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Catalase is a key antioxidant enzyme that promotes the conversion a harmful by-product of Hydrogen Peroxide (H2O2) to water (H2O) and Oxygen (O2). This enzyme that serves as a defense against oxidative stress is particularly located in a cell organelle called Peroxisome (1). This cell contributes to lipid metabolic pathway including the synthesis of cholesterol and bile acids and oxidation of fatty acids where Hydrogen peroxide is a main product. This hydrogen peroxide is released by the oxidation of fatty acids that can help eliminate bacteria in the body. But in some cases, this hydrogen peroxide can also harm the cell itself that is why the catalase has the responsibility to eliminate much of the hydrogen peroxide needed by catalyzing the compound and converting it to water and oxygen without producing free radicals. Catalase is one of the enzymes that have the highest turnover rates. It can change millions of hydrogen peroxide molecules to water and oxygen respectively in just one minute (2).
STRUCTURE OF CATALASE
Figure 1. Structure of Catalase (3)
Catalases are known to be a tetramer consisting 60kDa or 60,000Da comprising of four identical heme groups that is surrounded by the structure, but which is easily reached from the surface through the hydrophobic channels. Their constancy and resistance to proteolysis is an evolutionary improvement, particularly since they are formed during the motionless segment of the cell development while the levels of proteases are high and have a rapid rate of protein turnover.
MECHANISM OF HYDROGEN PEROXIDE CATALYSIS
Heme-containing catalases break down hydrogen peroxide by a two-stage mechanism in which hydrogen peroxide alternately oxidizes and trims down the heme iron at the active location. In the first step, one hydrogen peroxide molecule oxidizes the heme to an oxyferryl species. In the second step, a second hydrogen peroxide molecule is used as a reluctant to stimulate the enzyme, producing water and oxygen. Each heme groups have a tetramer with NADPH in its active site. (4) This NADPH is not really essential for the conversion of H2O2 to O2 and H2O but it does provide the protection to the catalase against the inactivation of H2O2. NADPH reduces the vulnerability of catalase to inactivation when the enzyme is exposed to small concentrations of H2O2. (5) Catalases may have one more role: the production of ROS, most probably hydroperoxides, during UVB irradiation. In this way, UVB light can be renewed through the creation of hydrogen peroxide, which can then be demeaned by the catalase. NADPH can participate in providing the electrons the required amount to lessen the molecular oxygen in the making of ROS. (6)
KINETICS OF REACTIONS
The study of the rate at which an enzyme works is described as enzyme kinetics. The rate at which an enzyme works is controlled by some aspects including the concentration of substrate (hydrogen peroxide in the case of catalase), temperature, pH, salt concentration and the occurrence of inhibitors or activators. Each enzyme has its finest range for each of these aspects. The activity reduces when an enzyme is exposed to circumstances that are outside the optimal range.
Substrate Concentration: If most of the conditions are held even, the rate of the response is supposed to increase with the rising concentrations of substrate. For very low values of the substrate, the reaction rate should raise very rapidly. At high substrate concentrations, the rate begins to fluctuate. Eventually, the highest rate for that reaction will be accomplished and the further increase in the substrate concentration will have no effect.
Temperature: In general, chemical reactions hasten as the temperature is raised. When the temperature increases, more kinetic energy is required for the reacting molecules to undergo the reaction. Enzyme catalyzed reactions also tend to go faster with the rising temperature until maximum temperature is reached. Above this value, the conformation of the enzyme molecule is disturbed. The alteration in the conformation of the enzyme will effect in less competent binding of the substrate. Temperatures beyond 40-50°C will surely denature many enzymes.
pH: pH is a determination of the acidity of hydrogen ion concentrations of a solution. It is measured on a scale of 0-14 with pH values of below 7 being acidic, and values above 7 being basic and a value of approximately 7 is neutral. As the pH falls into the acidic range, an enzyme tends to increase hydrogen ions from the solution. As the pH shifts into the basic range, the enzyme tends to lose hydrogen ions to the solution. In both cases, the changes created in the chemical bonds of the enzyme molecule effect in a change in conformation that diminishes its enzyme activity.
Salt Concentration: Each enzyme has an optimal salt concentration in which it can catalyze reactions. Too high or too low a salt concentration will surely denature the enzyme.
Presence of Inhibitors: A molecule that interacts with the enzyme and shrinks its activity is an inhibitor. Enzyme activity can be influenced by different ways. Competitive inhibition happens when the inhibitor has a like structure as the substrate, permitting it to compete for the active site on the enzyme molecule. In the case of catalase, the active spot is the heme group. Noncompetitive inhibition occurs when the inhibitor fuses somewhere other than the active site of the enzyme. This reasons for a change in the form of the enzyme molecule so that the substrate molecule will not have a chance to bind to the active site. Copper sulfate is one of the noncompetitive inhibitors of catalase while cyanide is a competitive inhibitor since it binds to the active site in the catalase molecule. Inhibitors other than cyanide are as follows: Ascorbate, Azide, Hydroxylamine, Aminnotrialzole, Mercaptoethanol, Peroxide, Nitro and Nitroso compounds.
Presence of Activators: A molecule that interrelates with an enzyme and raises its activity is an activator. Only Sodium arsenate is an activator for the catalase.