Catalytic site activity

Because of the distinct roles of the two different substrate cations in its reaction mechanism, the Na+,K+ pump most conveniently illustrates the mechanism of P-type pumps. The Na+,K+ pumps are driven by a cycle of conformational transitions that is driven by phosphorylation of their catalytic sites, activated by cytoplasmic Na+, and hydrolysis of the same phosphorylated sites, activated by extracellular K+ (Fig. 5-2). 

For heterogeneous reactions involving fluid and solid phases, the areal rate is a good choice. However, the catalysts (solid phase) can have the same surface area but different concentrations of active sites (atomic configuration on the catalyst capable of catalyzing the reaction). Thus, a definition of the rate based on the number of active sites appears to be the best choice. The turnover frequency or rate of turnover is the number of times the catalytic cycle is completed (or tumed-over) per catalytic site (active site) per time for a reaction at a given temperature, pressure, reactant ratio, and extent of reaction. Thus, the turnover frequency is ... 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

Just as is true for every zinc enzyme in which zinc is at the catalytic site, activity is lost if the metal is removed, and is restored by zinc uptake. The tertiary structure of carbonic anhydrase is maintained in the absence of zinc even the denatured apoprotein can refold spontaneously from a random coil to a native-like conformation. Although such a process is accelerated by zinc, ... 

Once properly bound, the hydrolysis actually involves two hydrolytic steps. The first step is the hydrolysis of acetylcholine by nucleophilic attack at the carbonyl carbon by the serine hydroxyl group, which liberates choline and leaves the enzyme acetylated. A triad formed between glutamine, histidine, and the serine at the catalytic site activates the serine for the nucleophilic attack. The second step is the hydrolysis of the acetylated enzyme by water to regenerate the free enzyme. The water is activated by hydrogen-bonding to the histidine residue, which increases the nucleophilic character of... 

Experiments indicatedthat the molar radioactivity function of H2S production on M0S2 can be described by one equation (A = 100, i= 1) biexponential equations were required for promoted catalysts with different Aj and Aj. This indicates that the catalyst sulfur was not bonded homogenously, like that of the observations with H2 S or H2 S exchange.An important difference from those observations is that, in this case, we deal with H2 S, replaced by sulfur, formed in HDS, so that the H2S molar radioactivity represents sulfur formed on the (or via) cat-al Tically active sites. Consequently, the fact that function (7) is described by a biexponential equation with two A-s and A-s, indicates that there exist two types of catalytic sites, active in HDS on promoted Mo-based catalysts. 

Here, the sulfoethyl group as the catalytic site activates the methanol and promotes the nucleophilic substitution at the atrazine. Consistently, catalytic activities were far smaller when the polymerization of 2-sulfoethyl methacrylate and methyl acrylate was achieved in the absence of the template. Furthermore, upon polymerizing only methyl acrylate in the absence of 2-sulfoethyl methacrylate, the resultant polymer-bound atrazine but showed no catalytic activity for its decomposition. Apparently, the binding sites were sufficiently formed in the polymers but they are not sufficient to decompose atrazine efficiently. 

Model 1 The number of catalytic sites active in methyl radical formation depends on dissocia-tively adsorbed-type oxygen (T > 1013 K)... 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

Fig. 5. Main reactions of catalytic reforming. Pt and acid refer to predominant active catalytic sites.

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

Triethyl aluminum, complexed with another electron donor, typically ethyl -anisate [94-30-4J, was used as cocatalyst with the FT-1 catalyst and acted to reduce and stabilize the active titanium-containing catalytic site. The early versions of the FT-1 catalyst required extremely high molar ratios (>400 1) of aluminum to titanium to obtain satisfactory activity and selectivity to isotactic polymer. This resulted in excessively high aluminum residues in the polymer. Later versions of the FT-1 catalyst attained much higher activity. 

Fig. 1. Inhibition of porcine pancreatic a-amylase. Substrates, an inhibitor, and their binding orientations in the active site are shown schematically. The arrows denote the catalytic site in each case, (a) The small substrate, G2PNP [17400-77-0] (3) (b) the large substrate, G OH [13532-61 -1] (4) and (c) the inhibitor, 4-phenyl imidazole (5) and the substrate G2PNP (3) in the binding orientation for noncompetitive inhibition. The binding orientation of G2PNP...

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

The basic kinetic properties of this allosteric enzyme are clearly explained by combining Monod s theory and these structural results. The tetrameric enzyme exists in equilibrium between a catalytically active R state and an inactive T state. There is a difference in the tertiary structure of the subunits in these two states, which is closely linked to a difference in the quaternary structure of the molecule. The substrate F6P binds preferentially to the R state, thereby shifting the equilibrium to that state. Since the mechanism is concerted, binding of one F6P to the first subunit provides an additional three subunits in the R state, hence the cooperativity of F6P binding and catalysis. ATP binds to both states, so there is no shift in the equilibrium and hence there is no cooperativity of ATP binding. The inhibitor PEP preferentially binds to the effector binding site of molecules in the T state and as a result the equilibrium is shifted to the inactive state. By contrast the activator ADP preferentially binds to the effector site of molecules in the R state and as a result shifts the equilibrium to the R state with its four available, catalytically competent, active sites per molecule. 

Figure 16.21 Structure of one subunit of the core protein of Slndbls virus. The protein has a similar fold to chymotrypsin and other serine proteases, comprising two Greek key motifs separated by an active site cleft. The C-terminus of the protein is bound in the catalytic site, making the coat protein inactive (Adapted from S. Lee et al., Structure 4 531-541, 1996.)...

FIGURE 16.16 Comparison of the amino acid sequences of chymotrypsinogen, trypsino-gen, and elastase. Each circle represents one amino acid. Nmnbering is based on the sequence of chymotrypsinogen. Filled circles indicate residues that are identical in all three proteins. Disnlfide bonds are indicated in yellow. The positions of the three catalytically important active-site residues (His, Asp °-, and Ser ) are indicated. 

There is a complication in choosing a catalyst for selective reductions of bifunctional molecules, For a function to be reduced, it must undergo an activated adsorption on a catalytic site, and to be reduced selectively it must occupy preferentially most of the active catalyst sites. The rate at which a function is reduced is a product of the rate constant and the fraction of active sites occupied by the adsorbed function. Regardless of how easily a function can be reduced, no reduction of that function will occur if all of the sites are occupied by something else (a poison, solvent, or other function). 

The deposition of carbon on the E-cat during cracking will temporarily block some of the catalytic sites. The carbon, or more accurately the coke, on the regenerated catalyst (CRC) will lower the catalyst activity and, therefore, the conversion of feed to valuable products (Figure 3-15). 

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