Catalase, activation energy

Water molecules flow through an AQP-1 channel at the rate of about 109 s 1. For comparison, the highest known turnover number for an enzyme is that for catalase, 4 X 107 s-1, and many enzymes have turnover numbers between 1 s 1 and 104 s 1 (see Table 6-7). The low activation energy for passage of water through aquaporin channels (AG < 15 kJ/mol) suggests that water moves through the channels in a continuous... 

The four curves show that various catalysts reduce the activation energy for the hydrogen peroxide decomposition reaction, but by different amounts. Notice that the enzyme catalase almost cancels the activation energy. 

Enzymes combine with their specific substrate in such a way that the activation energy a is decreased to a lower value of Ui. For example, for the decomposition of H2O2 without catalysis, the activation energy is 70 kJ/mol, whereas with the catalase (an enzyme with a very high turnover number) this becomes 7 kJ/mol. Since R = 8.314 J/mol-K, from eq. (1) it follows that the acceleration by the enzyme is ... 

The peroxidase activity of immobilized catalase on the oxidation of phenol has been studied. The immt ilization was carried out from catalase solutions with pH < 3,5 on two kinds of soot differing in the average size of the particles building them up. The effect of the initial concentration of ph iol on the rate of its peroxidase oxidation by catalase immobilized on the soot of finer-graWd structure has been studied. The relationships obtained are described by the equation of Michaelis-Menten. The kinetic parameters (the constant of Michaelis - Km. the maximum reaction rate - V, the rate constant - k and the activation energy - of the process were calculated. It was found that catalase adsorbed on the soot of larger globular particles does not take part in the peroxidase oxidation of phenol. 

The values obtained for E, hilly correspond to the values of the rate constants. For catalase adsorbed on "NORIT", the rate of the po oxidase oxidation of phenol is higher because the activation energy E, of this process is Iowct. 

Chi the basis of E. values we can make a conclusion about the difhision factors which are some of the most conqilicated points concerning catalysis with immobilized enzymes. The value for the activation energy on peroxidase oxidation of phenol with catalase immobilized on "NORIT" soot is E, =10.95 kJ.mof which is an indication that the process takes place under diSusion regime. The latter means that the enzymatic reaction rate is determined by the mass tranfer of substrate to the surfoce of the carrier particles and its diffiision into the carrier. 

The catalase like activity was tested with (48) as catalyst91. From the results it is evident that the polymer bond in phthalocyanine led to a lower activation energy due to higher concentration of not aggregated active centers than with low molecular phthalocyanines. Continuous flow experiments in a column show that (48) keeps 60% of its original activity. The polymer is more stable than the low molecular phthalocyanine. 

Table 10.1 gives values of rate constants, activation energies, and frequency factors for three enzyme-catalyzed reactions. For comparison, the values for other catalysts are included. Note that molecule for molecule, the enzymes are much more effective catalysts than the nonbiological catalysts. In urease and catalase this higher effectiveness is related to a much smaller activation energy, which is true for a number of other enzyme systems. Enzymes evidently exert their action by allowing the process to occur by a much more favorable reaction path. 

The activation energy of this reaction is lowered if the reaction is allowed to proceed on platinum surfaces, but it is lowered even more by the enzyme catalase. Table 6.1 summarizes the energies involved. 

Ans. The enzyme, catalase, is a catalyst. As such it speeds up the rate of the reaction, but is not consumed in the reaction. It forms an activated complex with H2O2, thereby providing a new and different reaction pathway possessing a signifieantly lower activation energy eompared with the decomposition of H2O2 alone. 

High activity. Enzymes can increase the rate of a reaction millions of times by lowering the activation energy of the reaction Uke conventional chemical catalysts. The maximum rate of conversion of a substrate to a product by a molecule of an enzyme is known as the turnover number Eor example, the /Qat for catalase, which catalyzes the conversion of hydrogen peroxide to O2 and H2O, is approximately 600,000 molecules per second per molecule of enzyme. 

The decomposition of hydrogen peroxide, for instance, requires a molar activation energy of 76 kJ mol which is why it runs so slowly at room temperature. When the enzyme catalase is added, the threshold reduces to 6 kJ mol, leading to extreme acceleration. However, the activation energy applies formally to the entire changed mechanism and cannot be attributed to an individual reactimi step as before. 

Shirai et al. also reported synthesis and catalase-like activity of Fe(III)-taPc bound to polystyrene. The activation energies of the polymer-bound Fe(III)-Pc are about half those with free Fe(III)-taPc. In continuous-flow experiments in a column, the polymer-attached catalyst was very stable compared with free Fe(III)-taPc [80,81]. 

Cobalt most often depresses the activity of enzyme including catalase, amino levulinic acid synthetase, and P-450, enzymes involved in cellular respiration. The Krebs citric acid cycle can be blocked by cobalt resulting in the inhibition of cellular energy production. Cobalt can replace zinc in a number of zinc-required enzymes like alcohol dehydrogenase. Cobalt can also enhance the kinetics of some enzymes such as heme oxidase in the liver. Cobalt interferes with and depresses iodine metabolism resulting in reduced thyroid activity. Reduced thyroid activity can lead to goiter. 

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