Catalysts biocatalyst

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

A microbial transformation is the conversion of one substance (substrate) to another (product) by a micro-organism. It is a chemical reaction, catalyzed by a particular cellular enzyme or by an enzyme originally produced within cells. Most such enzymes are necessary for the normal functioning of the biological processes of cellular metabolism and reproduction. In microbial transformation, however, these enzymes simply act as catalysts (biocatalysts) for chemical reactions. In addition to their natural substrates, many of these enzymes can utilize other structurally related compounds as substrates and therefore occasionally catalyze unnatural reactions when foreign substrates are added to the reaction medium. Thus, microbial transformation constitutes a specific category of chemical synthesis. 

The objectives of this account are to review the problems involved in tailoring man-made photosynthetic systems and to highlight the scientific accomplishments in artificial photosynthesis. The chemical methodology of linking catalysts, biocatalysts and photosystems into integrated photosynthetic assemblies will be discussed. 

Enzymes have been naturally tailored to perform under physiological conditions. However, biocatalysis refers to the use of enzymes as process catalysts under artificial conditions (in vitro), so that a major challenge in biocatalysis is to transform these physiological catalysts into process catalysts able to perform under the usually tough reaction conditions of an industrial process. Enzyme catalysts (biocatalysts), as any catalyst, act by reducing the energy barrier of the biochemical reactions, without being altered as a consequence of the reaction they promote. However, enzymes display quite distinct properties when compared with chemical catalysts most of these properties are a consequence of their complex molecular stracture and will be analyzed in section 1.2. Potentials and drawbacks of enzymes as process catalysts are summarized in Table 1.1. 

Notably, the use of liquid polymers in multiphase catalysis with SCCO2 is not restricted to transition metal catalysts. Biocatalysts can also be used in this environment, as demonstrated by the yeast-catalyzed reduction of a (i-ketoester that gave excellent (99%) enantioselectivity in PMPS-710 and the ionic liquid [P(Me)(Bu)3][ Bu)3][03SCgH4pMe] [Eq. (8)]. The product was isolated by extraction with water or SCCO2, respectively [54]. 

With the same catalytic/biocatalytic system different platform molecules can be obtained following a simple modification of the catalyst/biocatalyst design or of the reaction conditions. 

It is apparent that the use of enzymatic catalysis continues to grow Greater availabiUty of enzymes, development of new methodologies for thek utilization, investigation of enzymatic behavior in nonconventional environments, and the design and synthesis of new biocatalysts with altered selectivity and increased stabiUty are essential for the successhil development of this field. As more is learned about selectivity of enzymes toward unnatural substrates, the choice of an enzyme for a particular transformation will become easier to predict. It should simplify a search for an appropriate catalyst and help to estabhsh biocatalytic procedures as a usehil supplement to classical organic synthesis. 

One of the most direct questions to ask in the perspective of enzyme design is whether an already existing protein with a binding pocket might be turned into a new catalyst by introducing catalytic residues directly, rather than by the elaborated TSA mimicry approach used for catalytic antibodies, hoping to create a new biocatalyst that could harness both the activity and the selectivity, in particular stereoselectivity, that is possible with enzymes. 

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

Furthermore, the biocatalysts will be even more important with the shift of the raw materials from oil to biomass. Since biomass is a mixture of various multifunctional compounds, chemo-, regio-, and enantioselective catalysts will be... 

Compared to synthetic catalysts, enzymes have many advantages. First of all, being natural products, they are environmentally benign and therefore their use does not meet pubhc opposition. Enzymes act at atmospheric pressure, ambient temperature, and at pH between 4 and 9, thus avoiding extreme conditions, which might result in undesired side reactions. Enzymes are extremely selective (see below). There are also, of course, some drawbacks of biocatalysts. For example, enzymes are known in only one enantiomeric form, as they consist of natural enantiomeric (homochiral) amino acids their possible modifications are difficult to achieve (see Section 5.3.2) they are prone to deactivation owing to inappropriate operation parameters and to inhibition phenomena. 

At present, photosynthetic organisms are not generally used as biocatalysts for bioconversion of organic compounds except for bioremediation of pollutants in the environment, although they are environment-friendly catalysts, and they may contain unusual type of enzymes to establish new reactions. Development of bioreactors specially developed for photosynthefic organism-catalyzed reaction as well as finding effective photosynthetic organisms as a biocatalyst are required in the future. 

The catalysts was added after the reactants were fed in the tank reactor and pressure and temperature were set to the target values [84]. The study was performed using an immobilized lipase, Novozym-435 , as biocatalyst. The temperature was set to 65-75 °C and the pressure was reduced (60 mmHg). A catalyst concentration of 1-5% with an acid alcohol ratio of 1 3, 1 1 or 3 1 was used. 

The idea of using organic metal complexes as catalysts for electrochemical reactions (Jasinski, 1964) can be traced back to the biocatalysts in which such complexes often are the catalytically active sites and which are distinguished by a high catalytic activity. This area has seen a strong development starting in the 1960s. 

Figure 5.8 Environmental factors E (top figure) and cost indices Cl (bottom figure) for the biocatalytic (a) and chemical catalytic (b) syntheses of (5)-styrene oxide (Scheme 5.3) including the synthesis of the Jacobsen catalyst and of the bacteria (Scheme 5.4) as further syntheses. Waste produced during biocatalyst synthesis is indicated. However, it has to be considered that biocatalyst and product synthesis cannot be separated.

Catalytic transformations can be divided on the basis of the catalyst-type - homogeneous, heterogeneous or enzymatic - or the type of conversion. We have opted for a compromise a division based partly on type of conversion (reduction, oxidation and C-C bond formation, and partly on catalyst type (solid acids and bases, and biocatalysts). Finally, enantioselective catalysis is a recurring theme in fine chemicals manufacture, e.g. in the production of pharmaceutical intermediates, and a separate section is devoted to this topic. 

The above two processes employ isolated enzymes - penicillin G acylase and thermolysin, respectively - and the key to their success was an efficient production of the enzyme. In the past this was often an insurmountable obstacle to commercialization, but the advent of recombinant DNA technology has changed this situation dramatically. Using this workhorse of modern biotechnology most enzymes can be expressed in a suitable microbial host, which enables their efficient production. As with chemical catalysts another key to success often is the development of a suitable immobilization method, which allows for efficient recovery and recycling of the biocatalyst. 

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