Enzymatic catalysis reaction rate

Microreactor for heterogeneous and enzymatic catalysis reactions Nanostmctured peaks are created in Si substrate by deep reactive ion etching (black silicon) Increased reaction rates in the microreactor due to increased surface areas Roumanie et al. 2008... 

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 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. ... 

In this chapter we have seen that enzymatic catalysis is initiated by the reversible interactions of a substrate molecule with the active site of the enzyme to form a non-covalent binary complex. The chemical transformation of the substrate to the product molecule occurs within the context of the enzyme active site subsequent to initial complex formation. We saw that the enormous rate enhancements for enzyme-catalyzed reactions are the result of specific mechanisms that enzymes use to achieve large reductions in the energy of activation associated with attainment of the reaction transition state structure. Stabilization of the reaction transition state in the context of the enzymatic reaction is the key contributor to both enzymatic rate enhancement and substrate specificity. We described several chemical strategies by which enzymes achieve this transition state stabilization. We also saw in this chapter that enzyme reactions are most commonly studied by following the kinetics of these reactions under steady state conditions. We defined three kinetic constants—kai KM, and kcJKM—that can be used to define the efficiency of enzymatic catalysis, and each reports on different portions of the enzymatic reaction pathway. Perturbations... 

Zeffren and Hall (1973) have commented that, since reactions with polar transition states in nonpolar solvents can be accelerated by several orders of magnitude by the presence of low concentrations of salts (Winstein et al., 1959), the rate enhancement of tetramethylglucose mutarotation provided by the presence of acid-base pairs such as phenol and pyridine may be due to formation of ion pairs in benzene solution. Salts which do not act as acids and bases catalyse mutarotation of tetramethylglucose in aprotic solvents (Eastham et al., 1955 Blackall and Eastham, 1955 Pocker, 1960). The efficiency of enzymatic catalysis could arise largely from electrostatic catalysis... 

It is thus beneficial to work at fixed water activity in studies of the influence of solvents, snpports or other snbstances on enzymatic catalysis. Otherwise the effects due to differences in enzyme hydration will strongly influence the results and mask the effects songht. A typical example of this was seen when reaction rates were compared for the same reaction carried out in different solvents at varying water concentrations. In the different solvents, maximal reaction rate was observed at widely different water concentrations. However, when water was quantified in terms of water activity the optimum was observed at about the same water activity in all solvents (Valivety, Hailing and Macrae, 1992) (Figure 9.5). 

Selectivity is an intrinsic properly of enzymatic catalysis. [3] Following the nomenclature proposed by Cleland [24, 25], the pseudo second-order rate constant for the reaction of a substrate with an enzyme, kml/KM, is known as the specificity constant, ksp. [26] To express the relative rates of competing enzymatic reactions, involving any type of substrates, the ratio of the specificity constants appears to be the parameter of choice [3]. Since the authoritative proposition by Sih and coworkers [27], the ratio of specificity constants for the catalytic conversion of enantiomeric substrates, R and S, is commonly known as the enantiomeric ratio or E -value (Equation 1) ... 

The enzymatic catalysis of reactions is essential to living systems. Under biologically relevant conditions, uncatalyzed reactions tend to be slow—most biological molecules are quite stable in the neutral-pH, mild-temperature, aqueous environment inside cells. Furthermore, many common reactions in biochemistry entail chemical events that are unfavorable or unlikely in the cellular environment, such as the transient formation of unstable charged intermediates or the collision of two or more molecules in the precise orientation required for reaction. Reactions required to digest food, send nerve signals, or contract a muscle simply do not occur at a useful rate without catalysis. 

Although electron transfer as such is not considered as catalysis, most enzymatic redox reactions require the presence of electron-transfer proteins for fast and efficiently directed electron transfer to the active sites. The ferredoxins, azurins, and cytochromes are most well known in this respect. Variations of over 15 A in distance may occur, and as a consequence, the electron-transfer rate may vary over 10 orders of magnitude [35], Exciting developments are ongoing in this field, and are highly relevant for the bioinorganic catalytic subject. 

In recent years micellar emulsifiers have been found to affect the rate of many reactions (15,16). This phenomenon of micellar catalysis originally attracted attention as a model for enzymatically catalysed reactions although the analogy is... 

Enzymes are proteins that catalyze reactions. Thousands of enzymes have been classified and there is no clear limit as to the number that exists in nature or that can be created artificially. Enzymes have one or more catalytic sites that are similar in principle to the active sites on a solid catalyst that are discussed in Chapter 10, but there are major differences in the nature of the sites and in the nature of the reactions they catalyze. Mass transport to the active site of an enzyme is usually done in the liquid phase. Reaction rates in moles per volume per time are several orders of magnitude lower than rates typical of solid-catalyzed gas reactions. Optimal temperatures for enzymatic reactions span the range typical of living organisms, from about 4°C for cold-water fish, to about 40°C for birds and mammals, to over 100°C for thermophilic bacteria. Enzymatic reactions require very specific molecular orientations before they can proceed. As compensation for the lower reaction rates, enzymatic reactions are highly selective. They often require specific stereoisomers as the reactant (termed the substrate in the jargon of biochemistry) and can generate stereospecific products. Enzymes are subject to inhibition and deactivation like other forms of catalysis. 

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