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  1. 1. ENZYMESMode of Action of Enzymes ALBIO9700/2006JK
  2. 2. • Biological catalysts – a molecule which speeds up a chemical reaction but remains unchanged at the end of the reaction• Names end in –ase• Globular proteins (soluble)• Active site – region (cleft or depression) to which another molecule or molecules (substrate) can bind• Substrate fit perfectly and is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme’s amino acids (enzyme-substrate complex)• Specific - shape of active site will only allow one shape of molecule to fit ALBIO9700/2006JK
  3. 3. Mode of action of enzymes ALBIO9700/2006JK
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  5. 5. Activation energy• Enzymes increase the rate at which chemical reactions occur• Activation energy – extra energy temporarily given to substrate to convert it to product ALBIO9700/2006JK
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  7. 7. The course of a reaction• Oxygen that is released can be collected and measured• Initial rate of reaction is fastest (large volume of oxygen is collected in the first minute of the reaction)• Initial rate is measured by calculating the slope of a tangent to the curve, as close to the time 0 as possible• As reaction continues, the rate at which oxygen is released gradually slows down until it eventually stops completely ALBIO9700/2006JK
  8. 8. Effect of enzyme concentration• Reaction rate is directly proportional to the enzyme concentration• The more enzyme present, the more active sites will be available for the substrate to slot into• As long as plenty of substrate available, initial rate of reaction increases linearly with enzyme concentration ALBIO9700/2006JK
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  10. 10. Effect of substrate concentration• As substrate concentration increases, the initial rate of reaction also increases• In the same way that increasing the temperature increases the chances of a favourable collision, so increasing the substrate concentration increases the chances - because there are simply more substrate molecules kicking about!• Unfortunately this doesnt carry on forever. Eventually you reach an optimum value. If you increase the concentration of the substrate passing this point, it makes no difference.• Increasing the substrate concentration increases the activity of the enzyme, up to a point, after which increasing the substrate concentration has no effect ALBIO9700/2006JK
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  12. 12. Temperature and enzyme activity• Temperature increases reaction rate (temperature also increases the activity of an enzyme)• The more times they hit each other, the more likely it is that the substrate will slot into the active site, leading to the product being formed• The higher the temperature, the more energy the molecules have, so theyll be moving more quickly, and are more likely to collide successfully. So, the higher the temperature, the greater the activity of the enzyme• So as you increase temperature, the activity of an enzyme will increase, until you reach the optimum value. After that, any further increase in temperature will result in denaturation of the enzyme, and a steep drop in activity ALBIO9700/2006JK
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  15. 15. pH and enzyme activity• pH is a measure of the concentration of hydrogen ions in a solution• Hydrogen ions can interact with the R groups of amino acids, affecting the way in which they bond with each other and therefore affect 3D arrangement• If the pH is changed sufficiently, the enzyme will be completely altered due to this effect, and it is said to be denatured. However, unlike the effect of extreme heat, which causes the enzyme to be irreparably damaged, denaturation due to pH change is reversible. Restore the pH to its original level, and the enzyme will return to its original capability. ALBIO9700/2006JK
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  17. 17. Enzyme inhibitors• There are two types of inhibition – competitive inhibition – a substance that reduces the rate of activity of an enzyme by competing with the substrate molecules for the enzyme’s active site. Increasing the concentration of substrate reduces the degree of inhibition – non-competitive inhibition - a substance that reduces the rate of activity of an enzyme, but where increasing the concentration of the substrate does not reduce the degree of inhibition. Many non- competitive inhibitors bind to areas of the enzyme molecule other than the active site itself ALBIO9700/2006JK
  18. 18. • Competitive Inhibition This is where the inhibitor is a molecule which has a similar shape to the molecule which is supposed to be binding to the active site. In the case of enzymes, a competitive inhibitor may have the same shape as that of the substrate, but it doesnt react in the same way. Rather than turning into the product, it simply uses up time and prevents the substrate from getting to the active site. It blocks the way. ALBIO9700/2006JK
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  21. 21. • Non-competitive Inhibition This is the kind that exists when a molecule binds to a different site on the protein, rendering it inactive. Sometimes it does this before the substrate reaches the active site, sometimes afterwards, but in either case it stops the protein doing its job, and prevents a product being formed. ALBIO9700/2006JK
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  23. 23. • In both competitive and non-competitive inhibition, it is possible to have both reversible and irreversible inhibitors. As the name suggests, a reversible inhibitor does not have a permanent affect - it will stop the protein doing what it is supposed to do, but it will move off again and allow the protein to function later on; an irreversible inhibitor, on the other hand, permanently renders the protein inactive, so it will have to be replaced by a brand new one - the inhibitor will not budge ALBIO9700/2006JK
  24. 24. • Penicillin binds to the enzyme (transpeptidase) that links the peptidoglycan molecules in bacteria, and this weakens the cell wall of the bacterium when it multiplies ALBIO9700/2006JK
  25. 25. End-product inhibition ALBIO9700/2006JK