Lock and Key theory
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Either a single molecule with a complementary shape, or more than one molecule that together have a complementary site can fit into the active site.
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These substrate molecules form temporary bonds with the amino acids of the active site to produce an enzyme substrate complex.
- The enzyme holds the substrate molecules in such a way that they react more easily.
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When the reaction has taken place the products are released, leaving the enzyme unchanged.
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The substrate is often likened to a key, which fits into the enzyme lock, so this is known as the lock and key of enzyme reaction.
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Each enzyme will only catalyse one specific reaction because of its precisely shaped active site.
Finding rates of enzyme-controlled reactions
The rate of reaction is measured by determining the quantity of substrate used, or the quantity of a product formed in a given time. For example, when the example, when the enzyme catalase is used to break down the hydrogen peroxide H202, to water and oxygen, the rate of reaction can be found by measuring the volume of oxygen given off in a known time. If we mix a fixed quantity of enzyme and substrate, at first the reaction will proceed quickly, but as the substrate is used up, there will be fewer substrate molecules to bind with the enzyme and the reaction slows down and eventually stops. The slope of the rapid phase of this reaction is known as the initial rate of reaction.
How do enzyme and substrate concentrations affect the rate of reaction?
In A, the initial rate of reaction is directly proportional to the enzyme concentration because there are more enzymes present, so there are more active sites that are available to form enzyme substrate complexes. The increase it rate will continue because there is an excess amount of the substrate. In B you can see how at high substrate concentrations it is the enzyme concentration that limits the rate of reaction. Every active site is occupied and substrate molecules cannot enter an active site until one becomes free again.