Reaction of Catalase and Hydrogen Peroxide

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AS and A Level Science

PLANNING SECTION

Aim: - To find the optimum temperature for the enzyme catalyst.

Hypothesis: - I hypothesize that as the temperature increases the rate of enzyme catalysed reaction increases.

Enzymes

Enzymes are complex protein molecules produced by living organisms. They catalyse a vast range of chemical reactions without themselves being chemically changed at the end of the reaction. These biological catalysts are important because they speed up the rate of reaction they catalyse that would otherwise be too slow to support life. For example Catalase catalysis the decomposition of hydrogen peroxide into water and oxygen.

2H2O2 → 2H2O + O2

        Enzymes bind temporarily to one or more of the reactants of the reaction they catalyse. In doing so, they lower the amount of activation energy needed and thus speed up the reaction.

        Cofactors are non-proteins components required by enzymes for their efficient functioning. An enzyme-cofactor complex is called a holoenzyme. An enzyme without its cofactor is called apoenzyme.

The Chemical Structure Of Enzymes

All enzymes are globular proteins. Proteins consist of long chains of amino acids. In a globular protein the amino acid chain is folded and wound into spherical or globular shape. While the enzyme molecule is normally larger than the substrate molecule it acts upon, only a small part of the enzyme molecule actually comes into contact with the substrate. This region is called the active site. Only a few of the amino acids of the enzyme molecule make up the active site. These so-called catalytic amino acids are often some distance apart in the protein chain but are brought into close proximity by the folding of the chain.

        Hydrogen bonds, ionic bonds and disulphide bridges hold the amino acid chain in its distinct three-dimensional shape.

        Enzymes are very large and complex organic molecules that are synthesized by the cell to perform very specific functions. These biological catalysts are important because they speed up the rate of reaction they catalyze that would otherwise be too slow to support life. Catalase is an enzyme present in the cells of plants, animals and aerobic (oxygen requiring) bacteria. It promotes the conversion of hydrogen peroxide, a powerful and potentially harmful oxidizing agent, to water and oxygen molecule. Catalase is composed of four subunits. Each subunit contains a heme group. This heme group is responsible for carrying out catalase's activity. Catalase functions to break down hydrogen peroxide (H2O2) into water and oxygen:

2H2O2 → 2H2O + O2

This reaction is performed by oxidation, the loss of electrons, and reduction, which is the gain of electrons. Each of the sub-units in Catalase uses the energy from electrons to decompose the Hydrogen Peroxide.

Catalase also uses hydrogen peroxide to oxidize toxins including phenols, formic acid, formaldehyde and alcohols.

Enzyme act by binding substrates, forming a complex. The complex stresses chemical bonds forming a transition state. This makes the substrate more reactive. Energy is needed to form this state and the enzyme provides it. The enzyme’s site of attachment and the parts that stress the substrate’s bonds is known as the active site.

Properties Of Enzymes

  1. All are globular proteins consisting of coiled polypeptide chains.
  2. They increase the rate of a reaction without themselves being used up.
  3. Some enzymes require another compound called a coenzyme to be bound to them before they can catalyse reactions.
  4. Their presence does not alter the nature of the end products of the reaction.
  5. Their activity varies with changing pH, temperature, and substrate concentration.
  6. The generally catalyse one specific reaction.
  7. They catalyse a reaction in either direction according to prevailing conditions.
  8. They reduce the activation energy (Ea) required for a chemical reaction to take place.
  9. The names of most enzymes end in –ase, this suffixes often being added to the name of the substrate.

Where Is Catalase Found And What Does It Do?

The image above is the 3D structure of Catalase. The structure was found using x-ray crystallography.

All aerobic organisms use molecular oxygen, 3O2, for respiration or oxidation of nutrients. During reduction of molecular oxygen to water, hydrogen peroxide is generated. It can damage DNA, protein and lipid membranes and may even be a causative factor of cancer. For defence against H2O2 cells contain Catalase. Each type pf organism has its own Catalase, which probably differs at least slightly in other species of organism.

        The rate of which enzymes catalyse reaction varies with their environment. Enzymes work best within an optimum range of physical and chemical conditions.

Catalase is located in cell organelle called the peroxisome. Peroxisomes in animal cells are involved in the oxidation of fatty acids, and the synthesis of cholesterol and bile acids. Hydrogen peroxide is a by-product of fatty acid oxidation. White blood cells produce hydrogen peroxide to kill bacteria. In both cases Catalase prevents the hydrogen peroxide from harming the cell itself. Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of the nitrogen molecule N2 to retrieve nitrogen atoms). Hydrogen peroxide is produced as an intermediate during these chemical processes. And must be removed to prevent damage to cellular machinery.

        Prokaryotes organisms like bacteria that lack nuclear membrane also lack membrane bound organelles such as peroxisomes. Antioxidant enzymes like Catalase and superoxide dismutase are located in the periplasmic space, which is the space between the inner and outer membranes of the cell wall. There are numerous enzymes located here that would be toxic if they were found inside the cell. The Catalase found here can act on toxic molecules that are transported to the periplasm or the enzyme can be released outside the bacterial wall where it can act on toxic molecules in the environment. Catalase that is released by the bacteria plays a role in protecting the bacteria from being destroyed by white blood cells during an infection.

Mechanism Of Enzyme Action

Enzymes are thought to operate on a lock and key mechanism. In the same way that a key fits a lock very precisely, so the substrate fits accurately into the active site of the enzyme molecule. The two molecules form a temporary structure called the enzyme-substrate complex. The products have a different shape from the substrate and so, once formed, they escape from the active site leaving it free to become attached to another substrate molecules. The sequence is summarized in the figure below.

Effect Of Temperature On Enzyme Action

The temperature of a system is to some extent a measure of the kinetic energy of the molecules in the system. Thus the lower the temperature, the lower the kinetic energy of the system and likewise the higher the temperature the greater the kinetic energy of the system. Increase in the kinetic energy of a system results from increase in the kinetic energy of the system. This has several effects on the rates of reaction.

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More energetic collisions.

When molecules collide, the kinetic energy of the molecules can be converted into chemical potential energy of the molecules. If the chemical potential energy of the molecules becomes great enough, the activation energy of an exergonic reaction can be achieved and a change in chemical state will result. Thus the greater the kinetic energy of the molecules in a system, the greater is the resulting chemical potential energy when two molecules collide. As the temperature of a system is increased more molecules per unit time will reach the activation energy. Thus the rate of reaction may increase.

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Reaction of Catalase and Hydrogen Peroxide
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