Effect of Temperature on an Enzyme Controlled Reaction

To investigate the effect of temperature on an enzyme controlled reaction

Introduction and planning

For the investigation of enzymes, I am going to conduct an experiment to see how temperature can affect the rate of reaction of enzymes by testing it with starch.

The enzyme that we are going to use is called amylase. We are going to test this enzyme with starch. By mixing amylase and starch solutions together under different temperature conditions, we can record the rate of reaction by taking a sample out and test it with iodine solution to see if there is any remaining starch present.

We have to use the enzyme amylase because enzymes will only work on a specific substrate i.e. amylase will work on starch because of its special shape of active site. This is known as the 'enzymes specificity'.

The enzyme amylase is used for hydrolysing starch and glycogen to form glucose and maltose. This enzyme is found in human saliva for the use of breaking down starch in food, enabling the body to absorb and produce ATP energy.

Enzymes are in all living organisms, they are proteins made up of polypeptide chains (which are made up of many amino acids) with irregular components which give it a globular shape. This makes it a dense, small, compacted molecule that can move around very easily.

For the enzyme to work, it must collide with a substrate. If they collide at the right place, the substrate will then 'fit' into the active site of the enzyme forming enzyme-substrate complex; fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily. In order for any reaction to occur, there must be energy to allow it to happen; this is called the activation energy. As shown below.

http://en.wikipedia.org/wiki/Enzyme_reaction_rate

This is a general graph that shows how activation energy works. The top dotted line to the time line is the activation energy needed for the reaction without enzymes; and the second dotted line to the time line is the energy needed to start a reaction with enzymes. This clearly shows that the differences between using an enzyme to start the reaction and not using it. The y axis represents two different things on this graph; the positive represents the used energy to start the reaction, and the negative half is showing the energy released for the reaction. Overall, the reaction with the enzymes can work faster then the other one without, but they still reach the same energy out-put at the same time forming same the products, also with the help of enzymes, the energy needed to start the reaction reduces compared with the energy needed without the enzyme.

There are many factors can affect the rate of reaction in enzymes:

* Temperature

* pH

* Concentration of enzymes

* Concentration of substrate

Temperature

This factor has a big influence on enzymes' activity, because like any other chemical reaction, heat is needed to give the molecules more energy to move around - kinetic energy, so the molecules will move faster and therefore increase the chance of collision.

The following graph shows the general reaction rate of the enzymes against the temperature.

As the temperature increases, the reaction of enzymes also increases due to more kinetic energy; more vibrations, resulting an increased rate of collision with the substrate. But as it goes above the maximum temperature (optimum temperature), it starts to decrease rapidly. This is because the high temperature/ the high kinetic

energy breaks the bonds that hold the enzyme's tertiary structure together which change the shape of the active site and will not react with the substrate anymore - denatured.

Denaturation is often irreversible.

pH

There is also an optimum pH value where the enzymes work most efficiently. Most enzymes work best around pH 7, which is neutral.

pH is a measure of hydrogen ions in a solution, if it is too acidic, the enzymes will become denatured, if the enzymes are too alkaline, the enzymes will too be denatured. The H+ and the OH- ions can cause chaos to the ionic bonds between each polypeptide chain in the enzymes (protein), which then break up and make a permanent change to its tertiary structure, and therefore, enzyme denatures.

The rate of reaction did not start at the point zero, it started around 3 and reached its optimum at around pH 7 to 8. It started to decrease and finally there were no reactions at around pH 11 to 12.

Concentration of enzymes

If the concentration of enzymes increases, there is an increase of enzyme molecules and active sites available. This gives a higher chance of collision with a substrate and forms a product.

The graph should look like this:

The rate of reaction is directly proportional to the enzyme concentration.

As long as there is enough substrate available to bind with an active site, the rate of reaction will increase linearly with enzyme concentration.

Concentration of substrate

As the concentration of substrate increases, the rate of reaction will also increase because the substrates are binding with enzymes. At the beginning of the reaction, the line increases steeply because there is less substrate compared to the concentration of enzymes. They will bind with an enzyme very quickly, resulting in a steep linear at the beginning. But as soon as the amount of substrate increases to a certain point (saturation point), the rate of reaction starts to flatten out. This is because there aren't enough enzymes free for binding.

It will always get to a point where all the enzymes are being used and adding more substrate will have no effects on the rate of reaction.

By looking at the picture below, which I took from the Internet, the Lock and Key theory is about an area on the enzyme called the active site; this is an area which the substrate molecule fits into. The size and shape of the substrate matches exactly to a specific enzyme's active site, so they fit together like