Enzyme catalyzed conversion

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

The practical usefulness of Equations 11.46 through 11.53 has been demonstrated for the malic enzyme catalyzed conversion of L-malate to pyruvate (Equation 11.72). Table 11.1 lists experimentally determined isotope effects for this reaction. Comparison of carbon kinetic isotope effects for protio and deutero-malate substituted at position 2 (the carbon that undergoes sp3 to sp2 transition) rules out the possibility that the hydride transfer and the decarboxylation events are concerted. This conclusion follows from Equation 11.48 which, for a concerted reaction, predicts that 13(V/K) should be smaller than 13(V/K)D, which is opposite to the order observed experimentally. 

Table 11.1 Isotope effects on malic enzyme catalyzed conversion of L-malate to Isotope effect Experimental value...

The therapeutic class that uniquely exemplifies lactone prodrugs are the statins, i.e., the cholesterol-lowering agents that act by inhibiting 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.34). This microsomal enzyme catalyzes conversion of HMG-CoA to mevalonate, an important rate-limiting step in cholesterol biosynthesis. Cholesterol synthesis occurs mainly... 

Liynses. These enzymes catalyze conversions, which are more complex than those of the other groups, and must involve at least two separate stages in the reaction The overall result is the synthesis of a molecule I rnili two components w ith a coupled breakdown of adenosine triphosphate, or some oiher nucleoside triphosphate. In general, this may be written ... 

Glycolysis provides the main source of ATP in Trypanosoma brucei, E histolytica, and G lamblia, which possess pyruvate kinase as well as a pyruvate phosphate dikinase for converting phosphoenolpyruvate (PEP) to pyruvate and generating ATP. Pyruvate phosphate dikinase is not a homolog of pyruvate kinase but is closely related to PEP synthase from bacteria. The enzyme catalyzes conversion of PEP to pyruvate accompanied by the synthesis of ATP from AMP and pyrophosphate. Genes encoding the enzyme have been isolated from E histolytica and G lamblia and have demonstrated considerable structural divergences. No specific inhibitor of this enzyme has yet been identified. 

In a maltodextrin process using enzyme-catalyzed conversion, the starch slurry (30% to 40% dry solids) is first pasted at a temperature of 80-90°C, and is then treated with a heat-stable bacterial alpha-amylase for liquefication. When stabilized with calcium ions, alpha-amylases from B. licheniformis or B. stearothermophilus can withstand temperatures of 90-105°C for at least 30 minutes,10 allowing sufficient process time to split the 1,4 bonds and form maltose and limit dextrins (see Chapter 7). 

After conversion to the proper DE, the reaction is stopped in the neutralizer tank by raising the pH with soda ash (sodium carbonate) to 4.5-5.0. This pH is critical not only to optimize the conditions under which the proteins and fats can be removed, but also to reduce the risk of unnecessary color development. At this point, the liquor may be pumped to an enzyme tank for further enzyme-catalyzed conversion, or clarified, bleached and evaporated. 

Answer After an ear of corn has been removed from the plant, the enzyme-catalyzed conversion of sugar to starch continues. Inactivation of these enzymes slows down the conversion to an imperceptible rate. One of the simplest techniques for inactivating enzymes is heat denatura-tion. Freezing the corn lowers any remaining enzyme activity to an insignificant level. 

In biphasic reactors or two-phase partitioning bioreactors (TPPB), the substrate is located mostly in the immiscible phase and diffuses to the aqueous phase. The enzyme catalyzes conversion of the substrate at the interface and/or in the aqueous phase. The product/s of the reaction then may partition to the organic phase. The system is self-regulated, as the substrate delivery to the aqueous phase is only directed by the partitioning ratio between the two phases and the enzymatic reaction rate [53]. The use of ionic liquid/supercritical carbon dioxide for enzyme-catalyzed transformation is gaining attention [69]. 

Cho SH, Shim J, Yun SH et al (2008) Enzyme-catalyzed conversion of phenol by using immobilized horseradish peroxidase (HRP) in a membraneless electrochemical reactor. Appl Catal A 337 66-72... 

Starch conversion refers to the process of converting starch into other products. It involves gelatinization, liquefaction, and saccharification. Liquefaction refers to the acid-or enzyme-catalyzed conversion of starch into maltodextrin. Starch, usually from wet milling of com, is pumped in a slurry to the conversion plant, where it undergoes one or more hydrolytic processes to yield mixtures of various carbohydrates in the form of syrups. The kind and amount of the various carbohydrates obtained depend upon the type of hydrolysis system used (acid, acid-enzyme, or enzyme-enzyme), the extent to which the hydrolytic reaction is allowed to proceed, and the type of enzyme(s) used. The fact that most starches consist of two different kinds of polymers... 

A heterogeneous catalytic reaction involves adsorption of reactants from a fluid phase onto a solid surface, surface reaction of adsorbed species, and desorption of products into the fluid phase. Clearly, the presence of a catalyst provides an alternative sequence of elementary steps to accomplish the desired chemical reaction from that in its absence. If the energy barriers of the catalytic path are much lower than the barrier(s) of the noncatalytic path, significant enhancements in the reaction rate can be realized by use of a catalyst. This concept has already been introduced in the previous chapter with regard to the Cl catalyzed decomposition of ozone (Figure 4.1.2) and enzyme-catalyzed conversion of substrate (Figure 4.2.4). A similar reaction profile can be constructed with a heterogeneous catalytic reaction. 

Consider the reaction scheme of Michaelis and Menten (1913) for enzyme-catalyzed conversion of a substrate (reactant) S to a product P ... 

Catabolism of tyrosine and tryptophan begins with oxygen-requiring steps. The tyrosine catabolic pathway, shown at the end of this chapter, results in the formation of fumaric acid and acetoaceticacid, Iryptophan catabolism commences with the reaction catalyzed by tryptophan-2,3-dioxygenase. This enzyme catalyzes conversion of the amino acid to N-formyl-kynurenine The enzyme requires iron and copper and thus is a metalloenzyme. The final products of the pathway are acetoacetyl-CoA, acetyl-Co A, formic add, four molecules of carbon dioxide, and two ammonium ions One of the intermediates of tryptophan catabolism, a-amino-P-carboxyrnuconic-6-semialdchydc, can be diverted from complete oxidation, and used for the synthesis of NAD (see Niacin in Chapter 9). 

TABLE 13.7 Examples of Organic Chemicals Produced by Fermentation or Enzyme-Catalyzed Conversion of a Biomass Component, Primary or Secondary Biomass Derivative, or Other Organic Chemical ... 

Receptor theory is based on the classical Law of Mass Action as developed by Michaelis and Menten (20) for the study of enzyme catalysis. The extrapolation of classical enzyme theory to receptors is, however, an approximation. In an enzyme-substrate (ES) interaction, the substrate S undergoes an enzyme-catalyzed conversion to a product or products. Because of the equilibrium established, product accumulation has the ability to reverse the reaction process. Alternatively, the latter can be used in other cellular pathways and is thus removed from the equilibrium situation or can act as a feedback modulator (21) to alter the ES reaction either positively or negatively (Equation 10.2). 

In this chapter, two examples demonstrate that, in addition to the desired enzyme-catalyzed conversion of the substrate S to the product Pi, other reactions have to be considered, e. g. a parallel reaction of S to P2 or a consecutive reaction of Pi to P3. For a hypothetical reaction scheme, see (Eq. 4). 

Furthermore, in order to nullify the enzyme-catalyzed conversion or conversions, it may be required that the enzyme-breaking substance or poison be activated, or that various poison enhancers be added. The latter may be the role of creatine, for instance, in enhancing the effectiveness of urea as an anticancer agent In one way or another, this may also be the role of electromagnetic radiation as an anticancer agent. 

The inference from the foregoing, therefore, is that the enzyme-catalyzed conversion of pyruvic acid or pyruvate to lactic acid or lactate should somehow be... 

Catalysts for oxidative decarboxylation of the Wterminal aspartate residue contained in an oligopeptide are unprecedented in both biological and chemical systems. In biological systems, malic enzyme catalyzes conversion of aspartate to pyruvate (145). The substrate of malic enzyme is the free amino acid, whereas those of the angiotensin-cleaving catalysts discovered in this study are oligopeptides containing Wterminal aspartate residue. 

The Mlchaells-Menten reaction mechanism describes the enzyme-catalyzed conversion of substrate Into product as follows (, 22-24) ... 

Fig. 3.2 Representation of steady-state hypothesis for enzyme catalyzed conversion of S into P...

If one places the products in the sequence shown here, the elimination of the 7a-hydroxyl group in 2 need not necessarily be enzyme-catalyzed. Conversion of 3 into 4 follows logically and requires enzyme mediation. Other routes to the formation of the same products are not excluded. 

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