Mechanisms catalytic

We finally consider the EC catalytic mechanism in which the product of the electrode reaction transforms back to the initial electroactive reactant by means of a pseudo first-order chemical reaction [15,53,55]  

Note that the chemical step (2.75) is totally irreversible, attributed with a pseudo first-order rate constant kc (s ) defined as = fcc,rCx. where Cx has the same meaning as for the CE and EC mechanisms (Sect. 2.4.1). Although this is the simplest case of an electrode mechanism involving chemical reaction, it has particular analytical utility [53], The mass transport of the redox species is described by the following model  

By substituting (2.81) and (2.82) into the Nernst equation (1.8), one obtains an integral equation, as a solution for a reversible catalytic mechanism. The numerical solution for the reversible case reads  

The theory for catalytic reaction has been verified by studying the reductions of Ti + in presence of NH2OH and ClOj and the reduction of Fe in presence of NH2OH. In these studies the mercury electrode has been applied [53]. The properties of the experimental voltammograms confirm the theoretical predic- 

An impressive range of approaches have been applied over the years to investigating the molecular mechanism of catalysis by the radical copper 

Dissociation of the aldehyde product would leave a low-coordinate, Cu(I) redox center associated with two protonated tyrosine phenols in the active site. This complex is known to be very reactive toward dioxygen, the second-order kinetic constant for reoxidation of the reduced enzyme by O2 being nearly 8x10 s (Borman et aL, 1997 Whit- 

On the other hand, based on the 3IP-NMR analysis of the ternary complex of patato 

the cofactor phosphate acts as an electrophile to the phosphate oxygen of glucose 1-phosphate.I05,l06) 

the cofactor phosphate acts as an acid-base catalyst donating a proton to the phosphate group of glucose 1-phosphate.110 (From/. Biol. Chem., 268, 5574 (1993)). 

3 Understanding the Overall Structure of GpdQ and Metal Binding 

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Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Verschueren K H G, Seljee F, Rozeboom J, Kalk K H and Dijkstra B W 1993 Crystallographic analysis of the catalytic mechanism of haloalkane dehalogenase Nature 363 693-8... 

Engineering Substrate Specificity. Although the serine proteases use a common catalytic mechanism, the enzymes have a wide variety of substrate specificities. For example, the natural variant subtiHsins of B. amyloliquefaciens (subtiHsin BPN J and B. licheniformis (subtiHsin Carlsberg) possess very similar stmctures and sequences where 86 of 275 amino acids are identical, but have different catalytic efficiencies, toward tetraamino acid -nitroanilide substrates (67). 

Free radicals are molecular fragments having one or more unpaired electrons, usually short-lived (milhseconds) and highly reaclive. They are detectable spectroscopically and some have been isolated. They occur as initiators and intermediates in such basic phenomena as oxidation, combustion, photolysis, and polvmerization. The rate equation of a process in which they are involved is developed on the postulate that each free radical is at equihbrium or its net rate of formation is zero. Several examples of free radical and catalytic mechanisms will be cited, aU possessing nonintegral power law or hyperbohc rate equations. 

Convergent evolution has produced two different serine proteinoses with similar catalytic mechanisms... 

The subtilisin mutants described here illustrate the power of protein engineering as a tool to allow us to identify the specific roles of side chains in the catalytic mechanisms of enzymes. In Chapter 17 we shall discuss the utility of protein engineering in other contexts, such as design of novel proteins and the elucidation of the energetics of ligand binding to proteins. 

All the residues involved in important functions in the catalytic mechanism are strictly conserved in all homologous GTPases with one notable exception. Ras does not have the arginine in the switch 1 region that stabilizes the transition state. The assumption that the lack of this catalytically important residue was one reason for the slow rate of GTP hydrolysis by Ras was confirmed when the group of Alfred Wittinghofer, Max-Planck Institute,... 

In summary, structural studies of Ras and Gq with GTP-yS and a transition state analog have illuminated the catalytic mechanism of their GTPase activity, as well as the mechanism by which GTP hydrolysis is stimulated by GAP and RGS. In addition, these structural studies have shown how tumor-causing mutations affect the function of Ras and Gq. 

As indicated in the preceding section, amine hardeners will cross-link epoxide resins either by a catalytic mechanism or by bridging across epoxy molecules. In general the primary and secondary amines act as reactive hardeners whilst the tertiary amines are catalytic. 

The relative importance of the potential catalytic mechanisms depends on pH, which also determines the concentration of the other participating species such as water, hydronium ion, and hydroxide ion. At low pH, the general acid catalysis mechanism dominates, and comparison with analogous systems in which the intramolecular proton transfer is not available suggests that the intramolecular catalysis results in a 25- to 100-fold rate enhancement At neutral pH, the intramolecular general base catalysis mechanism begins to operate. It is estimated that the catalytic effect for this mechanism is a factor of about 10. Although the nucleophilic catalysis mechanism was not observed in the parent compound, it occurred in certain substituted derivatives. 

The change in mechanism with pH for compound 1 gives rise to the pH-rate profile shown in Fig. 8.7. The rates at the extremities pH < 2 and pH > 9 are proportional to [H+] and [ OH], respectively, and represent the specific proton-catalyzed and hydroxide-catalyzed mechanisms. In the absence of the intramolecular catalytic mechanisms, the... 

Aminotransferases Show Double-Displacement Catalytic Mechanisms... 

Uncovering of the three dimentional structure of catalytic groups at the active site of an enzyme allows to theorize the catalytic mechanism, and the theory accelerates the designing of model systems. Examples of such enzymes are zinc ion containing carboxypeptidase A 1-5) and carbonic anhydrase6-11. There are many other zinc enzymes with a variety of catalytic functions. For example, alcohol dehydrogenase is also a zinc enzyme and the subject of intensive model studies. However, the topics of this review will be confined to the model studies of the former hydrolytic metallo-enzymes. 

The cytosolic forms show no sequence homology to the secreted forms and have a different catalytic mechanism [1]. They are widely distributed and occur... 

Both for reaction in and IV the order with respect to catalyst is 0.5. The activation enthalpies are 96.6 3.4 and 97.6 3.4 kJ mol-1 respectively when Ti(OBu)4 is used as the catalyst. This is not too far from the activation enthalpies200 for the Sn(II)-cata-lyzed esterification of B with isophthalic acid (85.1 4.9) and with 2-hydroxyethyl hydrogen isophthalate (85.8 4.2). It is also close to the Ti(OBu)4-catalyzed esterification of benzoic acid with B (85.8 2.5)49. This is probably due to the formation of analogous intermediate complexes and similar catalytic mechanisms. On the other hand, the activation entropies of reactions III and IV are less negative than those of the reaction of benzoic or isophthalic acid with B. This probably corresponds to a stronger desolvation when the intermediary complex is formed and could be due to the presence of the sodium sulfonate group. 

Catalysts which enhance the burning rate of composite propellants are generally believed to accelerate the decomposition of ammonium perchlorate, but the catalytic mechanism is still not very clear. The important observed aspects of this catalysis can be summarized as follows ... 

Reversible electron transfer followed by an irreversible regeneration of starting materials, catalytic mechanism ... 

Improved sensitivity and scope can be achieved by coupling two (or more) enzymatic reactions hi a chain, cycling, or catalytic mechanism (9). For example, a considerable enhancement of the sensitivity of enzyme electrodes can be achieved by enzymatic recycling of the analyte in two-enzyme systems. Such an amplification... 

SITE-SPECIFIC MUTATIONS PROVIDE A POWERFUL WAY OF EXPLORING DIFFERENT CATALYTIC MECHANISMS... 

Site-Specific Mutations Provide a Powerful Way of Exploring Different Catalytic Mechanisms, 184... 

Abstract This review provides an overview of the development of viral protease inhibitors as antiviral drugs. We concentrate on HlV-1 protease inhibitors, as these have made the most significant advances in the recent past. Thus, we discuss the biochemistry of HlV-1 protease, inhibitor development, clinical use of inhibitors, and evolution of resistance. Since many different viruses encode essential proteases, it is possible to envision the development of a potent protease inhibitor for other viruses if the processing site sequence and the catalytic mechanism are known. At this time, interest in developing inhibitors is Umited to viruses that cause chronic disease, viruses that have the potential to cause large-scale epidemics, or viruses that are sufQciently ubiquitous that treating an acute infection would be... 

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