CATALASE

C A T A L A S E

.....Catalase is nearly ubiquitous among organisms that can grow in the presence of oxygen (air). ..The major function of catalase within cells is to prevent the accumulation of toxic levels of hydrogen peroxide formed as a by-product of metabolic processes - primarily that of the electron transport pathway. ..The only exceptions are the "lactic acid bacteria," which cannot synthesize the fundamental building block porphyrin, and hence do not even possess cytochromes that would otherwise make the toxic H2O2.

.....Each molecule of catalase has four polypeptide chains, each composed of more than 500 amino acids, and nested within this tetrad are four porphyrin heme groups - very much like the familiar hemoglobins, cytochromes, chlorphylls and nitrogen-fixing enzymes in legumes. ..(Catalase may also take part in some of the many oxidatic reactions that occur in all cells.)

Catalase, which normally accompanies SOD, is found in almost all cells, but may be found in its highest content in hydroponically grown, concentrated wheat sprouts.

Catalase was one of the first enzymes to be purified to homogeneity, and has been the subject of intense study. The enzyme is among the most efficient known, with rates approaching 200,000 catalytic events/second/subunit (near the diffusion-controlled limit). Catalase structure from many different species has been studied by X-ray diffraction. Although it is clear that all catalases share a general structure, some differ in the number and identity of domains.

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 is present in the peroxisomes,  Membrane-bound, microbody organelles that house various oxidation reactions in which toxic peroxides are generated as side products, of nearly all aerobic cells, serves to protect the cell from the toxic effects of hydrogen peroxide by catalyzing its decomposition into molecular oxygen and water without the production of free radicals, An atom or a group of atoms with an unpaired electron. Radicals are unusually reactive, strongly oxidizing species capable of causing a wide range of biological damage. The mechanism of catalysis is not fully elucidated, but the overall reaction is as follows:
 

2 H2O2 -- 2 H20 + O2

The protein exists as a dumbbell-shaped tetramer of four identical subunits (220,000 to 350,000 kD). Each monomer contains a heme, A prosthetic group consisting of a protoporphyrin ring and a central iron (Fe) atom <>. A protoporphyrin ring is made up of four pyrrole rings linked by methene bridges. Four methyl, two vinyl, and two propionate side chains are attached. The iron can either be in the ferrous (Fe++) or the ferric (Fe+++) oxidation state, prosthetic group, A tightly bound, specific non-polypeptide unit required for the biological function of some proteins. at the catalytic center. Catalase monomers from certain species (e.g. cow) also contain one tightly bound NADP per subunit. This NADP, Nicotinamide adenine dinucleotide phosphate, the major electron carrier in reductive biosynthesis, may serve to protect the enzyme from oxidation by its H2O2 substrate.


 

 

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


 

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.


 

Lysosomes and Peroxisomes: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/Lysosomes.html


 

Anatomy of the Cell, Peroxisomes: http://www.mythos.com/webmd/Content.aspx?P=CELLSA&E=10


 

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.


 

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.
 


 

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