Investigation into the effect of Temperature on the action of the Enzyme Lipase.

Planning

Background Information

Enzymes are large proteins that speed up chemical reactions. In their globular structure, one or more polypeptide chains twist and fold, bringing together a small number of amino acids to form the active site, or the location on the enzyme where the substrate binds and the reaction takes place. Enzyme and substrate fail to bind if their shapes do not match exactly. This ensures that the enzyme does not participate in the wrong reaction. The enzyme itself is unaffected by the reaction. When the products have been released, the enzyme is ready to bind with a new substrate.

Heat supplies kinetic energy to reacting molecules, causing them to move more rapidly. The chances of molecular collision taking place are thus increased at higher temperatures so it is more likely that enzyme-substrate complexes will be formed. However, heat energy also increases the vibration of atoms, which make up enzyme molecules. If the vibrations become too violent, chemical bonds in the enzyme break and the precise three-dimensional structure, so essential for enzyme activity, is lost. At high temperatures therefore enzymes become denatured.

When the effect of temperature on enzyme activity is investigated experimentally, a temperature usually called the optimum temperature is observed at which the reaction proceeds most rapidly. This temperature is not necessarily that at which the enzyme is most stable. It is the resultant of the contrary effects of temperature on the movements of reactants and of enzyme denaturing.

The reason denaturing has such a drastic effect on enzymes is explained in the Lock and Key method which enzymes use when breaking down their substrates. This method shows that enzymes with their precise structure fit perfectly into their substrate to break it down. If their structure changes just slightly then they will be unable to fit precisely into their substrate’s shape and they will be useless.

Background Information and secondary sources used:

Biological Studies by Henry Wilkinson, published by Nelson, page number 46.

Encarta ‘97

Prediction

In the experiment I shall be testing six different temperatures: 20°, 30°C, 40°C, 50°C, 60°C and 70°C. This leaves a range of 50°C and I feel that this is a wide enough variety to obtain a valid conclusion.

I predict that the optimum temperature will be 40°C since it is the closest to the human body temperature 37°C. 37°C has been mentioned in secondary sources as a possible optimum and out of the temperatures I am testing 40°C is the closest to this optimum. I therefore predict that 40°C will be the temperature at which the experiment has the maximum reaction rate.

In general, temperature and kinetic energy are proportional, therefore at higher temperatures the molecules possess a higher kinetic energy and therefore move faster. This has a direct effect on the rate of reaction since if there is less kinetic energy the enzyme and the substrate will not collide as often. If the chances of molecular collision are lower, there is also less chance of successful enzymes-substrate complexes, which therefore leads to lower reaction rates. High kinetic energy also impacts on the reactions in a different way since the molecules collide at higher speeds increasing the possibility of successful collisions since more collisions are likely to have the minimum energy required to begin the reaction (activation energy). All of this contributes to the idea that at lower temperatures the reaction time is slower than at the optimum and this is why I did not predict the optimum at a lower temperature than 40°C.*

At high temperatures the heat energy increases and causes the atoms that make up the enzyme molecules to vibrate. If these vibrations become too violent this eventually leads the enzyme structure to change from its precise three-dimensional shape because the chemical bonds inside it have broken down. This change in shape renders it useless to perform its function, as its specially structured form, essential for enzyme activity, is altered. In other words at higher temperatures bonds are broken which mean that the lipase enzyme key can no longer fit in the milk substrate lock (diagram of lock and key hypothesis ...