Processive enzymes

Steroids are synthetic products of cholesterol [57-88-5]. The chemical stmcture of a steroid hormone is determined by sequential enzymatic processing of the cholesterol molecule. Steroid products differ among steroid-secreting glands because of differences in enzyme processing, eg, the production of estrogen by the ovary requires enzymatic steps that do not occur in the adrenal cortex. 

Acetylsucrose [63648-81-7] has been prepared in 40% yield by direct acetylation of sucrose using acetic anhydride in pyridine at 40° C (36). The 6-ester has subsequently been obtained in greater than 90% yield, by way of 4,6-cycHc orthoacetate. Other selective methods for the 6-acylated derivatives include the use of alkyl tin reagents such as dibutyl tin oxide (37) and of dibutyl stannolane derivatives (38). Selective acetylation of sucrose by an enzymic process has also been described. Treatment of sucrose with isopropenyl acetate in pyridine in the presence of Lipase P Amano gave, after chromatography, 6-0-acetylsucrose (33%) and 4/6-di-O-acetylsucrose (8%). The latter compound has been obtained in 47% yield by the prolonged treatment (39). 

Enzyme—Heat—Enzyme Process. The enzyme—heat—enzyme (EHE) process was the first industrial enzymatic Hquefaction procedure developed and utilizes a B. subtilis, also referred to as B. amjloliquefaciens, a-amylase for hydrolysis. The enzyme can be used at temperatures up to about 90°C before a significant loss in activity occurs. After an initial hydrolysis step a high temperature heat treatment step is needed to solubilize residual starch present as a fatty acid/amylose complex. The heat treatment inactivates the a-amylase, thus a second addition of enzyme is required to complete the reaction. 

Dual-Enzyme Processes. In some cases, especially in symp production in Europe, a Hquefaction process is used that incorporates both a thermostable enzyme and a high temperature heat treatment. This type of process provides better hydrolyzate tilterabiHty than that attained in an acid Hquefaction process (9). Consequendy, dual-enzyme processes were developed that utilized multiple additions of either B. licheniformis or B. stearothermophilus a-amylase and a heat treatment step (see Eig. 1). 

Com symps [8029-43 ] (glucose symp, starch symp) are concentrated solutions of partially hydrolyzed starch containing dextrose, maltose, and higher molecular weight saccharides. In the United States, com symps are produced from com starch by acid and enzyme processes. Other starch sources such as wheat, rice, potato, and tapioca are used elsewhere depending on avadabiHty. Symps are generally sold in the form of viscous Hquid products and vary in physical properties, eg, viscosity, humectancy, hygroscopicity, sweetness, and fermentabiHty. 

There are two distinct classes of enzymes that hydrolyze nitriles. Nittilases (EC 3.5.5. /) hydrolyze nittiles directiy to corresponding acids and ammonia without forming the amide. In fact, amides are not substrates for these enzymes. Nittiles also may be first hydrated by nittile hydratases to yield amides which are then converted to carboxyUc acid with amidases. This is a two-enzyme process, in which enantioselectivity is generally exhibited by the amidase, rather than the hydratase. 

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

Fig. 9. The MoFe protein cycle of molybdenum nitrogenase. This cycle depicts a plausible sequence of events in the reduction of N2 to 2NH3 + H2. The scheme is based on well-characterized model chemistry (15, 105) and on the pre-steady-state kinetics of product formation by nitrogenase (102). The enzymic process has not been chsiracter-ized beyond M5 because the chemicals used to quench the reactions hydrolyze metal nitrides. As in Fig. 8, M represents an aji half of the MoFe protein. Subscripts 0-7 indicate the number of electrons trsmsferred to M from the Fe protein via the cycle of Fig. 8.

Stimulus molecules approach the receptor area in a random distribution. Therefore, there cannot be a homogeneous distribution of chemical or enzymic processing capabilities over the area, as this would produce a chaotic mass of information. The capabilities of such precise chemoreceptory discrimination that we observe can only arise from an ordered system in such a way that specific reaction-types would be localized, or at least be concentrated in specific areas of the epithelium. ... 

Egli T, S Stromeyer, AM Cook, T Leisinger (1990) Transformation of tetra- and trichloromethane to COj by anaerobic bacteria is a non-enzymic process. FEMS Microbiol Lett 68 207-212. 

Carbohydrate metabolism in the organism tissues encompasses enzymic processes leading either to the breakdown of carbohydrates (catabolic pathways), or to the synthesis thereof (anabolic pathways). Carbohydrate breakdown leads to energy release or intermediary products that are necessary for other biochemical processes. The carbohydrate synthesis serves for replenishment of polysaccharide reserve or for renewal of structural carbohydrates. The effectiveness of various routes of carbohydrate metabolism in tissues and organs is defined by the availability of appropriate enzymes in them. 

Compared with ketoreductases, the synthetic application of alcohol oxidases has been less explored. However, selective oxidation of primary alcohols to aldehydes is superior to the chemical methods in terms of conversion yields, selectivity, and environmental friendliness of reaction conditions. In addition, coupling of alcohol oxidase with other enzymes provides a tremendous opportunity to develop multi-enzyme processes for the production of complex molecules. Therefore, a growing impact of alcohol oxidases on synthetic organic chemistry is expected in the coming years. 

Disadvantages of Whole-Cell Process Compared with the Isolated Enzyme Process... 

Although, MediChem is a biosecurity products manufacturer, its biotechnology-based R D capabilities are worth mentioning here. The attended markets include Medical, Laboratory, Veterinary, and Environmental sectors. Medicinal chemistry services and drug discovery form the basis of the company, though their capabilities might be applied in a broader range of sectors. These capabilities comprise the areas of Proteomics, Combinatorial and Computational Chemistry, Medicinal Chemistry, Enzymes, Process Development, Analytical and Separations Chemistry, Chemical Synthesis and Scale Up. 

Cram and co-workers have been successful in modifying certain of their cavitands such that reactions with a bound substrate are promoted. Such systems provide a first step towards the synthesis of rudimentary enzymes (Cram, Katz, Dicker, 1984). One example of this type, involving a binding step followed by a fast acylation step, is illustrated by Figure 5.1. This sequence resembles part of the mechanism used by chymo-trypsin to cleave a peptide bond. Thus, the enzymic process entails several stages but, like the model system, begins with a binding step followed by a crucial transacylation step. 

The cytochromes are another group of haem proteins found in all aerobic forms of life. Cytochromes are electron carriers involving a Fe(ii)/Fe(m) redox system. They are a crucial part of the electron transfer reactions in mitochondria, in aspects of the nitrogen cycle, and in enzymic processes associated with photosynthesis. 

Maximo C, Costa-Ferreira MC (2004) Decolourisation of reactive textile dyes by Irpex lacteus and lignin modifying enzymes. Process Biochem 39 1475-1479... 

Toray (1) A large Japanese chemicals manufacturer, perhaps best known for its process for synthesizing /-lysine for use as a dietary supplement. The starting material is cyclohexene which is converted in five steps to racemic lysine. An enzymic process isolates the desired optical isomer, the other is recycled. 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

The URDP/PNP/Ara-U process is used to manufacture nelarabine, a water-soluble prodrug of 9-p-D-arabinofuranosylguanidine produced as a treatment for acute lymphoblastic leukaemia (Scheme 1.29). The two-enzyme process is run at 200 g... 

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