Active catalytic center

Luciferase turnover. The luciferase-catalyzed light-emitting reaction that forms oxyluciferin is fast, but the hydrolysis reaction of oxyluciferin into etioluciferin by luciferase is slow. The turnover rate (catalytic center activity) of luciferase was reported to be about 30/s for the luminescence reaction, and 0.03/s for the hydrolysis of oxyluciferin (Shimomura et al., 1969). 

In principle, the amount of a biocatalyst may be expressed as a number of elementary entities, an amount of substance, a mass, or by a catalytic effect. In many cases, due to the lack of information or for practical reasons, the later route is used. Then, the generally accepted term for the expression of enzyme activity is the Unit (U) defined in terms of reaction rate. One unit (U), accordingly to the International System of Units, is defined as the amount of enzyme that catalyzes the conversion of 1 pmol of substrate (or the formation of 1 pmol of product) in 1 min under standard conditions. The specific activity is the number of units per milligram of protein. If the relative molecular mass of the enzyme is accurately known it is then possible to express the activity as the molar catalytic activity, defined as the number of units per micromole of enzyme. This is, the number of moles of product formed, or substrate consumed, per mole of enzyme per minute. This may not correspond to the number of moles of substrate converted per enzyme active site since the enzyme may contain more than one active site. If the number of active sites per mole is known, then the activity may be expressed correspondingly as the catalytic center activity. 

In cases in which the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex still often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid, some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

The biologically uncommon Ni center associated with FeS clusters is a powerful and unique catalytic unity. In this chapter we have reviewed the structural and mechanistic aspects of three NiFeS centers the active site of hydrogenase and Clusters A and C of CODH/ACS. In the former, the association of a Ni center with the most unusual FeCOCN2 unit is a fascinating one. Model chemists, spectroscopists, and crystallographers have joined efforts to try and elucidate the reaction mechanism. Although a consensus is being slowly reached, the exact roles of the different active site components have not yet been fully established. Ni appears to be the catalytic center proper, whereas the unusual Fe center may be specially suited to bind a by-... 

A catalytic reaction is composed of several reaction steps. Molecules have to adsorb to the catalyst and become activated, and product molecules have to desorb. The catalytic reaction is a reaction cycle of elementary reaction steps. The catalytic center is regenerated after reaction. This is the basis of the key molecular principle of catalysis the Sabatier principle. According to this principle, the rate of a catalytic reaction has a maximum when the rate of activation and the rate of product desorption balance. 

Homogeneous catalysts are very often known as examples of single-site catalysts characterized by complete structural definition and (presumably) complete knowledge of the chemical processes occurring at their catalytic centers. It is a matter of fact that the homogeneous catalysts are molecular complexes constituted by an active core containing a single active atom (of-... 

True examples of single-site catalysts are enzymes, where active sites are made mainly by metalhc centers (mono- or polynuclear species) whose coordination sphere is completely defined by ligands [1-4]. The strength of enzymes is the combined effect of metal center activity with the specific behavior of metal coordination sphere hgands. These species play a key role, being optimized to create an environment suitable for (i) metal centers approaching and coordinating by reactants (ii) product removal from the catalytic centers at the end of the reaction in order to avoid further reactions. 

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

Molecular modeling studies revealed a similar binding mode for (5)-2-nitro-1 -phenylethanol in the catalytic center of ///)HNL as was determined experimentally for (5)-mandelonitrile, preserving all mechanistically important polar interactions with active-site residues. This implies that the mechanism for the cyanohydrin reaction applies to the nitroaldol reaction as well. 

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