Lipases, as biocatalysts

The enzymatic KR between racemic amines and nonactivated esters using a lipase as biocatalyst is shown in Scheme 7.15. In the same manner as in the transesterification of secondary alcohols, this process fits Kazlauskas rule [32], where normally if the large group (L) has larger priority than medium group (M), the (R)-amide is obtained. In general, major size differences between both groups result in better enantios-electivities ( ). 

Lipases (triacyl glycerol acyl hydrolases, E.C. 3.1.1.3) are a unique class of hydrolases i113-115 for asymmetric synthesis based on prochiral or racemic substrates. The application of lipases as biocatalysts has been reviewed emphasizing different... 

The hydrophilic/hydrophobic media provided by the host surface can also be of primary importance toward entrapped species reactivity and stabihty. Since pristine sihca surface is of hydrophilic natiue, it is possible to enhance its hydrophobic character using sihcon alkoxide bearing alkyl chain (C H2 +iSi(OR)3). This approach revealed to be particularly successful for the inunobihzation of lipases (Reetz, 1997) (see Section Entrapped Lipases as Biocatalysts ). 

Entrapped Lipases as Biocatalysts. Confinement within siUca gels does not only protect enzymes against denaturation. It can also provide a chemical surrounding that favors the enzymatic activity. The ability to tailor the matrix properties, by modifying sol-gel chemistry, enables optimization of the bioactivity of encapsulated enzymes. Hybrid materials can be used to control the polarity of the internal environment within the nanopores. Hydrophobic hybrid organic-inorganic materials can then be produced. They are more suitable for the encapsulation of hpophilic enzymes that would not remain functional in polar matrices (Gill, 1998 Brennan, 1999). 

Figure 7.11 Production process of biodiesel-like biofuels by selective ethanolysis of vegetable oils using lipases as biocatalyst.

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. 

Enzymes are widely recognized as valuable tools for the synthesis of optically active compounds [22]. Thus, lipase-catalyzed acylation or deacylation is one of the most efficient methods for the preparation of optically active alcohols, acids, and esters. Because lipases retain activity and selectivity in non-conventional media such as organic liquids, their use as biocatalysts in enantioselective synthetic reactions has considerably increased. 

The annual production would be 550 t of oleyl oleate with market price 5 EURO/kg. As biocatalyst, an immobilized Rhizomucor miehei lipase - Lipozyme IM - product from NOVO Nordisk was used. 

Several researches have reported an alternative method to produce esters through enzymatic reactions using lipases as catalysts (6-12). Because biocatalysts have high specific activity and a low impact on the environment, they have become increasingly important for industry. For example, immobilized lipases are used as catalysts for reactions involving biomodification of triglycerides (13). 

Lipases are commonly recognized as biocatalysts in hydrolysis and esterification reactions. The primary advantages of the used reactions are in asymmetric hydrolysis of chiral esters, as well as asymmetric esterification of a wide range of substrate... 

Chemical interesterification is conveniently achieved by using alkali metal methylates as a catalyst. Microbial lipases are also used as biocatalysts in enzymatic interesteiification. In contrast to the chemical process, the enzymatic process can be more selective if an enzyme with positional specificity is used, but this reaction is usually much slower and more sensitive to reaction conditions. Recent developments in lipase-catalyzed interesterification have resulted in new industrial applications of this process (255). Nevertheless, the high costs of enzymes and process equipment may limit widespread adoption of this process. 

Microbial lipases may be used as biocatalysts for interesterification. Lipase-assisted interesterification offers possibilities for transformation of lipids beyond those possible using chemical interesterification (11). Enzymatic interesterihcation has several advantages over the chemical-assisted reactions, such as mild reaction conditions leading to reduced energy consumption and less thermal damage to reactants and products, the possibility of lipase specificity toward their natural substrates, as well as high catalytic efficiency. Lipase-catalyzed interesterihcation reactions, in contrast to those carried out with chemical catalysts, yield different types of products depending on the specihcity of the lipase used. 

Enzyme-catalyzed reactions are used to produce human mUkfat substitutes for use in infant formulas (46-48). Acidolysis reaction of a mixture of tripalmitin and unsaturated fatty acids using a i -l,3-specific lipase as a biocatalyst afforded TAGs derived entirely from vegetable oils rich in 2-position palmitate with unsaturated fatty acyl groups in the sn- and sn-3 positions (44). These TAGs closely mimic the fatty acid distribution found in human mUkfat, and, when used in infant formula instead of conventional fats, the presence of palmitate in the sn-2 position of the TAGs improved digestibility of the fat and absorption of other important nutrients such as calcium (46, 49). 

The possible application of enzyme-assisted reactions for production of lower value nonspecialty lipids such as margarine hardstocks and cooking oils has been reported (50). When nonspecific lipases, such as those from Candida cylindraceae and C. antarctica, are used as biocatalysts for interesterification of oil blends, the TAG... 

If regio- or stereospecific lipases are used to interesterify oil blends, the products formed are different from those obtained by chemical interesterification, and may exhibit better functional properties. For example, interesterification of blends of canola and palm oils, using the in-1,3-specific Rhizopus delemar lipase as a biocatalyst, gave oils with improved fluidity compared with the original blends or chemically interesterified products. 

Enzymatic transesterification is under investigation [287, 288], but the cost of lipase production is the main hurdle for commercialization. Intracellular lipase as a vhole cell biocatalyst could lo ver the lipase production cost. Another problem is ho v to maintain lipase activity in the presence of a high concentration of methanol and glycerol. Industrialization is under investigation, but still not realized. 

Even though CaLB is the lipase most widely used in ionic liquids, other lipases such as Pseudomonas cepacia lipase (PcL) and Candida rugosa lipase (CrL) have been often used as biocatalyst for ester synthesis in ionic liquid [13, 14]. Nara et al. 

Yeast contains a variety of enzymes, and in some cases use of a single purified enzyme is preferable. These arc divided into oxidorcductases, transferases, hydrolases, lyases, isomerases, and lipases. Many of these are commercially available (but expensive). Purified reductases usually require expensive cofactors. In addition individual microbes can be used as biocatalysts. A general review of microbial asymmetric reductions is available.5 These reductions can be the opposite of those of yeast. 

Ferroquine possesses planar chirality due to the non-symmetrical 1,2-substitution of the ferrocene entity, and the pure enantiomers (+)35 and (—)35 were obtained by enzymatic resolution using the Candida rugosa lipase as a biocatalyst. The enantiomeric purity levels exceed 98%. However, the two optical isomers display identical activity in vitro at the nanomolecular level. In vivo, however, either of the enantiomers alone is less active than the racemic mixture against both chloroquine-sensitive and chloroquine-resistant strains. In addition, (4-)35 displays better curative effects than (—)35, suggesting different pharmacokinetic properties. The reasons for the enhanced behavior of racemic ferroquine have not yet been elucidated. It is still not clear whether 35 is oxidized by the parasite to give the ferricinium ion, thus initiating Fenton-type reactivity. Such a hypothesis is reasonable, given that reactive oxidative species can escalate in cancer cells due to the malfunction of mitochondria. ... 

Most commercial applications, such as enzyme preparations for detergents, do not require pure lipases, but a certain degree of purity simplifies their successfiil usage as biocatalysts because reduces side-product formation and simplifies product downstream. Extensive lipase purification should be considered when structural studies are going to be performed or when it will be used as biocatalyst in a synthetic reaction for the pharmaceutical industry. The main drawbacks of traditional purification strategies are low yields and productivities. The extent of purification varies with the number and the order of purification steps (see section 2.2.3) the importance of designing optimal purification schemes has been highlighted in several comprehensive reviews on this topic (Taipa et al. 1992 Aires-Barros et al. 1994 Palekar et al. 2000 Saxena et al. 2003). 

Lipases can be used as biocatalysts in hydrolytic and synthetic reactions. These enzymes are the most widely used biocatalysts in organic chemistry and they have... 

---