How do temperature and concentration affect the rate of decomposition of hydrogen peroxide by the enzyme catalase?

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Chemistry coursework                Yr 13 Major Project

How do temperature and concentration affect the rate of decomposition of hydrogen peroxide by the enzyme catalase?

Aims: This is an experiment investigating how the concentration of hydrogen

  peroxide (substrate) and temperature affect the rate of reaction of the  

  enzyme catalase.

Introduction

An enzyme is a globular protein found in living cells, they have the ability to catalyse an experiment, therefore they are often used to speed up reactions that don’t happen fast enough naturally. Enzymes speed up a reaction by lowering the activation enthalpy (Ea), thus meaning that more collisions between substrate molecules and enzyme molecules result in a reaction, see fig 1 below to see how the activation enthalpy of an experiment varies in the presence of an enzyme

Fig. 1: How Ea varies when a catalyst is and isn’t present

The graph shows that when there is no enzyme present then less of the molecules possess the activation energy so fewer collisions between particles will results in a reaction, however when the enzyme is present more of the molecules have the activation energy so more collisions results in a reaction.

All enzymes are specific, they are only able catalyse one or possible a few substrates. Enzymes can recognise its substrate from even isomers, they are that specific. This is because of its three dimensional shape. Only a small part of the enzyme binds with the substrate, this is the enzymes active site. The active site is formed from a few of the enzymes amino acids. Enzymes are specific because the substrate has to fit into the enzymes active site. As the substrate enters the active site, the enzyme changes shape slightly so that the substrate fits in the active site tightly to form an enzyme-substrate complex. Weak hydrogen and ionic bonds bind the substrate and enzyme. The enzyme-substrate complex brings chemical sites from the enzyme in a better position to break down the substrate. The products are then released from the enzyme and it is free to catalyse another substrate molecule because it is chemically unchanged by the reaction. Fig 2 on the next page shows us the induced fit model of an enzyme, on the diagram we can see how the enzymes shape changes slightly when it bonds to the substrate and how it returns to normal once the products have been released.

ENZYME+SUBTRATE → ENZYME-SUBSTRATE COMPLEX → ENZYME+PRODUCTS

Fig 2: The induced fit model of an enzyme and its substrate

Hydrogen peroxide is a harmful substance that is formed as a bi product of metabolism does break down naturally but for this experiment I am going to use a catalyst to speed the reaction up. The presence of this catalyst lowers the activation enthalpy by weakening the bonds present in the substrate so less energy is required to break the bonds and form new products.

The catalyst that I am going to use for the investigation is the enzyme catalase. Catalase is a breaker enzyme and performs the catabolic reaction (where a substrate is broken down into two products) of decomposing hydrogen peroxide. Catalase is found in food such as potato and liver. However for my experiment I will use yeast in the form of a liquid suspension as my source of yeast as it gives me a constant surface area for my enzyme.

For a reaction to compare between reactants two things must happen:

  1. The particles must collide.

  1. The total kinetic energy of the particles must at least equal the activation energy needed for them to enter the transition state. They must then have enough energy for the reaction to occur (activation energy).

There are factors that can affect how well an enzyme works, and therefore its rate of reaction, these are:

  • pH – The pH can affect the bonds in the secondary and tertiary structures of the enzyme. This happens because the charges on the ions present in the solution can attract the ions present in the enzyme. If this happens and the bonds break the shape of the active site will become distorted and the substrate will no longer fit into its active site, so the substrate will not get broken down. Every enzyme has its own optimum pH however most work best in neutral conditions, see fig 3 on the next page.

Fig 3 the effect of pH on rate of reaction

  • Temperature – As temperature rises, the rate of reaction also increases. This happens because the molecules have more kinetic energy so they collide more often, and more of these collisions result in the substrate hitting the enzymes active site. This means that there is more chance of a reaction occurring so the rate of the reaction increases. However if you increase the temperature too much the rate of reaction decreases, even if the molecules do have a higher kinetic energy and more collisions occur between them. This happens because the active site of the enzyme can begin to distort, just like with pH. The bonds present in the enzyme can break so the active site changes, no enzyme substrate-complexes are able to form and therefore no product is formed. We say that the enzyme has been denatured, this happens at around 40oC for most enzymes.

  • Surface area – Reactions happen faster when the surface area is increased. This is because there are more areas of contact available for the molecules to use. Therefore we can say that powders would react quicker than solid lumps of reactants, a liquid even faster because the surface area is even higher.

  • Concentration – When a solution is more concentrated there are more molecules present in the same volume. This means that there are more chances for collisions between catalyst and substrate, therefore there is a higher chance of a reaction between them, so the rate of reaction is increased. However a point will be reached, the saturation point, where no active sites are free for more substrate molecules to bind with at one time, therefore the rate can increase no further.

  • Pressure – A rate of reaction can also be increased by increasing the pressure, the molecules are pushed closer together therefore more collisions will occur between them, like when you increase the concentration, there is a higher chance of a reaction occurring because more collisions are happening.
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Prediction

The reaction for the decomposition of hydrogen peroxide is:

2H2O2(aq) + catalase(aq) → 2H2O(l) + O2(g)

I predict that the higher the concentration and the temperature, the faster the reaction will be because of the points that I have mentioned above. I also predict that as I increase the temperature and concentration the initial rate of the reaction will increase. This is because if the concentration is higher there are more active sites available for the hydrogen peroxide to bind with at one time so more products will be formed more quickly. More products will also be formed, more ...

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