Experiment to Investigate a Factor which Affects the Rate of Decomposition of Hydrogen Peroxide by Catalase.

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Phil Cooper L6

10.11.01.

Experiment to Investigate a Factor which Affects the Rate of Decomposition of Hydrogen Peroxide by Catalase

Aim

The aim of this investigation is to investigate a factor which affects the rate of decomposition of hydrogen peroxide using the enzyme catalase.

Background

Catalase is present in nearly all the peroxisomes of nearly all aerobic cells, serving to protect the cell from the toxic effects of hydrogen peroxide by catalysing its decomposition into molecular oxygen and water.  The overall reaction for this is:

2 H2O2      2 H20 + O2 

The enzyme is among the most efficient known, with rates up 200,000 catalytic events/second.

‘Hydrogen peroxide, H2O2, is a colourless, syrupy liquid that is a strong oxidising agent and, in water solution, a weak acid. It is miscible with cold water and is soluble in alcohol and ether. Although pure hydrogen peroxide is fairly stable, it decomposes into water and oxygen when heated above about 80°C; it also decomposes in the presence of numerous catalysts, e.g., most metals, acids, or oxidisable organic materials. A small amount of stabiliser, usually acetanilide, is often added to it. Hydrogen peroxide has many uses. It is available for household use as a 3% (by weight) water solution; it is used as a mild bleaching agent and medicinally as an antiseptic. The 3% solution is sometimes called ten volume strength, since one volume of it releases ten volumes of oxygen when it decomposes.’ ()

Variables

In this experiment, there are a number of variables which may affect the rate of reaction.  These are:

  • temperature at which the reaction takes place
  • pH at which the reaction takes place
  • concentration of the enzyme
  • concentration of the substrate
  • enzyme inhibition

  • Temperature – as the temperature at which the reaction takes place is increased, the rate of reaction will increase up to a point.  From this point the rate of reaction will decrease to a point at which it will be zero.  At low temperatures the reaction will take place slowly, for the reason that the molecules are moving slowly.  This means substrate molecules will not collide with the active site very often, so the substrate and enzyme rarely bind.  As the temperature of the reaction is increased, the kinetic energy is increased so the molecules move around faster, and therefore collisions are more frequent.  When they do collide, they collide with more energy, making it easier for bonds to be broken so that the reaction can take place.  As temperature continues to increase, so does the speed of the molecules.  However, at a certain temperature, being slightly different for each enzyme, the structure of the enzyme molecule starts vibrating too violently for the bonds holding the molecule together, so some of the bonds begin to break, especially the hydrogen bonds.  The enzyme molecule begins to deform and lose its shape, so the substrate molecule can no longer fit into the active site.  The enzyme is said to be denatured.  At first, the active site is only slightly deformed, so the substrate molecule does not fit in as well, so the rate of reaction decreases.  As the enzyme is heated more, more bonds begin to break and eventually the substrate cannot fit into the active site of the enzyme at all.  The optimum temperature of a reaction is the temperature at which the enzyme catalyses a reaction at its maximum rate.  This is usually around 40°C.  Below is a sketch graph of the effect of temperature on the rate of an enzyme controlled reaction:

Graph to show the effect of temperature on an enzyme controlled reaction (Cambridge Biology 1)

  • pH – most enzymes work fastest at around a pH of 7, so in neutral conditions.  pH is the measure of the concentration of hydrogen ions in a solution.  The lower the pH, the higher the hydrogen ion concentration.  Hydrogen ions can interact with the R groups of amino acids, affecting the way in which they bond with each other and therefore affect their 3D arrangement.  A pH which is very different from the optimum pH of the enzyme in question can cause denaturation.  Below is a sketch graph so show the effect of pH on an enzyme controlled reaction:

Graph to show the effect of temperature on an enzyme controlled reaction (Cambridge Biology 1)

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  • Concentration of the enzyme – at low concentrations of the enzyme, there are more substrate molecules than enzyme molecules to catalyse the decomposition.  This means that all the active sites are filled with substrate molecules, and there are substrate molecules ‘waiting’ for an empty active site to bind with, so there will be a fairly low rate of reaction.  At higher concentrations, there are more active sites available, so there are more catalytic events/second, so the rate of reaction will increase.  Doubling the number of enzyme molecules will double the number of active sites available for the substrate to ...

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