To find the effect of the enzyme concentration on the reaction between Catalase and hydrogen peroxide.

Hannah Davenport

The Effect of Enzyme Concentration on the Reaction Between Catalase and Hydrogen Peroxide

Aim

Background Knowledge and Variables

(Cambridge Advanced Sciences; Biology 1and Chemistry 1, www.coursework.info)

An enzyme, a biological catalyst, accelerates a chemical reaction without changing the reaction’s outcome and can be recovered from amongst the end products. For just about every reactions in a living organism there is an enzyme to catalyse it.

For chemical reactions to occur, two conditions must be obtained: the reacting molecules collide successfully and that this collision must have enough energy to produce a reaction to break the bonds, which need to be broken. The energy can come from heat but a high temperature is had to control and can be harmful or impractical for the other reactants which are present. Enzymes are globular protein molecules. They are made from protein produced by living cells. A protein molecule is made of polypeptides that are a series of amino acids linked together by peptide bonds. They can be made up of different configurations of polypeptide chains. There are secondary, tertiary and quaternary structures (i.e. haemoglobin) protein molecules which increase respectively in complexity. Proteins contain an ’R-group’ which is specific to each amino acid, this part of the molecule is the active site. Enzymes lower the activation energy needed for chemical reactions and allow the reactions to occur rapidly at the relatively low temperature of living things, without enzymes the high metabolic rates in organisms would not exist.

An enzyme decreases the activation energy so that the substrate molecules have energy greater than, or can easily reach the new activation energy and therefore the rate of reaction increases. For this to occur the catalase molecule must collide with the Hydrogen Peroxide molecule in the correct place (active site) for the collision to be successful and produce the products. The random movement of the molecules in the liquid solution results in these collisions. The enzyme must temporarily bond with the substrate in the active site for a fraction of a second for the reaction to occur. An enzyme has an active site as part of it’s tertiary structure which is a specific shape and will only fit on the particular molecule called it’s substrate therefore enzymes are specific.

The substrate is held in place by temporary bonds, the reaction takes place and then the products are released. The enzyme can then be used over again as it never changes. The rate at which substrate molecules can bind to the active site, be formed into products and leave can be very rapid. The enzyme Catalase, for example, can bind with hydrogen peroxide molecules, split them into water and oxygen and release these products at a rate of 50,000 molecules per second. This is the highest rate of all the enzymes.

Catalase (or Peroxidase) is very important in living cells, without this enzyme the hydrogen peroxide would cause severe damage to the cell but due to Catalase the hydrogen peroxide is decomposed into water and oxygen which are harmless in small amounts, but these products must be broken down again so that there is no damage to the cell.

There are some factors that, if changed, can affect the rate of reaction. Some of these are:

Enzyme Concentration

The above shows the results of an investigation in which different amounts of Catalase were added to the same amount of hydrogen peroxide. You can see that the shapes of all five curves are similar. In each case the reaction begins very quickly (steep curve) and then gradually levels off (curve levels off). Because the amounts of hydrogen peroxide are the same in each of the experiments the final volumes of oxygen released will be the same for all the experiments. In these conditions the reaction rate is directly proportional to the enzyme concentration.

Temperature

The rate of reaction increases up to 37ºC ad the decreases because up to 37ºC the molecules have more energy therefore there are more collisions between the enzyme and the substrate as the probability of the substrate and the enzyme meeting is much higher when the molecules are moving faster. After 37ºC the enzyme is denatured i.e. the bonds holding the tertiary structure vibrate so much they break. This is irreversible. The structure is changed therefore the active site is changed and the substrate no longer fits. At lower temperatures the molecules move much slower and collide with less energy therefore the collisions are not as effective.

A particular enzyme has a particular pH at which it works best i.e. a pH optimum.

A change in pH affects the charges on the enzyme molecule and so interferes with the H bonds and the ionic bonds. I.e. if the pH is low then the hydrogen ions can interact with the ionic bonds in the enzyme structure and distort the active site. This changes the tertiary structure which means the substrate no longer fits into the active site and therefore the substrate is not broken down.

Substrate Concentration

At a low substrate concentration the rate of reaction increases with increasing substrate concentration. When a certain substrate concentration is reached, increasing it further has no further effect on the rate of reaction. At the time when the rate of reaction is no longer increasing, all the active sites are filled.

At low substrate concentrations there are still spare enzyme active sites available therefore increasing the substrate concentration increases the collisions and increases the rate of reaction.

Inhibitors

Inhibitors ...