Does adding a noncompetitive inhibitor increases the rate of an enzymatic reaction?
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Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. There are three common types of enzyme inhibition - competitive, non-competitive and substrate inhibition. Show
Most theories concerning inhibition mechanisms are based on the existence of the enzyme-substrate complex ES. As mentioned earlier, the existence of temporary ES structures has been verified in the laboratory. Competitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. A theory called the "lock-key theory" of enzyme catalysts can be used to explain why inhibition occurs.  Figure 9: Lock and key theory – competitive analysis. The lock and key theory utilizes the concept of an "active site." The concept holds that one particular portion of the enzyme surface has a strong affinity for the substrate. The substrate is held in such a way that its conversion to the reaction products is more favorable. If we consider the enzyme as the lock and the substrate the key (Figure 9) - the key is inserted in the lock, is turned, and the door is opened and the reaction proceeds. However, when an inhibitor which resembles the substrate is present, it will compete with the substrate for the position in the enzyme lock. When the inhibitor wins, it gains the lock position but is unable to open the lock. Hence, the observed reaction is slowed down because some of the available enzyme sites are occupied by the inhibitor. If a dissimilar substance which does not fit the site is present, the enzyme rejects it, accepts the substrate, and the reaction proceeds normally. Non-competitive inhibitors are considered to be substances which when added to the enzyme alter the enzyme in a way that it cannot accept the substrate. Figure 10. Figure 10: Noncompetitive inhibition. Substrate inhibition will sometimes occur when excessive amounts of substrate are present. Figure 11 shows the reaction velocity decreasing after the maximum velocity has been reached. Figure 11: Substrate becoming rateinhibiting. Additional amounts of substrate added to the reaction mixture after this point actually decrease the reaction rate. This is thought to be due to the fact that there are so many substrate molecules competing for the active sites on the enzyme surfaces that they block the sites (Figure 12) and prevent any other substrate molecules from occupying them. Figure 12: Substrate inhibition. This causes the reaction rate to drop since all of the enzyme present is not being used. Enzyme inhibitorsVarious compounds can reduce the activity of enzymes. They may act in a variety of different ways, and indeed may be reversible or irreversible inhibitors of the enzyme. On this page there are notes about:
An irreversible inhibitor causes covalent modification of the enzyme, so that its activity is permanently reduced. Compounds that act as irreversible inhibitors are often useful as drugs that need be taken only every few days, although adjusting the dose to suit the patient’s response is a lengthy process with such compounds. By contrast, the effect of a reversible inhibitor can be reversed by removing the inhibitor, e.g. by dialysis or gel filtration. The normal sequence of an enzyme reaction can be represented as: where: There are three main types of reversible inhibitor:
They interact with the enzyme or enzyme-substrate complex at different stages in the sequence Competitive inhibitionA competitive inhibitor competes with the substrate for the active site of the enzyme: This means that increasing the concentration of substrate will decrease the chance of inhibitor binding to the enzyme. Hence, if the substrate concentration is high enough the enzyme will reach the same Vmax as without the inhibitor. However, it will require a higher concentration of substrate to achieve this and so the Km of the enzyme will also be higher. Reacting the enzyme with a range of concentrations of substrate at different concentrations of a competitive inhibitor will give a family of curves as shown below: The Lineweaver-Burk double reciprocal plot for this set of data shows a series of lines crossing the y (1/v) axis at the same point - i.e. Vmax is unchanged, but with a decreasing value of 1/Km (and hence a higher Km) in the presence of the inhibitor: top of page Non-competitive inhibitionA non-competitive inhibitor reacts with the enzyme-substrate complex, and slows the rate of reaction to form the enzyme-product complex. This means that increasing the concentration of substrate will not relieve the inhibition, since the inhibitor reacts with the enzyme-substrate complex. Reacting the enzyme with a range of concentrations of substrate at different concentrations of a non-competitive inhibitor will give a family of curves as shown below: The Lineweaver-Burk double reciprocal plot for this set of data shows a series of lines converging on the same point on the X (1/S) axis - i,.e. Km is unchanged, but Vmax is reduced: top of page Uncompetitive inhibitionThis is a very rare class of inhibition. An uncompetitive inhibitor binds to the enzyme and enhances the binding of substrate (so reducing Km), but the resultant enzyme-inhibitor-substrate complex only undergoes reaction to form the product slowly, so that Vmax is also reduced: Reacting the enzyme with a range of concentrations of substrate at different concentrations of an uncompetitive inhibitor will give a family of curves as shown below: The Lineweaver-Burk double reciprocal plot for this set of data shows a series of parallel lines - both Km and Vmax are reduced: top of page The choice of a competitive or non-competitive inhibitor as a drugIf the requirement is to increase the intracellular concentration of the substrate, then either a competitive or non-competitive inhibitor will serve, since both will inhibit the utilisation of substrate, so that it accumulates. However, if the requirement is to decrease the intracellular concentration of the product, then the inhibitor must be non-competitive. As unused substrate accumulates, so it will compete with a competitive inhibitor, and the final result will be a more or less normal rate of formation of product, but with a larger pool of substrate. Increasing the concentration of substrate does not affect a non-competitive inhibitor. Ki, the inhibitor constantThe inhibitor constant, Ki, is an indication of how potent an inhibitor is; it is the concentration required to produce half maximum inhibition. Plotting 1/v against concentration of inhibitor at each concentration of substrate (the Dixon plot) gives a family of intersecting lines. For a competitive inhibitor, the lines converge above the x axis, and the value of [I] where they intersect is -Ki For a non-competitive inhibitor, the lines converge on x axis, and the value of [I] where they intersect is -Ki top of page Does noncompetitive inhibitor affect enzyme activity?Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme whether or not it has already bound the substrate.
What happens when a noncompetitive inhibitor binds to an enzyme?Upon inhibitor binding to the enzyme or enzyme-substrate complex, the enzyme is prevented from forming its product. The resulting decrease in enzyme activity is independent of substrate concentration as the inhibitor does not compete with the substrate for active site binding.
How does a noncompetitive inhibitor decrease reaction rate?By decreasing the free-energy change of the reaction catalyzed by the enzyme. By binding to the substrate, thus changing its shape so that it no longer binds to the active site of the enzyme.
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