ENZYMESENZYMES

Mode of Action of EnzymesMode of Action of Enzymes

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• Biological catalystsBiological catalysts – a molecule which – a molecule which speeds up a chemical reactionspeeds up a chemical reaction but but remains unchangedremains unchanged at the end of the at the end of the reactionreaction

• Names end in –aseNames end in –ase• Globular proteins (soluble)Globular proteins (soluble)• Active siteActive site – region (cleft or depression) to – region (cleft or depression) to

which another molecule or molecules which another molecule or molecules ((substratesubstrate) can bind) can bind

• Substrate fit perfectly and is held in place Substrate fit perfectly and is held in place by temporary bonds which form between by temporary bonds which form between the substrate and some of the R groups of the substrate and some of the R groups of the enzyme’s amino acids (the enzyme’s amino acids (enzyme-enzyme-substrate complexsubstrate complex) )

• Specific Specific - shape of active site will only - shape of active site will only allow one shape of molecule to fit allow one shape of molecule to fit

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Mode of action of enzymesMode of action of enzymes

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Activation energyActivation energy

• Enzymes increase the Enzymes increase the rate at which rate at which chemical reactions chemical reactions occuroccur

• Activation energy – Activation energy – extra energy extra energy temporarily given to temporarily given to substrate to convert substrate to convert it to productit to product

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The course of a reactionThe course of a reaction

• Oxygen that is released can be collected Oxygen that is released can be collected and measuredand measured

• Initial rate of reactionInitial rate of reaction is fastest (large is fastest (large volume of oxygen is collected in the first volume of oxygen is collected in the first minute of the reaction)minute of the reaction)

• Initial rate is measured by calculating the Initial rate is measured by calculating the slope of a tangent to the curve, as close to slope of a tangent to the curve, as close to the time 0 as possiblethe time 0 as possible

• As reaction continues, the rate at which As reaction continues, the rate at which oxygen is released gradually slows down oxygen is released gradually slows down until it eventually stops completelyuntil it eventually stops completely

Effect of enzyme Effect of enzyme concentrationconcentration

• Reaction rate is directly proportional Reaction rate is directly proportional to the enzyme concentrationto the enzyme concentration

• The more enzyme present, the more The more enzyme present, the more active sites will be available for the active sites will be available for the substrate to slot intosubstrate to slot into

• As long as plenty of substrate available, As long as plenty of substrate available, initial rate of reaction increases linearly initial rate of reaction increases linearly with enzyme concentrationwith enzyme concentration

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• As substrate concentration increases, the initial As substrate concentration increases, the initial rate of reaction also increasesrate of reaction also increases

• In the same way that increasing the temperature In the same way that increasing the temperature increases the chances of a favourable collision, so increases the chances of a favourable collision, so increasing the substrate concentration increases increasing the substrate concentration increases the chances - because there are simply more the chances - because there are simply more substrate molecules kicking about!substrate molecules kicking about!

• Unfortunately this doesn't carry on forever. Unfortunately this doesn't carry on forever. Eventually you reach an optimum value. If you Eventually you reach an optimum value. If you increase the concentration of the substrate increase the concentration of the substrate passing this point, it makes no difference. passing this point, it makes no difference.

• Increasing the substrate concentration increases Increasing the substrate concentration increases the activity of the enzyme, the activity of the enzyme, up to a pointup to a point, after , after which increasing the substrate concentration has which increasing the substrate concentration has no effect no effect

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Effect of substrate concentrationEffect of substrate concentration

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Temperature and enzyme Temperature and enzyme activityactivity

• Temperature increases reaction rate (temperature Temperature increases reaction rate (temperature also increases the activity of an enzyme)also increases the activity of an enzyme)

• The more times they hit each other, the more likely it The more times they hit each other, the more likely it is that the substrate will slot into the active site, is that the substrate will slot into the active site, leading to the product being formedleading to the product being formed

• The higher the temperature, the more energy the The higher the temperature, the more energy the molecules have, so they'll be moving more quickly, molecules have, so they'll be moving more quickly, and are more likely to collide successfully. So, the and are more likely to collide successfully. So, the higher the temperature, the greater the activity of higher the temperature, the greater the activity of the enzymethe enzyme

• So as you increase temperature, the activity of an So as you increase temperature, the activity of an enzyme will increase, until you reach the optimum enzyme will increase, until you reach the optimum value. After that, any further increase in temperature value. After that, any further increase in temperature will result in denaturation of the enzyme, and a steep will result in denaturation of the enzyme, and a steep drop in activity drop in activity

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pH and enzyme activitypH and enzyme activity

• pH is a measure of the concentration of hydrogen pH is a measure of the concentration of hydrogen ions in a solutionions in a solution

• Hydrogen ions can interact with the R groups of Hydrogen ions can interact with the R groups of amino acids, affecting the way in which they bond amino acids, affecting the way in which they bond with each other and therefore affect 3D arrangement with each other and therefore affect 3D arrangement

• If the pH is changed sufficiently, the enzyme will be If the pH is changed sufficiently, the enzyme will be completely altered due to this effect, and it is said to completely altered due to this effect, and it is said to be be denatureddenatured. However, unlike the effect of extreme . However, unlike the effect of extreme heat, which causes the enzyme to be irreparably heat, which causes the enzyme to be irreparably damaged, denaturation due to pH change is damaged, denaturation due to pH change is reversible. Restore the pH to its original level, and reversible. Restore the pH to its original level, and the enzyme will return to its original capability. the enzyme will return to its original capability.

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Enzyme inhibitorsEnzyme inhibitors

• There are two types of inhibitionThere are two types of inhibition– competitive inhibitioncompetitive inhibition – a substance that – a substance that

reduces the rate of activity of an enzyme by reduces the rate of activity of an enzyme by competing with the substrate molecules for the competing with the substrate molecules for the enzyme’s active site. Increasing the concentration enzyme’s active site. Increasing the concentration of substrate reduces the degree of inhibition of substrate reduces the degree of inhibition

– non-competitive inhibitionnon-competitive inhibition - a substance that - a substance that reduces the rate of activity of an enzyme, but reduces the rate of activity of an enzyme, but where increasing the concentration of the where increasing the concentration of the substrate does not reduce the degree of inhibition. substrate does not reduce the degree of inhibition. Many non-competitive inhibitors bind to areas of Many non-competitive inhibitors bind to areas of the enzyme molecule other than the active site the enzyme molecule other than the active site itself itself

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• Competitive InhibitionCompetitive Inhibition This is where the inhibitor is a molecule which This is where the inhibitor is a molecule which has a similar shape to the molecule which is has a similar shape to the molecule which is supposed to be binding to the active site. In supposed to be binding to the active site. In the case of enzymes, a competitive inhibitor the case of enzymes, a competitive inhibitor may have the same shape as that of the may have the same shape as that of the substrate, but it doesn't react in the same substrate, but it doesn't react in the same way. Rather than turning into the product, it way. Rather than turning into the product, it simply uses up time and prevents the simply uses up time and prevents the substrate from getting to the active site. It substrate from getting to the active site. It blocks the way.blocks the way.

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• Non-competitive InhibitionNon-competitive Inhibition This is the kind that exists when a molecule This is the kind that exists when a molecule binds to a different site on the protein, binds to a different site on the protein, rendering it inactive. Sometimes it does this rendering it inactive. Sometimes it does this before the substrate reaches the active site, before the substrate reaches the active site, sometimes afterwards, but in either case it sometimes afterwards, but in either case it stops the protein doing its job, and prevents stops the protein doing its job, and prevents a product being formed.a product being formed.

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• In both competitive and non-In both competitive and non-competitive inhibition, it is possible to competitive inhibition, it is possible to have both have both reversiblereversible and and irreversibleirreversible inhibitors. As the name suggests, a inhibitors. As the name suggests, a reversible inhibitor does not have a reversible inhibitor does not have a permanent affect - it will stop the permanent affect - it will stop the protein doing what it is supposed to do, protein doing what it is supposed to do, but it will move off again and allow the but it will move off again and allow the protein to function later on; an protein to function later on; an irreversible inhibitor, on the other hand, irreversible inhibitor, on the other hand, permanently renders the protein permanently renders the protein inactive, so it will have to be replaced inactive, so it will have to be replaced by a brand new one - the inhibitor will by a brand new one - the inhibitor will not budge not budge

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• Penicillin binds to Penicillin binds to the enzyme the enzyme (transpeptidase) (transpeptidase) that links the that links the peptidoglycan peptidoglycan molecules in molecules in bacteria, and this bacteria, and this weakens the cell weakens the cell wall of the wall of the bacterium when it bacterium when it multiplies multiplies

End-product inhibitionEnd-product inhibition

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