Turnover number for

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. 

In summary, the order of reactivity for the most commonly used ruthenium-based metathesis catalysts was found to be 56d>56c>9=7. This order of reactivity is based on IR thermography [39], determination of relative rate constants for the test reaction 58—>59 (Eq. 8) [40], and determination of turnover numbers for the self metathesis of methyl-10-undecenoate [43]. 

True surface area, origination of method for obtaining (Leikis), 46 Turnover numbers, for minority carriers, 494... 

The sum of the individual turnover numbers for each oligomer also shows that the rate of hydrolysis of the oligomers with DP4 and DPS is much slower than of those with higher DP which is reflected in the product progression curves in Fig. 1. 

Grenoble and coworkers229 reported an important influence of the support on the water-gas shift activity of various metal catalysts. For example, the rate increased an order of magnitude when Pt was supported on alumina versus silica. Turnover numbers for alumina-supported metal catalysts decreased in the order Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir, whereby the activity varied by 3 orders of magnitude, suggesting a correlation between activity of the metal and the heat of adsorption. To describe these differences in activity, the authors used a bifunctional model, involving chemisorption of water on alumina and CO on the metal, followed by association of the CO with the water to form a formic acid-like formate species, with subsequent decomposition via dehydrogenation on the metal sites (Scheme 55). 

Run Catalyst Surfactant0 Solvent Atmosphere Turnover number for product Epoxycylohexane Cyclohexanone ... 

A poly-jr-nitrostyrene coated Pt electrode has been used for the electrocatalytic reduction of 1,2-dibromo-1,2-diphenylethane to stilbene in a CH3CN-R4NBF4 system. The turnover number for catalyst sites is estimated to be 10000 [453]. 

Under many circumstances, the behavior of a simple unireactant enzyme system cannot be described by the Michaelis-Menten equation, although a v versus [S] plot is still hyperbolic and can be described by a modified version of the equation. For example, as will be discussed later, when enzyme activity is measured in the presence of a competitive inhibitor, hyperbolic curve fitting with the Michaelis-Menten equation yields a perfectly acceptable hyperbola, but with a value for Km which is apparently different from that in the control curve O Figure 4-7). Of course, neither the affinity of the substrate for the active site nor the turnover number for that substrate is actually altered by the presence of a competitive... 

Thymidylate synthase also catalyzes an exchange of tritium of [5- H]dUMP for water protons in the absence of CH2H4folate. The turnover number for this exchange reaction is about 1/45,000 that of dTMP formation, and the Kjn is 1.2 X 10 M, about the same as the of the enzyme-dUMP complex estimated by equilibrium dialysis. The exchange reaction provided compelling evidence that the enzymic reaction involves attack of an enzyme nucleophile on the 6 position of dUMP to pro-... 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

Fig. 1. The testosterone 6j8-hydroxylase activity (expressed as turnover number) for cDNA-expressed CYP3A4 using yeast, bacteria and mammalian cell systems. Yeast-based expression with coexpressed human cytochrome P450 reductase and human cytochrome or native, yeast cytochrome P450 reductase and cytochrome bs (Peyronneau et al., 1992). E. co/i-GSH E. co/i-based expression where the enzyme was purified and reconstituted with and without the addition of glutathione (Gillam et al., 1993). Human lymphoblast-based expression with endogeneous cytochrome P450 reductase (Crespi et al., 1991a) and with coexpression of cytochrome P450 reductase cDNA (C. Crespi, unpublished observation). Vaccinia virus (vv)/HepG2 cell expression data from Buters et al. (1994).

The principal limitation of these data is the lack of definition of the individual forms for the CYP2C subfamily. Analysis of this subfamily has remained problematic due to high cross-reactivities of all of the distinct forms with most antibody preparations. In addition, Western blot analysis does not distinguish between active and inactive forms of the protein. Furthermore, distinct enzymes may have different affinities for coenzymes necessary for catalytic activity, which will serve to unlink abundance of the protein and its catalytic activity. Therefore the assumptions must be made that the ratios of active to inactive protein are similar for all forms and that all forms have similar affinities for coenzymes. These assumptions may not be justified. However, even with these limitations, the study of Shimada et al. (1994) contributes greatly to our understanding of relative enzyme abundance in human liver. In addition, the relative abundance data, coupled with the absolute P450 content (per unit protein) and the turnover numbers for enzyme-specific substrates (per unit protein), can provide an estimate of the turnover number for individual enzymes in the human liver membrane environment. This provides an important benchmark for evaluation of turnover number data from cDNA-expressed enzymes. 

CYPlAl has also been expressed in yeast (Eugster et al., 1990 Sengstag et al., 1994) and EROD activities of 223 pmol/(mg min) have been reported. The turnover numbers for EROD in yeast-expressed CYPlAl (1.4/min) were substantially lower than observed for human lymphoblast-expressed CYPlAl (7.6/min). Apparent KmS were quite similar (92 nM and 87 nM in yeast and human lymphoblasts, respectively). Modified CYPlAl has also been expressed in E. coli (Guo et al., 1994). The expression level of CYPlAl in E. coli (per mg membrane protein) was comparable to that obtained with human lymphoblasts (about 30 pmol/mg in both systems). The turnover number for EROD for E. co//-expressed CYPlAl was quite low in isolated membranes, but could be increased to 8/min if the protein was purified and reconstituted. The apparent for E. coli-exptessed CYPlAl EROD activity, 580 nM, was about 6-fold higher than that for the yeast- or human lymphoblast-expressed enzymes. It is not clear whether this difference is due to the base substitutions necessary to obtain expression in E. coli, or some other cause. 

Human CYP1A2 has been expressed in several microbial systems including bacteria and yeast (Sandhu et al., 1994 Sengstag et al., 1994 Guo et al., 1994). As with human CYPlAl, the turnover numbers for E. co/Z-expressed human CYP1A2 are higher upon purification and reconstitution. 

Fig 4. Turnover number for cDNA-expressed CYP2A6 and purified/reconstituted human liver CYP2A6. Data from human lymphoblasts ( lymphoblast , Crespi et al., 1991a), vaccinia virus using phosphate buffers ( vv Phosphate , Yamano et al., 1990), vv using Tris buffers ( vv Tris , C. Crespi and F. Gonzalez, unpublished observation) and purified/reconstituted human liver CYP2A6 ( purified , Yun et al., 1992). 

CYP3A4 has been expressed in human lymphoblasts (Crespi et al., 1991a). The expression level in this report was quite low. Recent modifications to the promoter for cDNA expression and coexpression of OR have led to a 40-fold increase in catalytic activity. The mean testosterone 6j8-hydroxylase activity in microsomes from h3A4/OR cells (cultured in the presence of dexamethasone to induce cytochrome P450 reductase activity) [1000 pmol/(mg min)] is comparable to the mean values observed in human liver microsomes [1070 pmol/(mg min) Yamazaki et al., 1993]. The turnover number for testosterone and CYP3A4 in the human lymphoblasts was 15/min for endogenous OR levels and increased to 44/min with OR coexpression. 

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 experimental results show that the turnover number for the hydrogenation of benzene changes according to the support used for the catalyst the variations are beyond experimental error since we have a fourfold increase from NaX to LaY. 

According to the relative importance of turnover number for hydrogenation, we can divide the supports roughly in two classes ... 

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