Allosteric phenomena activation

The kinetics of substrate hydrolysis by both AChEs and BuChEs deviates from Michaelis-Menten kinciic.s, A.s implicated in Fig. 2 for acetylthiocholinc, only at substrate concentrations lower than 1 mM docs the rise in enzyme activity follow MichaeliS -Mentcn kinetics. At higher concentrations, AChE activity decreases, inhibited by the excess. substrate, whereas BuChE activity increases, activated by the excess substrate. Hence, the terms substrate inhibition and substrate activation are respective hallmarks of catalysi,s hy AChE and BuChE, Both phenomena can be simply described as a consequence of the formation of a ternary complex between the enzyme and two substrate molecules and thus as an allosteric phenomenon. The ternary complex in AChE has reduced or no activity compared to the Michaelis-Mcntcn complex, whereas it appears more active in BuChE hydrolysis. It is imponant to emphasize that this is a substrate-specific phenomenon. Not all AChE and BuChE substrates exhibit substrate inhibition and sub-... 

In die metabolic pathway to an amino add several steps are involved. Each step is die result of an enzymatic activity. The key enzymatic activity (usually die first enzyme in the synthesis) is regulated by one of its products (usually die end product, eg die amino add). If die concentration of die amino add is too high die enzymatic activity is decreased by interaction of die inhibitor with the regulatory site of die enzyme (allosteric enzyme). This phenomenon is called feedback inhibition. 

When binding of a substrate molecule at an enzyme active site promotes substrate binding at other sites, this is called positive homotropic behavior (one of the allosteric interactions). When this co-operative phenomenon is caused by a compound other than the substrate, the behavior is designated as a positive heterotropic response. Equation (6) explains some of the profile of rate constant vs. detergent concentration. Thus, Piszkiewicz claims that micelle-catalyzed reactions can be conceived as models of allosteric enzymes. A major factor which causes the different kinetic behavior [i.e. (4) vs. (5)] will be the hydrophobic nature of substrate. If a substrate molecule does not perturb the micellar structure extensively, the classical formulation of (4) is derived. On the other hand, the allosteric kinetics of (5) will be found if a hydrophobic substrate molecule can induce micellization. 

The third type of inhibition is called allosteric inhibition, and is particularly important in the control of intermediary metabolism This refers to the ability of enzymes to change their shape (tertiary and quaternary structure, see Section 13.3) when exposed to certain molecules. This sometimes leads to inhibition, whereas in other cases it may actually activate the enzyme. The process allows subtle control of enzyme activity according to an organism s demands. Further consideration of this complex phenomenon is outside our immediate needs. 

Non-competitive inhibitors. These inhibitors bind to the enzyme or the enzyme-substrate complex at a site other than the active site. This results in a decrease in the maximum rate of reaction, but the substrate can still bind to the enzyme. An analogous concept is that of allosteric inhibition. The site of binding of an allosteric inhibitor is distinct from the substrate binding site. In this case, the inhibitor is not a steric analog of the substrate and instead binds to the allosteric site (the phenomenon was termed thus by Monod and Jacob). 

Of the various mechanisms of action ascribed to nerve CW agents in various in vitro model systems, most appear to be active at concentrations where significant inhibition of AChE would be expected to occur. These conditions are unlikely to be seen in the absence of recognizable cholinergic toxicity and hence would not be classified as a low-dose effect. The one effect that seems to occur at a sufficiently low concentration of organophosphorus anticholinesterase to be considered a low-dose phenomenon is the slow allosteric modulation of muscarinic receptors regulating presynaptic release of other neurotransmitters. The observation that this allosteric modulation requires several hours to develop suggests that this mechanism would most likely be applicable to a subacute or subchronic exposure rather than one seen after a brief, acute dose. 

Enzyme stimulation is an increase in enzyme activity resrrlting directly from the addition of a chemical. This is a somewhat imusual phenomenon in ertzymology, usually relegated to classically allosteric systems [200]. The concept is that a chemical stimulates the catalytic activity of an enzyme. This cooperativity may be considered in two aspects. One is homotropic cooperativity, in which a chemical stimulates its own biotransformation. This is usually manifested in sigmoidal (S-shaped) plots of v versus S. Heterotropic cooperativity is the stimulation of catalytic activity by direct addition of a different compound. 

A very distintive characteristic of the regulatory phenomenon already described is that the observed changes of the kinetic parameters require the integrity of the membrane, which means that the modification of the fatty acid composition change the behavior of the enzyme when membrane lipids are structuraly organized. This work shows that complementary information could be obtained from the studies of allosteric behavior and temperature-dependent activity... 

Not all enzymes show the simple hyperbolic dependence of rate of reaction on substrate concentration shown in Figure 2.8. Some enzymes consist of several separate protein chains, each with an active site. In many such enzymes, the binding of substrate to one active site causes changes in the conformation not only of that active site, but of the whole multi-subunit array. This change in conformation affects the other active sites, altering the ease with which substrate can bind to the other active sites. This is cooperativity — the different subunits of the complete enzyme cooperate with each other. Because there is a change in the conformation (or shape) of the enzyme molecule, the phenomenon is also called allostericity (from the Greek for different shape ), and such enzymes are called allosteric enzymes. 

Some comments are required on effectors which are of importance in metabolic and other regulation phenomena. The term allosteric inhibition was introduced by Monod, Changeux Jacob (1963) to explain the discovery of feedback inhibitors, which are competitive with the substrate but not closely structurally related to it. Such inhibitors were called allosteric in contrast to the usual competitive inhibitors which are isosteric with the substrate to fit competitively into the same binding site. Allosteric competitive inhibition occurs at a site (allosteric site) which is separate from the substrate binding sites. Conununication between these separate sites about respective occupancy occurs through conformation changes and these are transmitted through subunit interfaces. The term was extended to the related control phenomenon of allosteric activation, which occurs when a metabolite binds to an allosteric site and decreases the for... 

The widespread occurrence of mitochondria in eukaryotic cells indicates that segregation of some of the most important aspects of energy metabolism has been evolutionarily useful. Such segregation has been accompanied by modifications of regulatory interrelationships which depend upon the movement of metabolites into and out of mitochondria and upon the specific modification of regulatory enzymes by specific allosteric modifiers. For example, the activity of pyruvate carboxylase, an enzyme which fulfills an anaplerotic function in a wide variety of cells, depends upon allosteric activation by acetyl CoA. Since the inner mitochondrial membrane is virtually impermeable to coenzyme A and its esters, the activation of pyruvate carboxylase may be looked upon as a compartmentation phenomenon in which this acetyl CoA is separated from the cytosol acetyl CoA pool and must be formed intramitochondrially. In this case movement across the membrane is not involved, since both the activation and the formation of the activator occur within the same compartment. Although a distinct possibility. 

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