Enzymatic catalytic intermediates

Originally published in 1913 as a rate law for enzymatic sugar inversion [19], the Michaelis-Menten rate equation is also used frequently for describing homogeneously catalyzed reactions. It describes a two-step cycle (Eqs. (2.34) and (2.35)) the catalyst (the enzyme, E) first reacts reversibly with the substrate S, forming an enzyme-substrate complex ES (a catalytic intermediate). Subsequently, ES decomposes, giving the enzyme E and the product P. This second step is irreversible. 

Hydroperoxo-ferric intermediate, termed also Compound 0, is the immediate precursor of the main catalytic intermediate Compound I in peroxidase enzymatic cycle. Attempts to study this intermediate directly in reactions of hydrogen peroxide with HRP using fast kinetic methods have been inconclusive, possibly because it is not accumulated in sufficient concentrations.90,91 However, Compound 0 could be prepared and studied by EPR and optical absorption spectroscopy via cryoreduction of... 

We analyze a few of the simplest catalytic systems following. In the systems under consideration, the constituent elementary transformations usually are linear in respect to the concentrations of catalytic intermediates. Generally speaking, the results of this kind of analysis are only applicable when the inter action between the active centers, whether identical or different, of the catalyst can be ignored. This is the case mainly for homogeneous and enzymatic rather than heterogeneous catalysis. However, in some cases, the conclusions can be extended to more complex catalytic systems operating in the steady stationary states even with the lateral interaction between the active centers. 

Dinucleating ligand a ligand that binds two metals High valent iron centers in formal oxidation state (IV) Intermediate a species that is proposed to be present during the enzymatic catalytic cycle... 

Four-valent iron centers are well known as catalytic intermediates in enzymatic heme catalysis. The catalytic cycles of, for example, catalase, peroxidase, and cytochrome P450, all have in common an intermediate that is comprised of a ferryl Fe(lV) and a further oxidation equivalent located either on the heme ring or on the protein moiety. This reaction intermediate is called compound 1 (cpd 1) and has a system spin of 5 = 1/2 (e.g., in chloroperoxidase) or 5 = 3/2 (e.g.. 

Two mechanistic aspects of the cobalamin-dependent methyltransferases distinguish them from adenosyl-cobalamin-dependent enzymes. No methylcobalamin synthetases are known. Methylcobalamin is produced at enzymatic sites as an intermediate in the primary overall reaction. Second, methylcobalamin does not function as a coenzyme in the sense that it assists in catalysis to date, it is instead a catalytic intermediate in its enzymatic reactions. 

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

Enzymes are proteins catalyzing all in vivo biological reactions. Enzymatic catalysis can also be utilized for in vitro reactions of not only natural substrates but some unnatural ones. Typical characteristics of enzyme catalysis are high catalytic activity, large rate acceleration of reactions under mild reaction conditions, high selectivities of substrates and reaction modes, and no formation of byproducts, in comparison with those of chemical catalysts. In the field of organic synthetic chemistry, enzymes have been powerful catalysts for stereo- and regioselective reactions to produce useful intermediates and end-products such as medicines and liquid crystals. ... 

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

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