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| Enzymes and Inorganic Catalysts |
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| Enzymes resemble inorganic catalysts in several aspects and differ from them in many other features. These similarities and differences are summarised in the following table. |
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| fig. 14.12 - Similarities and Differences between Enzymes and Catalysts |
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| There are two different views to explain the mode of enzyme action the lock and key hypothesis and the induced-fit hypothesis. |
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| Lock and Key Hypothesis |
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| It was suggested by Emil Fischer in 1894. According to this view, the enzyme molecule operates by chemically uniting with the substrate molecule, forming an enzyme-substrate complex. The enzyme molecule provides a uniquely structured template on which the substrate molecules can become attached and interact subsequently. This brings about an interaction between the specific active sites in the enzyme molecule and the reactive sites in the substrate molecule. The enzyme now breaks down the substrate into products. The products initially remain attached to the enzyme for a short while forming an enzyme product complex. The products get released from the enzyme molecule subsequently. The enzyme is now ready to receive another substrate molecule again. Thus, the same enzyme can be used again and again. |
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| fig. 14.13 - Lock and Key Hypothesis |
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| Induced Fit Hypothesis |
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| It was proposed by Daniel Koshland in 1959. According to this hypothesis, there is an intermediate condition called transition state between the substrate and the products. It is highly unstable. |
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| When the substrate molecules bind to the enzyme molecule, a change is brought about in the active site to precisely fit the transition state (induced fit). This induced fit hold the substrates at the correct angle for the reaction to take place. |
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| The fact that an active site may also have a conformation to fit the product helps in explaining the role of enzymes in catalysing reversible reactions. |
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| fig. 14.14 - Induced Fit Hypothesis |
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| There are some enzymes which have slightly different molecular structure but exert similar catalytic action. Such enzymes are called isoenzymes or isozymes. More than 100 isozymes have been identified. The enzyme lactic dehydrogenase (LDH) in human skeletal muscle has five isozymes. |
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| Many enzymes can exhibit their catalytic activity only when they are in association with some non-protein substance. Such substances are called cofactors. The cofactors may be simple metal ions like Mg++ or complete organic compounds. |
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| An enzyme which functions only in the presence of a cofactor, is called apoenzyme. If the cofactor is an organic compound which can be easily separated from the apoenzyme, it is called coenzyme. If the cofactor is firmly bound with the enzyme, it is called prosthetic group. A working combination of an apoenzyme and its cofactor is called as holoenzyme. |
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