Other enzymes mediated processes

1 Fatty acid enrichment. Polyunsaturated fatty acids belonging to the co-6 series such as y-linolenic acid (y-18 3) and dihomo-y-linolenic acid (y-20 3) and those of the (0-3 series such as docosahexaenoic acid (DHA, 22 6) and eicosapentaenoic acid (EPA, 20 5), are of considerable importance due to their biomedical properties (Horrobin, 1982, 1992). Various plant seeds such as evening primrose Oenothera bienrus L.) and borage Borago officinalis L.) are good sources of y-linolenic acids while 

Similar approaches are feasible for enriching Y-linolenic and dihomo-y-linolenic acids. For example, the enrichment procedures for y-linolenic adds described by Hill et al. (1990) and by Mukherjee and Kiewitt (1991) 

2 Process catalysed by phospholipases. Soybean lecithin is a widely used natural emulsifier. Compared to synthetic emulsifiers, lecithin is inferior with respect to its emulsion stabilization potency. One of the ways to improve the stabilization properties of lecithin is to convert it to lysolecithin. By the loss of one of the fatty acids on the molecule, the hydrophilicity of the compound is generally improved, thus enhancing the stabilization of oil in water (OAV) types of emulsions. Such a transformation is performed industrially using phospholipases (Yamane, 1991). 

Soy lecithin is dispersed in water buffered at pH 8 with tris-HCl buffer in the presence of calcium chloride as Ca salt. Phospholipase A2 is added to the dispersed lecithin and the reaction is allowed to proceed. After completion, the water is removed by vacuum evaporation and dehydration and the oil removed by acetone extraction. The acetone is evaporated under partial vacuum and the lysolecithin is vacuum dried. 

Phospholipase D (PLD) hydrolyses phospholipids into phosphatidyl acid (PA) and a base (other component containing an -OH group). When the base is added to the phospholipid hydrolysing system, it catalyses the following type of reaction called phosphatidyl radical transfer reaction  

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As shown in the biosynthesis of granaticin, a hydride shift occurs intramolecularly. This process is mediated by an enzyme-bond pyridine nucleotide. A concerted abstraction of H-4 as a hydride in la and a C-5 deprotonation in 2a leads to the 4,5-enol ether 3a. The reduced form of the pyridine nucleotide transfers the hydride to C-6, simultaneously releasing a hydroxide to give 4a. Final tautomerization yields the dTDP-4-keto-6-deoxy-sugar in v-xylo configuration 4a. In other enzymes of the oxidoreductase type, the active site may show a different configuration. Thus, the intermediate 3a can be protonated from above at C-5 to yield the l-arabino isomer of 4a [2]. The stereochemistry of this mechanism was demonstrated by double labelling (cf. l-4b series), and as a net result proved a suprafacial 4—>6 hydride shift. 

DNA suitable for the insertion of foreign DNA is known as a vector the most commonly used vectors in bacteria are plasmids. Plasmids are small (2-3 kb) loops of DNA found in bacteria and yeast. They were first discovered when it was observed that bacteria could pass antibiotic resistance from one colony to another. This process was demonstrated to be mediated by plasmids containing genes for enzymes that inactivated the antibiotics. In addition to the work on plasmids, other research laboratories have isolated enzymes that cut DNA at specific sequences (restriction endonucleases) and other enzymes that can rejoin these cuts again (DNA ligases). 

All biochemical reactions are enzyme-mediated. The rate of an enzyme reaction depends on the substrate concentration at the location of the enzyme and thereby on the diffusion rate of a substrate to the enzyme. It is therefore important to permanently obtain an intimate contact between a cell or enzyme and substrate molecules. Additionally, the product generated in the bioreactor has to be extracted because it may under certain conditions inhibit its own production. In some processes there may also be even a prepurification in the bioreactor itself. If living micro-organisms have to be applied, it is necessary to provide sufficient nutrition and respiration gases in case of aerobic fermentation. All other reaction parameters such as temperature, pH-value and reaction time have to be controlled precisely. In many cases (generally with modem processes) the maintenance of microbiological integrity (sterile process) is absolutely mandatory for a successful fermentation. 

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