Reactant/substrate

Most biphasic systems use sparingly water-soluble substrates and yield hydrophobic products. Therefore, the aqueous phase serves as a biocatalyst container [34,35]. However, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38]. 

Biocatalysis localization in the biphasic medium depends on physicochemical properties of the reactants. When all the chemical species involved in the reaction are hydro-phobic, catalysis occurs at the liquid-liquid interface. However, when the substrate is hydrophobic (initially dissolved in the apolar phase) and the product is hydrophilic (remains in the aqueous phase), the reaction occurs in the aqueous phase [25]. The majority of biphasic systems use sparingly water-soluble substrates and yield hydrophobic products therefore, the aqueous phase serves as a biocatalyst container [34,35] [Fig. 2(a)]. Nevertheless, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38] (Fig. 2(b), (c)). 

There was therefore a clear need to assess the assumptions inherent in the classical kinetic approach for determining surface-catalysed reaction mechanisms where no account is taken of the individual behaviour of adsorbed reactants, substrate atoms, intermediates and their respective surface mobilities, all of which can contribute to the rate at which reactants reach active sites. The more usual classical approach is to assume thermodynamic equilibrium and that surface diffusion of reactants is fast and not rate determining. 

Addition of radicals to a different unsaturated substrate is an important class of organic reactions. To understand its regiochemistry, one needs to examine the condensed Fukui function (f°) or atomic softness (.v°) for radical attack of the different potential sites within the reactant substrate. We consider now a simple problem summarized in Example 3. 

As a practical matter of cost, studies on solvent isotope effects are usually limited to D/H substituted solvents, although recently a few lsO solvent effects have been measured. Interpretation of enzymatic solvent isotope effects is even more complicated than it is when the isotopic probe is incorporated in the substrate(s). This is because enzyme proteins have many exchangeable protons and, also, this is frequently true for reactants (substrates). Thus the observed isotope effect is the collective result of many different isotopic substitutions, each of which may influence... 

Almost all enzymes are proteins. They provide templates whereby reactants (substrates) can bind and are favorably oriented to react and generate the products. The locations where the substrates bind are known as active sites. Because of the specific 3D structures of the active sites, the functions of enzymes are specific that is, each particular type of enzyme catalyzes specific biochemical reactions. Enzymes speed up reactions, but they are not consumed and do not become part of the products. Enzymes are grouped into six functional classes by the International Union of Biochemists (Table 2.2). 

The action of enzymes is usually very specific. This applies not only to the type of reaction being catalyzed (reaction specificity), but also to the nature of the reactants ( substrates ) that are involved (substrate specificity see p.94). In Fig. B, this is illustrated schematically using a bond-breaking enzyme as an example. Highly specific enzymes (type A, top) catalyze the cleavage of only one type of bond, and only when the structure of the substrate is the correct one. Other enzymes (type B, middle) have narrow reaction specificity, but broad substrate specificity. Type C enzymes (with low reaction specificity and low substrate specificity, bottom) are very rare. 

Any conversion of reactant (substrate) into a particular product, irrespective of reagents or mechanisms involved. Transformations are distinct from reactions, be-... 

Metalloenzymes-Enzymes are large protein molecules so built that they can bind at least one reactant (substrate) and catalyse a biochemically important reaction. Some enzymes incorporate one or more metal atoms in their normal structure they are called metalloenzymes. The metal not merely participate during the time that the enzyme-substrate complex... 

Apparent reaction rates with immobilized enzyme particles also decrease due to the mass transfer resistance of reactants (substrates). The Thiele modulus of spherical particles of radius R for the Michaelis-Menten type reactions is given as... 

Many enzymes appear to be tailor-made for one specific reaction involving only one reactant, which is called the substrate. Others can function more generally with different reactants (substrates). But there is no such thing as a universal enzyme that does all things for all substrates. However, nothing seems to be left to chance even the equilibration of carbon dioxide with water is achieved with the aid of an enzyme known as carbonic anhydrase.8 Clearly, the scope of enzyme chemistry is enormous, yet the structure and function of relatively few enzymes are understood in any detail. We can give here only a brief discussion of the mechanisms of enzyme action—first some general principles then some specific examples. 

The effectiveness of enzymes is due to the fact that they bring the reactants (substrates) together in a location where they are ideally oriented with respect to each other and the catalytic groups of the enzyme. 

In the determination of steady state reaction kinetic constants of enzyme-substrate reactions, FABMS also provides some very unique capabilities. Since these studies are best performed in the absence of glycerol in the reaction mixture, the preferred method is that which analyzes aliquots which are removed from a batch reaction at timed intervals. Quantitation of the reactants and products of interest is essential. When using internal standards, generally, the closer in mass the ion of interest is to that of the internal standard, the better is the quantitative accuracy. Using these techniques in the determination of kinetic constants of trypsin with several peptide substrates, it was found that these constants could be easily measured (8). FABMS was used to follow the decrease in the reactant substrate and/or the increase in the products with time and with varying concentrations of substrate. Rates of reactions were calculated from these data for each of the several substrate concentrations used and from the Lineweaver-Burk plot, the values of Km and Vmax are obtained. 

Fig. 7. General scheme of an artificial photosynthetic device S — photosensitizer, A — electron acceptor, D — electron donor, and P2 — reactants (substrates) for the reduction and oxidation processes, respectively...

Proteins are macromolecules (molecular weights from about 5000 to 106 [3]). There are thousands of different types of proteins, each with a particular biological function, often extremely specific. Thus, a particular enzyme will often recognize only one or a very narrow class of compounds as reactants and catalyze reaction of those to particular products. Other enzymes, typically those that catalyze hydrolysis or other degradative reactions, recognize a particular bond type but will act upon a broad class of reactants (substrates). These protein catalysts typically operate effectively at ambient temperature and pressure. Unique catalytic capabilities give enzymes their niche in bioprocessing. 

Since the Mannich reaction is a condensation involving three reactants (substrate, aldehyde, and amine), pathways a or b (Fig. 24) can be followed, if one excludes a rather unlikely trimolecular mechanism ... 

In a base-free medium (dry MeCN), Fe Ch activates HOOH to form a reactive intermediate that oxygenates alkanes, alkenes, and thioethers, and dehydrogenates alcohols and aldehydes. Table 11 summarizes the conversion efficiencies and product distributions for a series of alkene substrates subjected to the Fe Cfi/HOOH/MeCN system. The extent of the Fe Cb-induced monooxygenations is enhanced by higher reaction temperatures and increased concentrations of the reactants (substrate, Fe Cls, and HOOH). For 1-hexene (representative of all of the alkenes), a substantial fraction of the product is the dimer of 1-hexene oxide, a disubstituted dioxane. With other organic substrates (RH), Fe Cb activates HOOH for their monooxygenation the reaction efficiencies and product distributions are summarized in Tables 11(b). In the case of alcohols, ethers, and cyclohexane, a snbstantial fraction of the product is the alkyl chloride, and with aldehydes, for example, PhCHO, the acid chloride represents one-half of the product. In the absence of snbstrate the Fe Cls/MeCN system catalyzes the rapid disproportionation of HOOH to O2 and H2O. 

The exploration of low-valent lanthanide chemistry opened up a wide range of possibilities in organolanthanide chemistry. These studies demonstrated that unsaturated hydrocarbons were clearly viable reactants/ substrates, that a variety of reactions with C—H and C—C bonds were... 

Here the reactants (substrates) are glucose (CH20), 02, NH3, and a sulfur-providing nutrient S3, and the products are biomass X, C02, metabolic product P and H20. 

A major disadvantage of the gel entrapment route to immobilization is the potential for physical loss of the enzyme as time elapses. To circumvent this problem, cross-linking agents, such as A,A -methylene-bis-acrylamide or glutaraldehyde, may be used to more firmly immobilize the enzyme or to provide mechanical stability. However, the more rigid the matrix the greater is the possibility that diffusional resistance to transport of reactants (substrates) to the site of the enzyme and of products out of the gel will limit the reaction rate. 

The detailed kinetic studies on the oxidation of saturated hydrocarbons cyclohexane [4,5] and adamantane [26] and epoxidation of unsaturated hydrocarbons [4,5,25] cis-cyclooctene, cyclohexene, styrene and trans-stilbene were done by measuring the rate of reaction with respect to the concentration of each reactant, substrate, catalyst, ascorbic acid, hydrogen ion and molecular oxygen. The dependence of the reaction rate at various initial concentrations of the reactants were determined. While varying the concentration of a particular reactant, the concentrations of other reactants were kept constant under identical physical conditions. 

Free energy as a function of the reaction coordinate. In both panels, the purple curve shows how the free energy of a system of reactants (substrate plus enzyme, if present, plus product) changes as substrate is converted to product. In the upper panel, there is no catalysis and the curve shows a high transition-energy barrier inhibiting the conversion of substrate to product. In the lower panel, enzyme catalysis has broken the reaction into steps so that the transition energy for any one step is much less. 

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