Enzymatic synthetic reactions

Aldol Additions. These reactions catalyzed by lyases are perhaps the most synthetically useful enzymatic reactions for carbon—carbon bond formation. Because of the broad synthetic utiUty of this method, the enzymatic aldol reactions have received considerable attention in recent years and have been extensively covered in a number of books and reviews (10,138—140). 

Adenosine triphosphate (ATP) is one of the most important cofactors involved in many of the synthetic reactions going on within the cell. Its recent large scale in vitro enzymatic synthesis from adenosine and acetylphosphate is of particular interest. Three enzymes immobilized in polyacrylamide gel were used adenosine kinase, adenylate kinase and acetate kinase (lip. ... 

Figure 4.3 The synthesis of an oligonucleotide from an activated mononucleotide, (a) Adenonine triphosphate (ATP), the substrate of enzymatic nucleic-acid synthesis. (b) An imidazolide of a nucleotide of the kind used in many non-enzymatic template-directed reactions, (c) The synthetic reaction leading to the formation of a trinucleotide. (Modified from Orgel, 2002.)...

Based on the stereospecific transketolase-catalyzed ketol transfer from hydroxy-pyruvate (20) to D-glyceraldehyde 3-phosphate (18), we have thus developed a practical and efficient one-pot procedure for the preparation of the valuable keto-sugar 19 on a gram scale in 82% overall yield [29]. Retro-aldolization of D-fructose 1,6-bisphosphate (2) in the presence of FruA with enzymatic equilibration of the C3 fragments is used as a convenient in-situ source of the triose phosphate 18 (Scheme 2.2.5.8). Spontaneous release of CO2 from the ketol donor 20 renders the overall synthetic reaction irreversible [29]. 

It is generally stated that biocatalysis in organic solvents refers to those systems in which the enzymes are suspended (or, sometimes, dissolved) in neat organic solvents in the presence of enough aqueous buffer (less than 5%) to ensure enzymatic activity. However, in the case of hydrolases water is also a substrate and it might be critical to find the water activity (a ) value to which the synthetic reaction (e.g. ester formation) can be optimized. Vahvety et al. [5] found that, in some cases, the activity of Candida rugosa lipase immobihzed on different supports showed the same activity profile versus o but a different absolute rate. With hpase from Burkholderia cepacia (lipase BC), previously known as lipase from Pseudomonas cepacia, and Candida antarctica lipase B (CALB) it was found that the enzyme activity profile versus o and even more the specific activity were dependent on the way the enzyme was freeze dried or immobihzed [6, 7]. A comparison of the transesterification activity of different forms of hpase BC or CALB can be observed in Tables 5.1 and 5.2, respectively. 

Biocatalytic synthetic reactions also include carbon dioxide fixation with the production of methanol in artificial multi-enzyme systems [188]. Formate dehydrogenase (FDH, EC 1.2.1.2) can catalyze the reduction of carbon dioxide to formate, and methanol dehydrogenase (MDH, EC 1.1.99.8) can catalyze the reduction of formate to methanol. Both of these enzymes require NAD+-NADE1 cofactor, and in the presence of the reduced dimethyl viologen mediator (MV+), they can drive a sequence of enzymatic reactions. The cascade of biocatalytic reactions results in the reduction of CO2 to formate catalyzed by FDEI followed by the reduction of formate to methanol catalyzed by MDH. A more complex system composed of immobilized cells of Parococcus denitrificans has been demonstrated for the reduction of nitrate and nitrite [189]. 

Considering the complex kinetic behavior of most enzymatically controlled reactions (41), the formal treatment of simple catalytic analogues should not pose additional problems. However, one consequence of the less perfect, but for most practical and mechanistical purposes sufficient performance of synthetic catalysts in comparison to enzymes is that in many kinetic studies, a large excess of substrate over the catalyst cannot be used, because then the uncatalyzed reaction will be too fast. Consequently, kinetic studies under catalyst saturation, or the steady-state methods that are most often used in the investigation of enzymes (18, 19, 41), are not suitable here. The formal treatment of the resulting, often quite complex, kinetics is greatly facilitated by computer-aided numerical simulations, which also help to design proper experimental conditions. 

Broad interest in the possible biodegradation of synthetic polymers has developed only in recent years and primarily in response to the growing problem of the waste disposal of plastics. Essentially all biopolymers are susceptible to enzymatic degradation because the enzymatic polymerization reactions responsible for their synthesis in nature have closely related counterparts in nature for their enzymatic depolymerization what nature creates, nature can destroy . If it were not so, polymers could not be utilized as reserve materials and waste... 

Proteolytic reactions take place in general in aqueous media, with the equilibrium in the direction of hydrolysis [87]. However, with decreased water activity, the equilibrium of the reaction is shifted toward synthetic reactions [74]. The use of organic media provides a potentially useful approach to the enzymatic modification of food protein. It has been already established that proteinases are active in organic media. They catalyze ester synthesis or aminolysis by synthesizing new peptide bonds preferentially to hydrolysis. 

Enzymatic reactions are characterized by their high stereospecificity. The enzyme exhibits activity only towards one of the optical antipodes of the substrate. In reactions where an asymmetric center is newly formed, the product is only one of the optical antipodes. Such a high stereospecificity in the reaction is a reflection of the asymmetric primary and higher order structures of the enzyme molecule, which is polypeptide. In this connection, it is of much interest to investigate the asymmetric reactions in which a synthetic polypeptide takes part, with respect to the effect of the primary and higher order structures. Such studies will not only serve as models for enzymes in order to throw light on the mechanism of their stereospecificity, but also open a way to develop specific catalysts for synthetic reactions. 

The activity of enzymes can be regulated by a number of means. The enzymes involved in the synthesis and hydrolysis of glycogen can be activated by phosphorylation, and deactivated by dephosphorylation. This is an example of covalent modification. Amino acids are synthesized by sequences of up to 15 separate enzymatically catalyzed reactions. If the end product is present in high concentrations, it combines with the first enzyme in the synthetic sequence and shuts it down. This is an example of feedback inhibition. Other kinds of feedback inhibition will prevent the synthesis of the enzyme itself by interfering with the transcription step producing messenger RNA. 

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