Catalase is a common enzyme found in nearly all living organism exposed to oxygen. It catalyze the decomposition of hydrogen peroxide to water and oxygen. It is formed as a toxic waste product of metabolism. It must be converted to less dangerous chemicals. To manage the problem the enzyme catalase is used to rapidly catalyse the decomposition of hydrogen peroxide into harmless oxygen and water. Free radicals are constantly produced as reactive oxygen. When they are produced the free radicals are removed by antioxidant defense. Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long. It contains four porphyrin heme group that allow the enzyme to react with the hydrogen peroxide.
Oxygen is very important to life but when we use oxygen our bodies always produce free radicals. Free radicals are chemically unstable molecules. They can also influence other molecules in the body to be unstable and can result to damaged proteins, cell membrane or even DNA structures. Ot is a process that can lead permanent damage to cells and tissue which can result to infection and other diseases. Free radicals also play an important role in aging process. Catalase is always the one who fight against free radicals in our body. It transforms harmful superoxide radicals to hydrogen peroxide which breaks down to water and oxygen.
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Structure of a catalase monomer:
The catalase monomer consists 0f 506 amino acidpolypeptide chain plus one heme group and one NADH molecule. Only about 60% of catalase structure is composed of regular secondary structural motifs. α-helices account for 26% of its structure and β-structure for 12%. Irregular structure includes a predominance of extended single strands and loops that play a major role in the assembly of tetramer. Each monomer has four domains. The first domain is made up of the amino-terminal 75 residues. These form an arm with two α-helices and a large loop extending from the globular subunit. The second and largest domain contains the heme moiety. It is composed of residues 76 to 320 and may be classified as an α+β type domain. It includes a β-barrel, several helical segments of three to four turns each and various loops. The β-barrel consist of two four stranded anti parallel β-sheets that twist to form a closed cylindrical surface. The third domain consists of residues 321-436 and is referred to as the wrapping domain. It lacks discemable secondary structure except for two helices, the largest of which contains the heme phenolic ligand, Tyr357. The carboxy-terminal portion of the molecule contains residues437 to 506 and is folded into a four helical domain similar to the globin folds. Along with three α-helicesfrom the heme-containing domain, these helices from one surface of the enzyme.
Quaternary Structure: Assembly of the Catalase Monomer
Functional catalase is a tetramer of four identical holo subunits. Each monomer harbors a single heme and NADP. Whereas the NADPs lie on the surface, the heme moieties are embedded in the middle of each monomer. The assembly of the multimeric complex is presumably more complicated than a simple combination of monomers, with changes in the folding pattern of each monomer occurring so as to optimize packing interactions. Most intersubunit contacts are confined to the amino-terminal arms and the wrapping domains. The most flexible parts of the protein are thus responsible for most of the quaternary structure interactions. The amino-terminal domain becomes almost completely buried between neighboring subunits in the tetramer. Β-strands from two pairs of adjacent wrapping domains form inter-subunit anti-parallel β-sheets. There are numerous salt bridges at the interfaces between monomers, mostly involving arginine, asparagines and glutamic acid partners.
Figure 1. Structure of Catalase
Mechanism of Catalysis
The reaction is believed to occur in two stages:
H2O2 + Fe(III)-E → H2O + O=Fe(IV)-E(.+)
H2O2 + O=Fe(IV)-E(.+) → H2O + Fe(III)-E + O2
The iron center of the heme attached to the enzyme is represented by Fe()-E. the mesomeric form of Fe(V)-E is Fe(IV)-E(+) that means the iron is not completely oxidized to V+ but it receives some supporting electron from heme ligand. This should be drawn as a radical cation (+)
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Hydrogen peroxide interacts with amino acids asparagine147 and histidine74 after entering the aactive site and it causes a proton (hydrogen ion) to transfer between oxygen atoms. This oxygen atoms coordinates that frees the newly formed water molecule and Fe(IV)=O. This reacts with second hydrogen peroxide molecule to reform Fe(III)-E and produce oxygen an water. The presence of phenolate ligand of tyrosine357 improves the reactivity of the iron center and it can assist the oxidation of the Fe(III) to Fe(IV). The interaction of His74 and Asn 147 improves the efficiency of reaction with reaction intermediates. The Mechaelis-Menten equation determines the rate of reaction.
Catalase can also catalyze the oxidation, by hydrogen peroxide of various metabolites and toxins including formaldehyde, formic acid, phenols, acetaldehyde and alcohols. It does so according to this reaction:
H2O2 + H2R → 2H2O + R
Any heavy metal ion can act as a noncompetitive inhibitor of catalase. Poison cyanide is a competitive is a competitive inhibitor of catalase, strongly binding to the heme of catalase and stopping the enzyme's action.
Figure . Mechanism of Catalysis
Kinetics of Reaction
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, chlorophylls and nitrogen-fixing enzymes in legumes.
Figure 2. Reaction of Catalase
In the absence of catalase, this reaction occurs spontaneously, but very slowly. Catalase speeds up the reaction rate many thousands of fold. If the amount of oxygen formed is measured at regular intervals and this quantity is plotted on a graph, a curve like the one that follows is obtained:
Study the solid line on the graph of this reaction. At time 0 there is no product. Product only starts accumulating when the system has begun working. In this graph there is shown a lag, which may be caused by several factors. One is that for a few seconds any produced oxygen is dissolving into and saturating the water. Only after the water has become saturated does gas start coming out of solution, to fizz and be collected. Another factor is how long it takes for H2O2 to diffuse into the cell and then for oxygen to diffuse out. Thus on this graph, it took 10 seconds for gas to start being produced. After 15 seconds, 25 millimoles (mmoles) have been formed ; after 20 seconds, 50 mmoles; after 30 seconds, 100mmoles. The rate of this reaction could be given as 5 mmoles of product formed per second for this initial period. Note, however that by the 40th and 50th second, only a few if any additional mmoles of oxygen have been formed. After the lag and then during the first thirty seconds, the rate is constant. From thereon, the rate is changing; it is slowing down. For each successive period time after the 30 seconds, the amount of product formed in that interval is less than in the preceding 5 seconds. From the second half-minute onward, the reaction rate is very slow and stopping asymptotically.
Therefore the rate of the reaction is the slope of the linear portion of the curve. To determine a rate, pick any two points on the straight-line portion of the curve. Divide the difference in the amount of product formed between these two points by the difference in time between them. The result will be the rate of the reaction which if properly calculated, can be expressed as mmoles product/sec. The rate then is:
Figure 3. Rate
In comparing the kinetics of one reaction with another, a common reference point is needed. Using known amounts of enzymes, it is best to compare their reactions when each of their rates are constant. In the first moments of an enzymatic reaction such as this, the number of substrate molecules is usually so large compared with the number of enzyme molecules that changing the substrate concentration does not affect the number of successful collisions between substrate and enzyme. During this early period, the enzyme is acting on substrate molecule at a nearly constant rate. The slope of the graph's line during this early period is called the initial rate of the reaction. The initial rate of any enzyme-catalyzed reaction is determined by the characteristics of the enzyme molecule. It is always the same for any enzyme and its substrate at a given temperature and pH. This also assumes that the substrate is present in excess.
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Mode of Regulation
Aerobic respiration is a powerful source of reactive oxygen species. One of this is hydrogen peroxide. When hydrogen peroxide oxidize thiol group and iron-sulfur it can damage enzymes. It can produce mutagenic and lethal lesions when conveted to hydroxyl radicals on the [resence of metal ions. Although there are many hindrances to cells, they have many protective mechanism against toxic products. Catalase perform enzymatic detoxification of hydrogen peroxide. There are three classes of catalase which are monofunctional catalase, catalase-peroxidases and manganese catalase. Only monofunctional catalase and catalase-peroxidase contain heme as a prosthetic group. Monofunctional catalase fprm prokaryotes to eukaryotes and catalase-peroxidase are found in bacteria and minor have been found in fungi and other eukaryotic organisms. Bacteria express one or more catalase in response to oxidative stress.
Catalase is a very useful enzyme that is widely used all over the world. It is use in food industry to remove hydrogen peroxide in making cheese. It is also used in food wrappers to prevent oxidizing of food. It is used in textile industry to remove hydrogen peroxide from fabrics to make sure it is peroxide-free. For minor uses catalase is also used in contact lens hygiene wherein a few lens cleaning products disinfect the lens using hydrogen peroxide solution catalase is used to decompose the hydrogen peroxide before the lens is use again. Catalase was also used in aesthetics industry. The enzyme with hydrogen peroxide was combined by several mask treatment to increase cellular oxygenation in the upper layer of epidermis. Catalase is also used as a test to identify species of bacteria. Hydrogen peroxide detects the presence of catalase enzyme in the test isolate. Low levels of catalase also help in greying process of hair. Hydrogen peroxide is naturally produced in our body and catalase breaks it down .Hydrogen peroxide cannot be broken down as well if catalase levels decline. This allows the hydrogen peroxide to bleach the hair inside and out.
Although catalase is a very useful enzyme it can also be dangerous to our health. One of which is acatalasemia which is lack of erythrocyte catalase that affect lipid metabolism.