desired activity will be the fastest developing direction. The use of mutants of simple serine-hydrolases, which exhibit the phosphotriesterase activity (in contrast to the native enzymes, which are irreversibly inhibited under such conditions), clearly shows that practically any kind of substrates can be enzymatically transformed. The... 

Reversible inhibition occurs rapidly in a system which is near its equilibrium point and its extent is dependent on the concentration of enzyme, inhibitor and substrate. It remains constant over the period when the initial reaction velocity studies are performed. In contrast, irreversible inhibition may increase with time. In simple single-substrate enzyme-catalysed reactions there are three main types of inhibition patterns involving reactions following the Michaelis-Menten equation competitive, uncompetitive and non-competitive inhibition. Competitive inhibition occurs when the inhibitor directly competes with the substrate in forming the enzyme complex. Uncompetitive inhibition involves the interaction of the inhibitor with only the enzyme-substrate complex, while non-competitive inhibition occurs when the inhibitor binds to either the enzyme or the enzyme-substrate complex without affecting the binding of the substrate. The kinetic modifications of the Michaelis-Menten equation associated with the various types of inhibition are shown below. The derivation of these equations is shown in Appendix S.S. 

Selective hydrogenation catalysts are susceptible to contact with a wide variety of contaminants (S, Hg, As, Si, Na, Fe, etc.) during their lifetime. Their influence on catalytic performance depends on the type of reaction, the operating conditions, the active phase and the contaminant itself [1-3]. The effects range from a simple reversible inhibition to an irreversible poisoning and consequently the necessity to change the catalyst. 

Further characterization of the chemical outcome of the tritium label revealed that the tritium was only found in the reduced product dihydrofinasteride and not in finasteride. This suggested that finasteride binding was irreversible and all bound finasteride went through catalysis. The possibility that the product dihydrofi-nasteride was the actual inhibitor was ruled out when it was determined to be a simple, irreversible inhibitor of the enzyme with a A) = 1 nmol H. A more vexing problem for the researchers was that while the overall release of tritium from the H-fmasteride-enzyme complex was >98%, only 30 5% could be recovered as H-dihydrofmasteride. This, combined with the inhibition kinetics of dihydrofmasteride, suggested that the potent inhibitor compound had not yet been isolated. 

Intraperitoneal and Intravenous toxicity of phenols to the mouse depend on small positive slopes of HI with no observable optimum. This simple behavior (Class 1) cannot be causally defined but suggests absorption-desorption from a lipid pool as the rate-limiting step. All other toxicities explored (oral, dermal, sc) to mouse, rat, guinea pig and chicken correlate with positive slopes of Zo and/ or 2,6-effects (Class 2). Diarylamines, anilines and pyrldines also appear to behave as Class 2 toxicants against mice. These reactivity factors Indicate target site expression, consistent with death by irreversible inhibition. 

We present some simple models that can be used to analyze irreversible inhibition data. In aU of these treatments, the concentration of inhibitor will be considered to be in excess of that of enzyme (i.e., [I] [E]). 

The initial compounds, both from natural sources and synthesis were relatively simple structures consisting of a zinc-binding warhead (a hydroxamate or epoxide), a 4-carbon linker (mimicking the e-amino side chain of lysine) and an aromatic end . However, within the original natural product structures was a tetrapeptide known as trapoxin A (Figure 1.8, 66), first reported by Itazaki et al. in 1990, with recognition of the mechanism of action, irreversible inhibition of deacetylation of acelylated histone molecules, being published by Kijima et al. three years later. ... 

We have already dealt with the subject of irreversible inhibitors under enzyme titration and location of the active site (Section 11.4.3.2). The phenomenon of reversible inhibition involves simple complexation of the inhibitor with the enzyme at a site which modifies the reactivity of the enzyme catalysis. 

Although a radical must be reactive in order to damage other components of a system, there is not necessarily a simple correlation between reactivity and the ability to cause irreversible damage to a complex structure. For example, the activity of certain enzymes is found to be inhibited more effectively by radicals of relatively low reactivity than by the reactive hydroxyl radical. This is because reactive radicals are not very selective in their reactions, having a tendency to react at many different sites in a molecule. Less reactive radicals are more selective and can be more effective at damaging a specific site. If this site happens to be essential for activity, then the less reactive radical will be more damaging. 

Most biosensors based on AChE have the enzymes immobilized on the surface of the sensor. The inhibition reaction being irreversible, the membrane with immobilized enzyme has to be replaced after several measurements or the biosensor can be use for only one determination. Due to this fact, the researchers tried to realize pesticide biosensors with a renewable surface or disposable biosensors based on screen-printed electrodes (SPE). The screen-printing technology provides a simple, fast and inexpensive method for mass production of disposable biosensors for different biomolecules starting with glucose, lactate and finishing with environmental contaminants as pesticides (Kulys et al., 1991) and herbicides (Skladal, 1992). 

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