Catalysis, enzymic

Enzyme catalysis Enzyme electrode Enzyme immobilization Enzyme immunoassay Enzyme inhibitors... 

Enzymatic Catalysis. Enzymes are biological catalysts. They increase the rate of a chemical reaction without undergoing permanent change and without affecting the reaction equiUbrium. The thermodynamic approach to the study of a chemical reaction calculates the equiUbrium concentrations using the thermodynamic properties of the substrates and products. This approach gives no information about the rate at which the equiUbrium is reached. The kinetic approach is concerned with the reaction rates and the factors that determine these, eg, pH, temperature, and presence of a catalyst. Therefore, the kinetic approach is essentially an experimental investigation. 

Enzymes accelerate reaction rates by lowering the activation barrier AGp. While they may undergo transient modification during the process of catalysis, enzymes emerge unchanged at the completion of the reaction. The presence of an enzyme therefore has no effect on AG for the overall reaction, which is a function solely of the initial and final states of the reactants. Equation (25) shows the relationship between the equilibrium constant for a reaction and the standard free energy change for that reaction ... 

Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which catalyze reactions in living organisms-biochemical and biological reactions. The systems involved may be colloidal-that is, between homogeneous and heterogeneous. Some enzymes are very specific in catalyzing a particular reaction (e.g., the enzyme sucrase catalyzes the inversion of sucrose). Enzyme catalysis is usually molecular catalysis. Since enzyme catalysis is involved in many biochemical reactions, we treat it separately in Chapter 10. 

CDs are cyclic oligosaccharides comprised of a-l,4-linked glucopyr-anose units (37- 0). The following properties make CDs attractive components in organic chemistry and supramolecular catalysis/ enzyme mimics in particular (i) CDs are water soluble (ii) their hydrophobic cavity can host a variety of lipophilic guest molecules ... 

Micellar catalysis, conducted in the absence of Lewis acid tends to inhibit the Diels-Alder reaction, relative to the reaction in water. The reason is that the local reaction medium in the Stern region is less favorable than bulk water. However, by combining Lewis-acid and micellar catalysis, enzyme-hke rate accelerations can be obtained (Table 7.5) in case the Lewis acid acts as the counterion for the miceUe. " ... 

Catalysis. Enzymes, with more than 2000 known representatives, are the largest group of proteins in terms of numbers (see p.88). The smallest enzymes have molecular masses of 10-15 kDa. Intermediatesized enzymes, such as alcohol dehydrogenase (top left) are around 100-200 kDa, and the largest-including glutamine synthetase with its 12 monomers (top right)—can reach more than 500 kDa. 

I 7 AsYininetric Transformations by Coupled Enzyme and Metal Catalysis enzyme... 

Catalysis Enzymes such as alcohol dehydrogenase, acetyl cholinesterase,... 

The chemoenzymatic synthesis of chiral alcohols is a field of major interest within biocatalytic asymmetric conversions. A convenient access to secondary highly enan-tiomerically enriched alcohols is the usage of alcohol dehydrogenases (ADHs) (ketoreductases) for the stereoselective reduction of prochiral ketones. Here, as in many other cases in asymmetric catalysis, enzymes are not always only an alternative to chemical possibilities, but are rather complementary. Albeit biocatalysts might sometimes seem to be more environmentally friendly, asymmetric ketone reduction... 

H. Wada, H. Kagamiyama, T. Watanabe, In Pyridoxal Catalysis Enzymes and Model Systems (E. E. Snell, ed.) Wiley Interscience, New York, p. Ill (1966). 

Catalysis, enzyme-substrate and intermediate compound theory in homo-and heterogeneous, V, 51 Catalysts, for acetonation, III, 51 for acetylation of starch, I, 284, 286 Bourguel s, II, 109, 110, 113 for esterification of cellulose, I, 312 in oxidation of carbohydrates by halogens, III, 177... 

Enzymatic catalysis. Enzymes are protein catalysts, capable of enhancing rates of reactions by factors of up to 1012. 

In heterogeneous catalysis these models are generally referred to as the Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. The term Michaelis-Menten kinetics is often used in homogeneous catalysis, enzyme reactions and reactions of microbial systems. 

Keywords Catalysis Enzyme Metal nanoparticles Microgels Thermosensitive... 

Under saturating substrate concentrations, the rate of the enzyme-catalyzed reaction will be governed by the activation energy for the conversion of the ES complex to the EP complex. It is clear that if the substrate is bound more tightly by the enzyme, then the size of this activation energy barrier wiU increase, which leads to a reduced rate. Therefore, for optimum rates of catalysis, enzymes should bind the substrate fairly weakly, but they should selectively bind the transition state of the reaction. 

Many enzymes use coenzymes to achieve the detailed transformations they catalyze but the enzyme proteins themselves also supply important elements of the catalysis. Enzyme proteins are the source of the entire catalytic effect when coenzymes are not involved. As one common process, acid and base groups in enzymes perform proton transfers that are critical to the catalytic mechanism. A particularly informative example is observed in the enzyme ribonuclease A, which catalyzes the cleavage of RNA (16). The catalytic process (Fig. 4) involves the imidazole ring of the amino acid histidine that removes the proton from the 2-hydroxyl of the ribose. A different protonated histidine transfers a proton to the RNA to promote the cleavage process. Studies with D2O-H2O mixtures established that the two proton transfers occur at the same time (17). 

---