Investigate how the concentration of the substrate hydrogen peroxide (H2O2) affects the rate of reaction of the enzyme catalase in yeast cells.

Introduction

Enzymes were discovered be Edward Buncher towards the 19th century. Enzymes such as catalase are globular protein molecules found in all living cells. They are biological catalysts, which are used to speed up a reaction rate within the cell. They reduce the level of activation energy needed in a reaction, increasing the rate of reaction by doing that. The higher the activation energy, the slower the reaction is going to be. Figure 1, below shows the amount of activation energy needed with and without an enzyme.

Figure 1: The activation energy of a reaction is smaller than in the presence of an enzyme

Enzyme-substrate complex

An area on the surface of an enzyme is known as the active site. The substance that the enzyme acts on is called the substrate. The substrate fits into the active site (shapes fit together); therefore their shapes are complimentary to each other. Each enzyme has a different active site; making enzymes specific, because only a substrate with a complimentary shape to the active site will fit.

When enzymes denature, the bonds that are holding the active site together start to break making the shape of the active site no longer complimentary to the substrate's shape. The denaturing of enzymes means that the tertiary structure of the polypeptide chain has changed and caused a change in its three dimensional shape. The 'Lock and Key' hypothesis states that the enzyme is like a lock and the substrate like a key; so to lock or unlock something you will have to have the exact lock and the exact key or else you would not be able to do anything with either the key or the lock. This is exactly how the enzyme-substrate complex works. Figure 2 shows how the enzyme breaks a large molecule into a smaller molecule. The same enzyme will also catalyse the reverse reaction; it will then join the smaller molecules together again to form larger molecules. The formation of the enzyme-substrate complex reduces the activation energy of the reaction.

Figure 2: The 'Lock and Key' mechanism of enzyme action

Competitive and non-competitive inhibition

Inhibition is when some substances prevent enzyme-substrate complexes being formed, or slow down their formation. Competitive inhibitors have a shape similar to that of the substrate. They 'compete' with the substrate for the enzyme's active site; this slows down the reaction, because the inhibitor blocks the active site, not allowing the substrate to bind. The effect of the competitive inhibition can be reversed by increasing the amount of inhibitor. Figure 3 shows the effect of a competitive inhibitor on an enzyme-controlled reaction over a range of different amounts of substrate (to speed up the reaction). The graph below shows that when the inhibitor is present, the reaction is slower with small amounts of substrate than when there is no inhibitor present in a reaction with large amounts of substrate.

Figure 3: Effects of a competitive inhibitor on

an enzyme-controlled reaction

Non-competitive inhibition is when the inhibitor binds to an area on the enzyme away from the active site; this causes the enzyme's shape to change, as well as the active site's to change. This means that the substrates can no longer bind; therefore enzyme-substrate complexes can no longer form. Increasing the amount of substrate present does not overcome the effect of a non-competitive inhibitor. This is shown in Figure 4 below.

Figure 4: The effect of a non-competitive inhibitor

on the rate of an enzyme-controlled reaction

Collision theory

The collision theory explains chemical reactions and the way in which the rate of reaction alters when the conditions alter. For a reaction to occur the reactant particles must collide. Only a fraction of the total collisions cause a chemical change. These are called fruitful collisions. The fruitful collisions have sufficient energy activation energy to break the existing bonds and to form new bonds, which then form the products of the reaction. Not all collisions, which are taking place between particles, end up with a reaction. This is because in the middle of a reaction, there is a shape of the particle, which is difficult to compete. This is called the transition state. For a reaction to occur there must be successful collisions in which; particles must have enough energy for the reaction to take place. A reaction speeds up if the numbers of successful collisions are increased. An increase in temperature increases collisions (kinetic energy; movement energy); an increase of concentration of one of the reactants or both increases the chance of particles colliding. An increase in surface area also increases the rate of reaction (more chance of collisions).

Factors affecting the rate of an enzyme-catalysed reaction

These are four factors that can affect the rate of an enzyme reaction:-

The pH
The temperature
The concentration of the enzyme solution
The concentration of the substrate solution

The pH can affect the rate of an enzyme-catalysed reaction by affecting the bonds in the tertiary structure of the enzyme's active site. When bonds are broken, the shape of the active site is distorted, so the substrate molecule cannot bind to the enzyme causing no enzyme-substrate complex to be formed, therefore no products are ...

Investigate how the concentration of the substrate hydrogen peroxide (H2O2) affects the rate of reaction of the enzyme catalase in yeast cells.

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