Catalase turnover number

An enzyme electrode is basically a dense package of dialyzer, enzyme reactor, and electrode (detector). Enzymes introduce analytical selectivity due to the specificity of the signal-producing interaction of the enzyme with the analyte. They enhance the equihbrium formation of chemical reactions. For example, splitting of H2O2 is accelerated by a factor of 3 x 10 in the presence of catalase. Turnover numbers can be as fast as 6 X 10 s (carbonic anhydrase) where cat/ m approaches the diffusion limited value of 10 s ... 

The term represents the kinetic efficiency of the enzyme. Table 14.4 lists turnover numbers for some representative enzymes. Catalase has the highest turnover number known each molecule of this enzyme can degrade 40 million molecules of HgOg in one second At the other end of the scale, lysozyme requires 2 seconds to cleave a glycosidic bond in its glycan substrate. 

As an example, we mention the enzyme catalase, which catalyzes the decomposition of H2O2 to H2O and O2 at a turnover number of kcat = 10 and a high specificity constant of kcat/f M = 4 x 10 mol s . Such activities are orders of magnitude higher than those of heterogeneous catalysts. 

The maximum rate is directly related to the rate at which the enzyme processes or permits conversion of the reactant molecule(s). The number of moles of reactants processed per mole of enzyme per second is called the turnover number. Turnover numbers vary widely. Some are high, such as for the scavenging of harmful free radicals by catalase, with a turnover number of about 40 million. Others are small, such as the hydrolysis of bacterial cell walls by the enzyme lysozyme, with a turnover number of about one half. 

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... 

It was once thought that the rate of equilibrium of the catalytic acid and basic groups on an enzyme with the solvent limited the rates of acid- and base-catalyzed reactions to turnover numbers of 103 s 1 or less. This is because the rate constants for the transfer of a proton from the imidazolium ion to water and from water to imidazole are about 2 X 103 s 1. However, protons are transferred between imidazole or imidazolium ion and buffer species in solution with rate constants that are many times higher than this. For example, the rate constants with ATP, which has a pKa similar to imidazole s, are about I0 J s 1 M-1, and the ATP concentration is about 2 mM in the cell. Similarly, several other metabolites that are present at millimolar concentrations have acidic and basic groups that allow catalytic groups on an enzyme to equilibrate with the solvent at 107 to 108 s-1 or faster. Enzyme turnover numbers are usually considerably lower than this, in the range of 10 to 103 s-1, although carbonic anhydrase and catalase have turnover numbers of 106 and 4 X 107 s 1, respectively. 

Turnover numbers for some representative enzymes are listed in table 7.2. The enormous value of 4 x 107 mole-cules/s achieved by catalase is among the highest known the low value for lysozyme is at the other end of the spectrum. As is the case with Km, the relationship of kcal to individual rate constants, such as k2 and k3, depends on the details of the reaction mechanism. 

Catalase, a heme-containing enzyme with tyrosine as the proximal heme ligand, decomposes hydroperoxides and peracids by this reaction. Catalase is one of the most efficient enzymes known, with maximum turnover numbers on the order of 107 s 1. In... 

Catalase (EC 1.11.1.6) is a heme containing oxidoreductase that acts on peroxides liberating oxygen and water. The enzyme is a very fast biocatalyst, i.e., Pichia pastoris catalase has a turnover number of 3 x 10 H2O2 s [199]. The enzyme consists of multiple subunits containing each heme as active site. 

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 ... 

In addition to Km and ymax, the turnover number (molar activity) and the specific activity are important parameters for the characterization of enzyme reactions. Both are determined under substrate saturation. With highly purified enzymes the turnover number reflects the number of substrate molecules converted in unit time by a single enzyme molecule (or a single active center). Catalase, one of the most potent enzymes, has a turnover number of 2-105/s. 

The parameter (, .31 is also referred to as the turnover ntonber. It is the number of sub.strate molecules converted to product in a given time on a single-enzyme molecule when the enzyme is saturated with substrate (i.e,. ail the active sites on the enzyme are occupied. S Kvi). For example, turnover number for the decomposition H Oi by the enzyme catalase is 40 x 10 s" . That is, 40 million molecules of are decomposed every second on a single-enzyme molecule saturated wdth HiO . The constant (mol/dm ) is called the Michael is constant and for simple. systems is a measure of the... 

This is accelerated by the iron heme protein catalase, a particularly efficient enzyme with one of the highest turnover numbers of all known enzymes (at 4x 107 molecules per second). This high rate reflects the important role for the enzyme, and its capacity for detoxifying hydrogen peroxide. 

The TON for O2 production by dimanganese(III) chloride complexes of both DPX (11) and DPD (19) are graphically represented in Fig. 20. Both systems are poor catalase mimics, displaying turnover numbers (TONs) for O2 evolution that are <40 (139). Given that the etioporphyrin-type substitution patterns of the parent DPX and DPD systems are generally imstable to oxidizing conditions. 

Water molecules flow through an AQP-1 channel at the rate of about 10 s. For comparison, the highest known turnover number for an enzyme is that for catalase, 4 X 10 s, and many enzymes have turnover... 

An extremely small aniount of a catalyst frequently causes a considerable increase in the rate of a reaction. Colloidal palladium at a concentration ofl mol in 10 dm will cause hydrogen peroxide to decompose at a measurable rate. The effectiveness of a catalyst is sometimes expressed in terms of its turnover number, which is the number of molecules of substrate decomposed per minute by one molecule of the catalyst. For example, the enzyme catalase has, under certain conditions, a turnover number of 5 000 000 for the decomposition of hydrogen peroxide (2H2O2 =... 

In view of the well-known radical nature of the ferrous and ferric ion decomposition of hydrogen peroxide (60, 61) Chance s conclusion is of great importance, that neither the kinetics, nor paramagnetic resonance, reveal radicals with the enzyme. Catalase, one of the most active enzymes, has a turnover number of 5 X 10 , and it is of great interest that Wang (62) has... 

Turnover numbers are a particularly dramatic illustration of the efficiency of enzymatic catalysis. Catalase is an example of a particularly efficient enzyme. In Section 6.1, we encountered catalase in its role in converting hydrogen peroxide to water and oxygen. As Table 6.2 indicates, it can transform 40 million moles of substrate to product every second. The following Biochemical Connections box describes some practical information available from the kinetic parameters we have discussed in this section. 

As can be seen from Table 6.2, the first two enzymes are very reactive catalase has one of the highest turnover numbers of all known enzymes. These high numbers allude to their importance in detoxifying hydrogen peroxide and preventing formation of CO2 bubbles in the blood these are their respective reactions. 

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. 

By catalase, which has one of the highest turnover numbers of all enzymes one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second. 

When the molecular weight of a pure enzyme is known it is possible to determine the molecular activity or turnover number,i.e. the number of molecules of substrate transformed per minute per molecule of enzyme. These are usually of the order of several thousand, although acetyl cholinesterase has a value of 950 000 and catalase 5 000 000. 

The dimerization of propylene on a ( rr-C3H5)Ni-Br(PCy3) (Cy = crotyl) [590] in the presence of EtAlClj proceeds with a turnover number of 60 at —75°C and 230 at — 55°C. Extrapolated to 25 C, this gives a turnover number of 60,000, which is comparable to the activity of catalase. At higher conversions the catalyst has diminished activity. The product formed included 10-15% higher olefins, 18% 4-methyl-l-pentene, 1-3% ds-4-methyl-2-pentene, 76% 2,3-dimethyl-l-butene, and 4% 2-methyl-l-pentene as well as traces of other isomers. 

Even under these conditions, the value of k should increase linearly with the donor concentration and no maximum value of k exists as it does for the simpler mechanism of equations (3) and (4). Experimentally, maximal values of k are often quoted in various papers, but they may be attributed to insufficient substrate concentration (the inequality A)Xo kiOo is violated), or to enzyme inactivation due to the excess peroxide concentration. For example, catalase inactivation can be caused by the formation of the inactive catalase complex II. In these cases it is desirable to use a lower value of substrate concentration and a smaller enzyme turnover number in order to avoid the inactivation. 

In this case the reaction is of the first order and the turnover number increases linearly with the hydrogen peroxide concentration. There is no experimental condition under which ki or k can be studied independency from measurements of the over-all reaction velocity. Thus direct studies of the enzyme-substrate complex are essential in order to determine the relative magnitudes of fci and W and to study the effect of physical and chemical factors upon them. It is ironic that catalase, for which the over-all reaction has been studied in more detail than for any other eni me, should be one for which such studies cannot give incisive data on the effect of environmental factors upon a single reaction velocity constant two constants are always involved. 

Catalase is an enzyme which breaks HsOs into H O and O. Its active group is protohemin. It has an unusually high activity (as measured by the turnover number). The molecular weight of the crystalline protein comes to 240,000. There are four heme groups in each molecule. 

To gain insight into the mechanisms of these enzymes, a variety of Mn complexes that mimic the active site have been developed [28]. Dismukes and coworkers reported the first functional catalase model that exhibits high activity towards H2O2 decomposition even after turnover numbers of 1000, no loss of activity towards H2O2 decomposition was observed [29]. The dinuclear Mn -complex is based on ligand 1... 

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