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Irreversible and Reversible Inhibition (Part II)
Many biological systems that utilize enzymes must be able to regulate their activity. One means of enzyme regulation involves using special agents called inhibitors (molecules or sometimes ions) that bind onto the enzymes and inhibit their activity. There are two categories of inhibitors - irreversible inhibitors and reversible inhibitors. In irreversible inhibition, the inhibitor binds very tightly to the enzyme either via covalent or non-covalent means and ultimately does not dissociate very easily, if at all, from the enzyme. Some examples of irreversible inhibitors include nerve gas, penicillin and aspirin. In reversible inhibition, the inhibitor binds onto the enzyme but can dissociate relatively easily under the proper conditions. There are four major subdivisions of reversible inhibition - competitive inhibition, uncompetitive inhibition, non-competitive inhibition and mixed inhibition. In competitive inhibition, the inhibitor resembles the substrate and binds directly to the active site. Increasing the concentration of the substrate can overcome the competitive inhibitor. In uncompetitive inhibition, the substrate must bind onto the active site before the inhibitor can bind onto the enzyme. This is because the binding of the substrate onto the enzyme creates an allosteric site on that enzyme that was not previously there. The inhibitor can now bind onto that allosteric site and create the enzyme-substrate-inhibitor complex. This complex will not go on to produce the product. Increasing the substrate concentration will not overcome an uncompetitive inhibitor. In non-competitive inhibition, the enzyme has a permanent allosteric site that the inhibitor can bind to. In addition, the inhibitor can bind to the allosteric site regardless of whether or not the substrate is bound to the active site. Increasing the substrate concentration will not effect the non-competitive inhibitor. Mixed inhibition is a more complex form of reversible inhibition in which the binding of the inhibitor essentially decreasing the affinity of the active site for the substrate and decreases the ability of the substrate to produce product molecules.
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