Turnover definition

Although we will stick to the IL-6 gene, it should be mentioned at the side that two other RNA polymerases exist in mammalian cells responsible for the synthesis of RNA molecules, which are not translated into proteins ribosomal (rRNA), transfer (tRNA), small nuclear (snRNA), small nucleolar (snoRNA), and some of the recently discovered microRNAs and piRNAs. These RNA molecules act in the process of translation and mRNA turnover. Micro and piRNAs are probably extremely important in the definition of stem cells and of differentiation programs. Some of them are synthesized by RNA polymerase II. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

The definition of turnover time is total burden within a reservoir divided by the flux out of that reservoir - in symbols, t = M/S (see Chapter 4). A typical value for the flux of non-seasalt sulfate (nss-SOl"") to the ocean surface via rain is 0.11 g S/m per year (Galloway, 1985). Using this value, we may consider the residence time of nss-S04 itself and of total non-seasalt sulfur over the world oceans. Appropriate vertical column burdens (derived from the data review of Toon et ai, 1987) are 460 fxg S/m for nss-801 and 1700 jig S/m for the sum of DMS, SO2, and nss-S04. These numbers yield residence times of about 1.5 days for nss-S04 and 5.6 days for total non-seasalt sulfur. We might infer that the oxidation process is frequently... 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

The turnover number of an enzyme is defined as the maximum number of moles of substrate reacted per mole of enzyme (or molecules per molecule) per minute under optimum conditions (i.e., saturating substrate concentration, optimum pH, etc). If 2 mg/cm3 of a pure enzyme (50,000 molecular weight, Michaelis constant Km = 0.03 mole/m3) catalyzes a reaction at a rate of 2.5 jumoles/nUksec when the substrate concentration is 5 x 10 3 moles/m3, determine the turnover number corresponding to this definition and the actual number of moles of substrate reacting per minute per mole of enzyme. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Catalyst stability can be defined in terms of turnover number (TON). A textbook definition of this is ... 

The results of adsorption and desorption of CO mentioned above suggest that for the reaction at low temperature, the sites for relatively weakly chemisorbed CO are covered by the deposited carbon and the reaction occurs between molecularly adsorbed CO and oxygen on the carbon-free sites which are the sites for relatively strongly chemisorbed CO. Therefore, the definition of the turnover rate at 445 K remains as given in Equation 1. For the reaction at 518 K, however, this definition becomes inappropriate for the smaller particles. Indeed, to obtain the total number of Pd sites available for reaction, we now need to take into consideration the number Trp of CO molecules under the desorption peak. Furthermore, let us assume that disproportionation of CO takes place through reaction between two CO molecules adsorbed on two adjacent sites, and let us also assume that the coverage is unity for the CO molecules responsible for the LT desorption peak, since this was found to be approximately correct on 1.5 nm Pd on 1012 a-A O (1). Then, the number Np of palladium sites available for reaction at 518 K is given by HT/0 + NC0 LT s nce t ie co molecules under the LT desorption peak count only half of the available sites. Consequently, the turnover rate at 518 K should be defined as ... 

Enzyme activity refers to the rate at which a particular enzyme catalyzes the conversion of a particular substrate (or substrates) to one or more products under a given set of conditions. Usually, activity refers to the contribution of many enzyme molecules (often expressed simply as activity per mg of protein or similar) but, in its simplest form, activity refers to the contribution of a single enzyme molecule. The turnover number of an enzyme-substrate combination refers to the number of substrate molecules metabolized in unit time (usually a period of 1 s) under a given set of conditions (see later). These definitions appear, at first glance, to be largely self-explanatory. However, many factors contribute to the final activity of an enzyme, and these must be considered during any assessment of such activity. 

The basic principle for definition of the term fine chemicals is a three-tier segmentation of the universe of chemicals into commodities, fine chemicals, and specialty chemicals (see Figure 1.1). Fine chemicals account for the smallest part, about 4-5% of the total 1.8 trillion turnover of the chemical industry (see Section 9.1). 

By definition, the turnover frequency is expressed per number of active sites. So, catalytic samples that differ only in the amount active sites must exhibit the same values of turnover frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct measurement of intrinsic kinetic data in heterogeneous catalysis is difficult due to the effect of heat and mass transfer, especially inside the pores of high specific-area materials. The turnover frequency reveals these phenomena. In other words, in the case of supported... 

The remarkable efficacy of topical corticosteroids in the treatment of inflammatory dermatoses was noted soon after the introduction of hydrocortisone in 1952. Numerous analogs are now available that offer extensive choices of potencies, concentrations, and vehicles. The therapeutic effectiveness of topical corticosteroids is based primarily on their antiinflammatory activity. Definitive explanations of the effects of corticosteroids on endogenous mediators of inflammation await further experimental clarification. The antimitotic effects of corticosteroids on human epidermis may account for an additional mechanism of action in psoriasis and other dermatologic diseases associated with increased cell turnover. The general pharmacology of these endocrine agents is discussed in Chapter 39. 

Correlation between the two observed Mn species and catalytic activity properties was attempted. For this purpose, turnover frequencies (TOF) referred to the bulk content of the different Mn species were derived from the results of catalytic activity tests in CH4 combustion. TOF referred to Mn in Al(2) site was found to be almost constant on varying the overall Mn content. This suggested a possible correlation between catalyst activity and this Mn species. However, an alternative correlation was found by normalizing the catalytic activity to the surface area. Such normalized activity correlated well with the overall Mn content. No further evidence was found in favor of either these two alternatives, so that no definitive conclusion could be drawn. 

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. 

As is the case with all biologically active chemicals (i.e., enzymes and substrates), activity (i.e., turnover number and Km) is the relationship between a defined response and the concentration that causes it. Table G.0.1 shows some flavor activities and their definitions. 

The turnover frequency allows performance comparison between different catalyst systems, biological and/or non-biological. Its threshold is at 1 event per second per active site. According to the definition, a turnover frequency can be determined only if the number of active sites is known (Chapter 9, Section 9.2.3). For an enzyme reaction obeying Michaelis-Menten kinetics, Eq. (2.15) holds. 

Figure 6.8. Change in atmospheric 14C with time in the Northern Hemisphere (heavy solid line) since 1955. Radiocarbon values are expressed as the per mil variation in 14C/12C ratio relative to a standard (see Appendix for definition of units).The lighter lines show the evolution of 14C for homogeneous, steady-state reservoirs with turnover times of 5,15, 60, and 120 yr.

The catalyst turnover number (TON) and the turnover frequency (TOF) are two important quantities used for comparing catalyst efficiency. Their definitions, however, vary slightly among the three catalysis fields. In homogeneous catalysis, the TON is the number of cycles that a catalyst can run through before it deactivates, i.e., the number of A molecules that one molecule of catalyst can convert (or turn over ) into B molecules. The TOF is simply TON/time, i.e., the number of A molecules that one molecule of catalyst can convert into B molecules in one second, minute, or hour. In heterogeneous catalysis, TON and TOF are often defined per active site, or per gram catalyst. This is because one does not know exactly how many... 

Dopamine (0.1 /iM) inhibits the release of [3H]-serotonin (71), while apomorphine (50 M) stimulates the release of Q3H]]-sero-tonin (72). These apparently conflicting observations preclude any definitive conclusions being drawn about the involvement of dopamine receptors in regulating the turnover or release of serotonin in the substantia nigra. 

---