Xylans, synthesis

Ebringerova, A. Hromadkova, Z. Kacurakova, M. Natal, M. Quartemized xylan synthesis and structural characterization. Carbohydr. Polym. 1994, 24, 301-308. 

Pure xylan is not employed in industry. but crude xylan or pentosans are of industrial importance. Xylan has been proposed as a textile size but is not employed as yet for this purpose.130 Perhaps the largest use of pentosans is in their conversion to furfural, which has many applications and serves as the source of other furan derivatives. At the present time, large quantities of furfural are used in the extractive purification of petroleum products, and recently a large plant has been constructed to convert furfural by a series of reactions to adipic acid and hexamethylene-diamine, basic ingredients in the synthesis of nylon. In commercial furfural manufacture, rough ground corn cobs are subjected to steam distillation in the presence of hydrochloric acid. As mentioned above, direct preferential hydrolysis of the pentosan in cobs or other pentosan-bearing products could be used for the commercial manufacture of D-xylose. 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Selective Production of Xylanases by Cellulolytic Microorganisms. Until recently there was little information on common or separate genetic control of cellulase and xylanase synthesis in microorganisms (60). Studies on this subject were complicated by the fact that numerous microbial ceUulases and xylanases are non-specific with respect to cellulose and xylan as substrates. As could be expected from a comparison of both polysaccharide structures, non-specificity is more frequently observed with cel-lulases, because their substrate binding sites can easily accommodate substrate using an unsubstituted p-(l 4)-linked chain of D-xylopyranosyl units. 

Low-Molecular Weight Xylanase Inducers. The response of cellulolytic fungi to low-molecular weight fragments of xylan and ceUulose confirmed the separate regulatory control of the formation of xylanases and ceUulases. Xylobiose [Xylp-p-(l- 4)-Xylp] was a specific inducer of xylanases in T. reesei (61) and Aspergillus terreus (67). Sophorose and otho glucobioses selectively induced the synthesis of... 

Such enzymatic catalyzed polycondensations have allowed the synthesis of a number of natural polysaccharides, but has also allowed the production of nonnatural polysaccharides such as cellulose-xylan hybrids and functionalized hyaluronan, chondroitin sulfate, and chondroitin. Such work illustrates the ever-narrowing bridge between natural and synthetic polymers and polymer syntheses. 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

T. Uryu, Y. Koyama, and K. Matsuzaki, Selective ring-opening polymerization of 3,5-anhydro-1,2-0-isopropylidene-o -D-xylofuranose synthesis of (3—> 5)-D-xylan, Makromol. Chem. Macro. Chem. Phys., 185 (1984) 2099-2107. 

P-Xylosidases are being used commonly in paper industry for complete degradation of xylans. The enzymes used in the synthesis of xylose-containing compounds have also been reported. For example, Eneyskaya et al. (50) have used />-nitrophenyl P-D-xylopyranosides (XyiPpNP) as a donor and a 3-xylosidase ixom Aspergillus sp. in the synthesis of several 31,4-D-xylooli-gosaccharides as P-xylanase substrates. 

Kobayashi S. Wen X, Shoda S. Specific preparation of artificial xylan a new approach to polysaccharide synthesis by using cellulase as catalyst. Macromolecules 1996 29 2698-2700. 

Scheme 66. Synthesis of xylans 332 using an engineered xylanase.

Nakamura, T. Toshima, K. Matsumura, S. One-step synthesis of n-octyl (3-D-xylotrioside, xylobioside and xyloside from xylan and n-octanol using acetone powder of Aureobasi-dium pullulans in supercritical fluids. Biotechnol. Lett. 2000, 22 (14), 1183-1189. 

Xylanase synthesis on maltose medium was more sensitive to glucose repression when compared to the enzyme synthesis on the xylan medium (Fig. 3a). At 24 h, xylanase production was at the same level in the medium with maltose and maltose plus glucose at 1.0 g/L. However, glucose at 3.0 or 5.0 g/L repressed enzyme production after 24 h of eultivation, leading to a reduction of 55% on specific activity (about 2.30 U/mg dry biomass Fig. 3a). 

The repressive effect of xylose on xylanase synthesis was milder on the maltose medium than on the xylan medium. Xylanase production was induced, at 24 h, when the maltose medium was supplemented with xylose at 1.0 g/L, and xylanase repression was observed, at 24 h, only when 3.0 or 5.0 g/L of xylose was added, leading to a reduction of 22 and 57%, respectively, on the productivity (Fig. 3b). 

Xylanase production on maltose has been cited for some microorganisms [27, 28], but the authors did not provide details about the enzyme production on this substrate. The data reported in the present work indicated that xylanase synthesis on the maltose medium was more sensitive to glucose repression and less sensitive to xylose repression, when compared to the enzyme synthesis on the xylan medium. From these data, we can infer that xylanase synthesis by B. circulans D1 is regulated by different ways when the microorganism is cultivated on xylan or maltose. 

The results showed in this research indicate that B. circulans D1 is a promising microorganism regarding to xylanase production, as the amounts of enzyme obtained were similar to or higher than those cited in the scientific literature in works dealing with Bacillus species. The enzyme synthesis is not necessarily related to the presence of xylan in the cultivation medium. Maltose was the best inducer of xylanase production, and the microorganism showed versatility in the utilization of other carbon sources. 

The participation of uridine 5-(D-xylopyranosyl pyrophosphate) in the metabolic pathway leading to the synthesis of xylan appears to be well established. This glycosyl nucleotide has been isolated from seedlings, and synthesized enzymically from uridine 5-(a-D-glucopyranosyl pyrophosphate) by the following reaction sequence. ... 

Here, compounds 10 and 13 were chemically synthesized as monomers for the synthesis of cellulose-xylan hybrid polysaccharide 11, and chitin-xylan hybrid polysaccharide 14, respectively (Figure 8). 

The results of later work with plant enzymic preparations in vitro agreed with the postulated mechanism of synthesis of xylan in vivo. Particulate enzyme preparations from com shoots and immature com-cobs were capable of transferring D-xylosyl- C groups from... 

Xylo OS [Xyl(pi] - 4)Xyl Enzymatic synthesis of, for example, corncob xylan... 

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