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Current Trends in Biotechnology and Pharmacy Vol. 6 (2) 145-165 April 2012, ISSN 0973-8916 (Print), 2230-7303 (Online) number prior to 2006, a pointer to the increasing interest for tannases. Recombinant Tannases : The tannases of wild microbial sources do not satisfy all requirements for optimal versatility in industrial processes. They exhibit rather limited substrate spectra and are relatively expensive to purify because they are secreted only at low levels by their microbial producers. Obviously, the high-yield production of fully active recombinant tannases is an attractive goal, both for basic research and for industrial purposes. There is therefore an ongoing search for new sources of tannases and recombinant technology to produce the same with more desirable properties for commercial applications (27). Over-expression of TAN genes in different hosts seems to be relatively difficult. Hatamoto et al. (28) cloned and sequenced the gene-encoding tannase from A. oryzae. Later, the Aspergillus tannase gene was heterologously expressed in Saccharomyces cerevisiae, although with a low yield of protein production. Conversely, large quantities of enzyme were obtained when tannase gene was cloned in Pichia pastoris (29). In addition, A. adeninivorans is thermo and osmotolerant and can be cultured at temperatures up to 48 ºC in media containing up to 20% NaCl. These unusual properties make A. adeninivorans an ideal host for heterologous gene expression as well as a useful source of the same genes for biotechnological significance (27). The secreted Arxula tannase was glycosylated with a carbohydrate content of 31.2%, higher than that of the A. oryzae tannase (22.7%) but close to the level found in the recombinant tannase of the A. oryzae produced in P. pastoris (29). Tannases are often posttranslationally modified. In A. oryzae, the product of the tannase gene is translated as a single polypeptide and then cleaved into two subunits linked by disulphide bonds (28, 29). In contrast to the case in A. oryzae, the A. adeninivorans tannase subunit Atan1p does not undergo post- Dinesh Prasad et al 146 translational cleavage (except for the removal of the secretion signal). This may be one of the reasons why the tannase from Arxula exhibits a higher specific activity than its counterpart from A. oryzae (27). Zhong et al. (29) have described the successful expression of an A. oryzae TAN gene in the methylotrophic yeast P. pastoris. For this purpose the gene was fused to the open reading frame (ORF) coding for the α-mating-typespecific genes (MAT-α) sequence and placed under the control of the methanol-inducible alcohol peroxidase 1 (AOX1) promoter from P. pastoris. Recombinant tannase was successfully secreted by P. pastoris and the productivity of recombinant tannase was found to be approximately 3.5-fold higher compared to that of solid-state fermentation with wild strain (29). These features will enhance future acceptance of the transformants as tannase producers in industrial applications. In another report the identification and cloning of a gene (tanLpl) encoding tannase from Lactobacillus plantarum ATCC 14917 and subsequent expression in P. pastoris was carried out (30). Curiel et al. (15) have reported the production and characterization of recombinant tannase from L. plantarum. The physicochemical characteristics exhibited by L. plantarum recombinant tannase make it an adequate alternative to the currently used fungal tannases (15). Substrates for Tannase production : There are two main categories of tannase substrates reported i.e. natural and synthetic. Propyl and methyl gallate are synthetic while tannins are naturally occurring substrates. Tannins are plant secondary metabolites and also the second most abundant group of plant phenolics after lignin (9). They are defined as naturally occurring watersoluble polyphenols of varying molecular weight ranging from 500 to 3000 Da and used as substrates for tannase production by microbial fermentation (3, 31).
Current Trends in Biotechnology and Pharmacy Vol. 6 (2) 145-165 April 2012, ISSN 0973-8916 (Print), 2230-7303 (Online) Tannins are widely distributed in different parts (barks, needles, heartwood, grasses, seeds and flowers) of vascular plants (32) and they are further classified into two major groups: hydrolysable and condensed tannins (33). The hydrolysable tannins (Fig. 1) are constituted by several molecules of organic acids such as gallic, ellagic, digallic and chebulic acids, esterified to a core molecule of glucose. Also molecules with a core of quinic acid instead of glucose also considered as hydrolysable tannins. Condensed tannins or proanthocyanidins are complex compounds constituted by flavonoid groups (2 to 50) which are considered not to be hydrolysable (Fig. 2). Their major constituents are cyanidin and delphinidin which are also responsible for the astringent taste of fruit and wines (26, 34). The selection of a substrate for tannase production by fermentation depends on several Fig. 1. Structure of hydrolysable tannins [Gallotannin (A), Ellagitannins (B), Ellagic acid (C), Hexahydroxyphenic acid (D). 147 Fig. 2. Structure of a typical condensed tannins. Condensed tannins are all oligomeric and polymeric proanthocyanidins. ‘n’ denotes number of flavonoid groups (2 to 50). factors viz., cost, availability and suitability of the substrate for obtaining the desired yield of tannase, and thus requires screening of several agro-industrial and forest residues (35, 36) Mukherjee and Banerjee (35) evaluated the various substrates for production of tannase and gallic acid on the basis of their tannin contents as 30-41% in case of myrobalan fruit (Terminalia chebula), 10-14.1% for tea leaf and 40-52% in case of teri pod (Caesalpinia digyna) cover powder (37). Optimum tannase activity and yield were reported with a mixed substrate of myrobalan and teri pod powder at a fixed ratio (4:6) with maximum gallic acid production. Microbial degradation of condensed tannins is less documented than that of gallotannins; however, it is known that the selective hydrolysis of galloyl groups of the ellagitannins is catalyzed by tannase (4). Ellagic acid production from cranberry pomace (Vaccinium microcarpum) by SSF using a fungus Lentinus edodes has been reported, attributing the catalysis to the enzyme β-glucosidase (38). A review work of Li et al. (3), presents information on tannase production by species of Lactobacillus, Leuconostoc, Oenococcus, and Pediococcus, using substrate Overview on production and characterization of tannases
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6th Annual Convention of Associatio
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