for this experiment my main aim is to investigate the effect of temperature on enzyme activity

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BIOLOGY COURSEWORK

My Aim: for this experiment my main aim is to investigate the effect of temperature on enzyme activity. In this case Catalase is my enzyme and Hydrogen peroxide is my substrate, I will be reacting both of them at different temperatures.

Introduction

Catalase is an enzyme found in tissues of most living things. It promotes the conversion of hydrogen peroxide, a powerful and potentially harmful oxidizing agent, to water and molecular oxygen.

2H2O2   to   2H2O + O2

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

H2O2 + RH2   to   2H2O + R

What does Catalase look like?

Each molecule of Catalase is a tetramer of four polypeptide chains. Each chain is composed of more than 500 amino acids. Located within this tetramer are four porphyrin haem groups that are very much like the familiar haemoglobins, cytochromes, chlorophylls and nitrogen-fixing enzymes in legumes. The haem group is responsible for catalase’s enzymatic activity. Catalase has one of the highest turnover rates for all enzymes: one molecule of Catalase can convert 6 million molecules of hydrogen peroxide to water and oxygen each minute.

Enzymes (protein molecules) are biological catalysts that help speed up the rate of chemical reaction and they remain unchanged after reaction. Almost every metabolic reaction that takes place within a living organism is catalysed by an enzyme. It is also important that we know the shape of enzymes, they are globular proteins and like all other globular proteins, enzymes take 3-D shape with a hydropholic R groups on the outside of the molecule ensuring that they are soluble.

        It is also important to recognise one of the main feature enzymes possess which is their Active Site. This active site helps enzymes catalyse reaction. It is usually a depression to which another molecule or molecules can bind. The molecules that bind with the active site of an enzyme are called Substrate. The combination of an enzyme and a substrate is called an enzyme-substrate complex.

        The shape of the active site on an enzyme allows a particular type of substrate to fit perfectly into it.

                                                                           Catalase-hydrogen peroxide complex

Enzymes catalyses a reaction either by breaking down bonds or by forming bonds. After the reaction the products leave the active site for another substrate to bind with.

NB: Enzymes are specific in action as only one type of substrate can fit into their active sites.

        The rate at which substrate molecules can bind to the enzyme’s active site, be formed into products and leave can be rapid.

        Enzyme Catalase contains four subunits. Each subunit containing a haem group, each of the subunits in Catalase uses the energy from electrons to decompose hydrogen peroxide.

        Enzyme has various factors affecting its activity. But the one to be considered in this experiment is Temperature: Temperature can directly affect the rate of an enzymatic reaction. Firstly, all chemical reactions are affected by temperature, according to the laws of thermodynamics; in general chemical reactions speed up as the temperature is raised. The increased molecular motion that occurs as a result of increased temperature makes collisions between the Enzyme and substrate more likely, and therefore the reaction will occur at a greater rate. As temperature increases so does the rate of reaction. However one must also bear in mind that high temperatures can cause thermal Denaturation of the enzyme and freezing may also damage an enzyme. Denaturation is a change in the tertiary structure of an Enzyme. An enzyme's function is related to its 3-dimensional (Tertiary) structure. This structure can be altered by heat, thus causing the enzyme to lose function. Temperatures above 40-50°C denature many enzymes.

Effect of temperature

From the graph above we can see that the rate of reaction increased as temperature increases (from 30- 40oC rate of reaction increases, this is because as temperature increases the kinetic energy of the reacting particles increases), and also that the optimum rate of reaction was at 40oC before it started to reduce, this is because when temperature goes past the optimum temperature the conformation of the enzyme is disrupted and it starts to denature (change shape). The optimum temperature for Catalase depends on the source of the Catalase i.e. Catalase derived from a potato or from yeast might “prefer” slightly different condition than Catalase derived from beef liver, which means they prefer different temperatures.

The effect of enzyme on activation energy

        I find it very interesting  finding out that most of the reactions which occur in living cells would occur so slowly without enzymes, in fact they would virtually not happen at all .In many reactions, the substrate  will not be converted to a product unless it is temporarily given some extra energy. This energy is called Activation Energy.

Shown above are two plots of the energy levels of molecules involved in a reaction. The thin curve depicts the course of the reaction in the absence of an enzyme which facilitates the reaction while the thick curve depicts the course of the reaction in the presence of the reaction specific enzyme. The difference in energy level between the beginning state (left side) and the energy necessary to start the reaction (peaks of the curves) is the activation energy of the reaction. The presence of the enzyme lowers the energy of activation.

        One way of increasing the rate of many chemical reactions is to increase the energy of the reactants by heating them. Mammals, such as humans also use this method of speeding up their metabolic reactions. Our body temperature is maintained at 37oC which is usually much warmer than the temperature of air around us. But even raising temperature of cells is not enough to give most substrates the activation energy which they would to need to change into products. We cannot raise body temperature much more than this, as temperatures above about 40oC begin to damage too many of the molecule from which we are made, especially protein molecules. Enzymes are a solution to this problem because they decrease the activation energy of the reaction which they catalyse. They do this by holding the substrate or substrates in such a way that their molecules can react more easily. Reactions catalysed by enzymes will take place rapidly at a much lower temperature than they would without them.

        Apart from temperature, the rate at which an enzyme works is influenced by several other factors including the concentration of substrate, pH, salt concentration and the presence of inhibitors or activators. Every enzyme has an optimal range for each of these factors. Activity decreases when an enzyme is exposed to conditions that are outside the optimal range.

Substrate Concentration: At lower concentrations, the active sites on most of the enzyme molecules are not filled because there is not much substrate.  Higher concentrations cause more collisions between the molecules.  With more molecules and collisions, enzymes are more likely to encounter molecules of reactant. The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate.  Reaction rate therefore increases as substrate concentration is increased but it eventually levels off. For my experiment, my substrate concentration is going to be constant

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Enzyme Concentration: If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because there are not enough enzymes for all of the reactant molecules. As the amount of enzyme is increased, the rate of reaction increases. If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate. Reaction rate therefore increases as enzyme concentration increases but then it levels off. For my experiment, my enzyme concentration is going to be constant.

pH Concentration: pH is a measure of the acidity or hydrogen ion concentration ...

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