What is Catalase?
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 the 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 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
Catalase – An Extraordinary Enzyme: http://www.catalase.com/cataext.htm
Where is it found and what does it do?
Catalase is located in a 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 byproduct 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 reactive nitrogen atoms). Hydrogen peroxide is produced as an intermediate during these chemical processes and must be removed to prevent damage to cellular machinery. Aerobic (oxygen requiring) bacteria produce hydrogen peroxide as a byproduct of metabolism. This fact is used when identifying bacteria. If hydrogen peroxide is added to a bacterial colony and bubbles are produced, this is evidence of oxygen production and confirms that the colony is aerobic.
Prokaryotes, organisms like bacteria that lack a 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.
Lysosomes and Peroxisomes:http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/Lysosomes.html
Anatomy of the Cell, Peroxisomes: http://micro.magnet.fsu.edu/cells/peroxisomes/peroxisomes.html
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 heme groups that are very much like the familiar hemoglobins, cytochromes, chlorophylls and nitrogen-fixing enzymes in legumes. The heme 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.
Catalase: H2O2 Oxidoreductase:http://www.clunet.edu/BioDev/omm/catalase/frames/cattx.htm#Topic4
What factors affect the activity of Catalase?
The study of the rate at which an enzyme works is called enzyme kinetics. The rate at which an enzyme works is influenced by several factors including the concentration of substrate (hydrogen peroxide in the case of catalase), temperature, 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: If all other conditions are held constant, the rate of the reaction should increase with increasing concentrations of substrate. At very low values of substrate the reaction rate will increase very rapidly. At higher substrate concentrations the rate begins to level off. Eventually the maximum rate for that reaction will be achieved and further increases in substrate concentration will have no effect.
Temperature: In general, chemical reactions speed up as the temperature is raised. When the temperature increases, more of the reacting molecules have the kinetic energy required to undergo the reaction. Enzyme catalyzed reactions also tend to go faster with increasing temperature until a temperature optimum is reached. Above this value the conformation of the enzyme molecule is disrupted. Changing the conformation of the enzyme results in less efficient binding of the substrate. Temperatures above 40-50°C denature many enzymes.
pH: pH is a measure of the acidity or hydrogen ion concentration of a solution. It is measured on a scale of 0-14 with pH values below 7 being acidic, values above 7 being basic and a value around 7 is neutral. As the pH drops into the acidic range an enzyme tends to gain hydrogen ions from the solution. As the pH moves into the basic range the enzyme tends to lose hydrogen ions to the solution. In both cases the changes produced in the chemical bonds of the enzyme molecule result in a change in conformation that decreases enzyme activity.
Salt Concentration: Every enzyme has an optimal salt concentration in which it can catalyze reactions. Too high or too low a salt concentration will denature the enzyme.
Presence of Inhibitors: A molecule that interacts with the enzyme and decreases its activity is an inhibitor. Enzyme activity can be affected in different ways. Competitive inhibition occurs when the inhibitor has a similar structure as the substrate, allowing it to compete for the active site on the enzyme molecule. In the case of catalase the active site is the heme group. Noncompetitive inhibition occurs when the inhibitor binds somewhere other than the active site of the enzyme. This causes a change in the shape of the enzyme molecule so that the substrate molecule can no longer bind to the active site. Copper sulfate is a noncompetitive inhibitor of catalase. Cyanide is a competitive inhibitor because it binds to the active site in the catalase molecule.
Presence of Activators: A molecule that interacts with an enzyme and increases its activity is an activator.
In living systems the optimal ranges of temperature, pH and salt concentration for a given enzyme are the ranges found in that system. When determining various optimum conditions for a catalase solution it is important to consider the source of the catalase. Catalase derived from a potato or from yeast might “prefer” slightly different condition than catalase derived from beef liver. For example, mammalian enzymes have a temperature optimum of about 40°C, but there are enzymes that work best at very different temperatures, e.g. enzymes from the arctic snow flea work at -10°C, and enzymes from thermophilic bacteria work at 90°C. The optimal pH range is about 7-8 (physiological pH of most cells), but a few enzymes can work at extreme pH, such as protease enzymes in animal stomachs, which have an optimum of pH 1.
Enzyme Kinetics: http://themedicalbiochemistrypage.org/enzyme-kinetics.html
Experiments to do with Catalase!
Catalase is a popular enzyme to use for the study of enzyme kinetics. It is found in a wide variety of organisms and is easily isolated. The following websites have protocol for several interesting catalase experiments.
Catalase Kinetics: http://www.science-projects.com/catalasekinetics.htm
Access Excellence Experiment 1, Catalase Activities: http://www.accessexcellence.org/LC/TE/PW/EXP/ENZ/default.php
Enzyme Model – Catalase: http://mvhs1.mbhs.edu/mvhsproj/enzyme/enzstu.pdf