Substrate inhibition will sometimes occur when excessive amounts of substrate are present. 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. If a dissimilar substance which does not fit the site is present, the enzyme rejects it, accepts the substrate, and the reaction proceeds normally. Hence, the observed reaction is slowed down because some of the available enzyme sites are occupied by the inhibitor. When the inhibitor wins, it gains the lock position but is unable to open the lock. However, when an inhibitor which resembles the substrate is present, it will compete with the substrate for the position in the enzyme lock. 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. The substrate is held in such a way that its conversion to the reaction products is more favorable. 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. A theory called the "lock-key theory" of enzyme catalysts can be used to explain why inhibition occurs.įigure 9: Lock and key theory – competitive analysis. As mentioned earlier, the existence of temporary ES structures has been verified in the laboratory.Ĭompetitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. Most theories concerning inhibition mechanisms are based on the existence of the enzyme-substrate complex ES. There are three common types of enzyme inhibition - competitive, non-competitive and substrate inhibition. Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis.
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