Polysaccharides synthesis with enzymes

Another problem for biosynthesis concerns the fact that certain other, native plant-polysaccharides, most notably the xyloglucans found in higher-plant cell-walls,6 contain backbone structures of (1— 4)-/3-D-glucan, and it is necessary to consider whether separate enzymes synthesize these structures, and, if so, whether these enzymes could be confused with enzymes specifically involved in the synthesis of cellulose. 

The continuing interest of Bourne in the chemistry of polysaccharides and associated enzymes originated from the work of Haworth and Peat directed towards the enzymic synthesis and degradation of starch. The impetus for this work was given by the discovery, made by C. S. Hanes in 1940, that a phosphorylase isolated from the potato and pea effects the synthesis, from D-glucosyl phosphate, of starch, later shown (by Haworth, Heath, and Peat) to be amylose. In his first paper (with Haworth and Peat) in 1944, Bourne described the isolation of the Q-enzyme which, in conjunction with phosphorylase, effects the conversion of D-glucosyl phosphate into the major component of whole starch, namely, amylopectin. He had discovered the Q-enzyme in a fraction discarded by previous workers. Already, the quintessence of his mind was revealed in this work meticulous attention to detail, and perception of essentials. 

In addition many of the glycosyltransferases and glycosidases used so far are not very stable. However, it can be expected that with biotechnological methods it will become possible to obtain enzymes with enhanced stability that can be used more frequently in polysaccharide synthesis. One example is for instance the new mutants of glycosidases seen in Section 9.3.3. 

Despite some unexplained results concerning cellulose synthesis with a cell-free system in plants, the data seem, in general, to be consistent with the assumption that GDP-D-glucose is the sugar nucleotide from which the /3-D-(l — 4)-linked chain of cellulose originates. However, there is evidence for the presence of another enzyme system that, in some plants, produces from UDP-D-glucose a polymer that appears to be a mixed, )8-D-(l - 4), /3-D-(l - 3)-linked polysaccharide. - ... 

A very similar reaction was observed with enzyme preparations from chick-embryo cartilage.Most of the reaction product contained D-xylose in an alkali-labile linkage, and a xylosyl-serine was isolated from the pronase-digested incorporation product. As the enzyme had been prepared from a tissue very active in chondroitin 4-sulfate synthesis, this reaction is assumed to be the first step in the synthesis of the polysaccharide moiety of this compound. Here, again, after centrifuging at 105,000 g, the supernatant fraction was at least as active as the particulate fraction. 

Fermentation processes can be a valuable alternative to the conventional chemical synthesis, particularly when the finished product contains specific and complex stereochemistry. Fermentation technology in the industrial synthesis of chemicals started to be used in the first decades of the twentieth century. Industrial production of citric acid by fermentation, achieved by Pfizer in 1923, was an early success in this field. However, it was only with the production of penicillin during the Second World War that the whole sector took off. Today, besides ethanol, the range of products that are produced by fermentation includes antibiotics, organic acids, amino acids, polysaccharides, vitamins, and enzymes. 

Amylosucrase 173) from Neisseria perflava utilizes sucrose for the synthesis of polysaccharides with properties intermediate between those of glycogen and amylopectin 174). Apparently, there are two enzymes responsible for this synthesis one enzyme converts sucrose into an amylose-type polysaccharide and the other enzyme, which is of the Q-enzyme type, breaks the straight chains to produce highly branched molecules. The resultant branched polysaccharides have about 12 D-glucose units for each nonreducing end unit. 

A more specific catalysis was developed using enzymes as catalysts for polymerizations ( enzymatic polymerization ) starting in the mid-1980s, which can now be regarded as a third stream of polymerization catalyst. Normally, monomers of enzymatic polymerization are to be recognized and activated by the enzyme for the polymerization to occur, and thus polymers with precisely controlled structiues are expected. This articles concentrates on in vitro polysaccharide synthesis via enzymatic polymerization. 

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