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Effect of pH and Temperature
Enzymes in Living Things
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The following article is incomplete and will be updated shortly.


Enzymes are biological catalysts.  Catalysts increase the rate of chemical reactions without being changed themselves - i.e. you get the catalyst back at the end.  Enzymes are protein molecules and are made up of longs chains of amino acids.  These long chains are folded to produce a special shape (called the active site) which enables other molecules to fit into the enzyme.  This shape is vital for the enzyme's function.  High temperatures destroy this special shape - and the enzyme is denatured.  (Never say enzymes have been killed by high temperatures as they are not living - only living things can be killed or die).


How enzymes work

The substrate (reactant) fits into the active site of the enzyme - just like a key fits into a lock.  The enzyme and the substrate bind together to form an enzyme-substrate complex.  The binding of the substrate to an enzyme helps a chemical reaction to take place, often making it occur thousands of times faster than if the enzyme was not present.  The reaction occurs rapidly and the products are released from the surface of the enzyme.  Enzymes are used in living organisms to join together small molecules to make larger ones as well as breaking up larger ones.  The diagrams below show how enzymes work.




Factors Affecting Enzyme Action

Like most other chemical reactions, the rate of enzyme-controlled reactions increases with temperature.  However, this is only true up to temperatures of about 40oC.  Above this temperature the amino acid chains which make up the enzyme begin to unravel and the shape of the active site changes.  It has been denatured.  The enzyme no longer works as a catalyst and the rate of reaction drops rapidly.

Investigating the Effect of Temperature on an Enzyme Controlled Reaction

The enzyme amylase, which is contained in your saliva, breaks down starch into sugar.  (See the section on digestion for the structure of starch and sugar molecules).  The presence of starch can be tested using iodine solution.  In the presence of starch iodine turns blue/black and if no starch is present it is a pale yellowy-brown colour.  Iodine can therefore be used to monitor the breakdown of starch into sugar.  When the iodine no longer changes colour, you know all of the starch has been broken down.

In this experiment, test tubes containing 5 ml of starch were pre-warmed in a water bath at the appropriate temperature for 5 minutes.  1 ml of amylase was also placed in each water bath in a separate test tube.  After 5 minutes, when the starch and amylase had reached the temperature at which we were working, the two tubes were mixed together and a stopwatch started.  The tube containing the reaction mixture was then kept in the waterbath at the appropriate temperature.  After one minute a few drops of the reaction mixture were added to iodine on a spotting tile to test for the presence of starch.  When the iodine no longer changed colour (i.e. no starch present) the time was recorded.  The results of the experiment are shown below.


The results show that the reaction was most rapid at 40oC and the starch remained undigested at 10oC and 60oC after 12 minutes.  These results can be plotted on a graph to show how the rate of reaction varies with temperature:

Note that the rate of reaction increases from 10oC to 40oC.  Increasing the temperature causes the reactants to move faster making a collision between the enzyme and the substrate more likely.  At 40oC the rate of reaction reaches a maximum.  Above this temperature, the enzyme becomes denatured and it can no longer catalyse (speed up) the reaction.

Effect of pH on Enzyme Action

The pH of the solution surrounding an enzyme can also affect its activity.  Changes in pH can also cause enzymes to become denatured.  Many enzymes have optimum pH values at which they work best e.g. pepsin  a protease found in the stomach works best in acidic conditions, whereas amylase found in the small intestine works best under slightly alkaline conditions.  In the experiment shown below, photographic film was added to tubes containing biological washing powder at different pHs.  Biological washing powder contains enzymes which break down protein molecules in stains.  The photographic film has a coating which contains protein.  If this is digested, the film becomes transparent.  In this case it can be seen that the enzymes in the washing powder work best under slightly alkaline conditions.




What Are Enzymes Used For In Living Things?